Astronomy

Why does the Moon's terminator look “wrong” in this image?

Why does the Moon's terminator look “wrong” in this image?


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There is a picture of the moon in National Public Radio's on-line article Get Ready For Halloween By Watching The Moon's 'Occultation' Tonight. It looks wrong to me - specifically, the brightness gradation near the terminator - or lack thereof.

The image is credited "A waning gibbous moon occultation will be visible Tuesday night in parts of the United States. JPL/NASA" but I wonder if this is true. Something just looks "wrong" here.

In the original article the JPL/NASA contains a link to this page, which currently contains this image also shown below. It looks more like an actual photograph.

And here is a NASA/JPL image from https://svs.gsfc.nasa.gov/4404 for 2016-10-19 04:00 UT, which if I uderstand correctly is not actually a photograph, but simulated from LRO data. It also illustrates that the terminator is expected to be graded from light to dark and contain contrast from shadowing.


above: terminator from all three images. Realistic terminator on the right show graded intensity and strong shadowing from the highly oblique incident light.


The reason that moon image looks wrong is because it is wrong. It is not a real image of the moon $-$ at least the terminator is not real.

The original article you cite has a link just below their image indicating the source of their image of the moon. That source is the night sky planner, hosted by JPL. You'll find the same image on that website, albeit slightly darker (it seems the NPR people lightened up the image a bit).

If you do some more digging, you'll see that the night sky planner got its image of that moon from someone else. Within the html code, they have the image defined as:

the moonUnited States National Observatory.

After a bit of digging, I found out what exactly is going on here. The purpose of this site is to show you the current phase of the moon. To do this, they take a single image of the full moon and artificially shade out a region to make it appear as the current phase of the moon. You can see their process here and how it was done by some guy named R. Schmidt. They've broken down the moon phases into 181 images which you can download here, if you're interested.

As you can see, the terminator on that image is wrong because it is not a real image of the current phase of the moon, but rather a computer generated "shading" of the full moon to indicate the current moon phase.


Elementary Astronomy Hybrid/On-Campus Laboratory (108)

There is a lot you can learn by simply observing the Moon without a telescope, night after night, month after month, and even year after year. On September 8, 2013, from outside the Physics and Astronomy building on campus, this is how it appeared just after sunset.


The bright "evening star" next to it is Venus. At the time it was still on the far side of the Sun, beginning to approach Earth on its own faster orbit. For months afterward, when the Moon reappeared in the evening, Venus would be there too until April of 2014 when it passed up Earth and then moved to the morning sky. If you had a telescope and could see Venus up close on the night this photo was taken, it would have been half illuminated, like a first quarter Moon. Just before it passed by Earth, in April of the next year, it too was a thin crescent slice of sunlight.

The appearance of the Moon (and Venus and Mercury) depends on where it is with respect to the Sun and the Earth. For understanding this image, imagine that the Sun is below the horizon to your right. It is illuminating the spherical Moon from a very great distance, lighting up half of the Moon's surface that is on the side nearest the Sun. From Earth we see only part of that illuminated sphere, and to us it looks like this crescent. Every day the Moon progresses farther along in its orbit around Earth, and would nightly move more toward the east, showing more and more of the illuminated surface for those of us on Earth.

The Details

The motions of the Earth and Moon determine how it looks to us. If you could put yourself out into space looking onto our solar system you would see the Earth and Moon both orbiting the Sun, but in different planes and different rates.

  • Earth rotation on its axis every 23 hours 56 minutes
  • Earth revolution around the Sun 365.256 days
  • Moon rotation on its axis every 27.3 days
  • Moon revolution around Earth 27.3 days
  • Moon revolution with respect to Earth-Sun line 29.5 days
  • Tip of Earth's axis to the plane of its orbit 23.5 degrees
  • Tip of Moon's orbit to Earth's orbit 5 degrees
  • Tip of Moon's axis to Moon's orbit 6.7 degrees
  • The Moon's orbit is elliptical, whether viewed with respect to Earth, or with respect to the center of gravity of Earth and Moon
  • Its closest approach to Earth of about 360,000 km and most distant from Earth about 406,000 km. The "geocentric" orbit has a semi-major axis of 384,400 km.


There is a lot here to process, so let us focus on some essential ideas that you may have heard about in an astronomy course

  • Every 29.5 days on average the Moon reappears after new Moon in the evening sky
  • Dividing by 4, first quarter, full, last quarter, and new again are separated by a little more than 7 days
  • Seen from space, the Moon orbits the Earth in less time, about 27 days, but it takes 2 more days to catch up to the Earth-Sun line and get to new again
  • The Moon rotates as it orbits, and on average keeps the same face toward Earth (the "near" side) and away from Earth (the "far" side)
  • The rotation is at a steady rate, but the orbital motion speeds up and slows down, we get to see a little more east and west of the near side during the month
  • The rotation axis is tipped to the orbital plane, we get to see a little more north and south than just 50% of the Moon during the month
  • The orbital plane is tipped to Earth's equator, the rising and setting points of the Moon on the horizon vary during the year
  • The direction of the orbital plane slowly "precesses", taking 18.6 years to complete a full turn as it maintains a nearly constant tip to the plane of Earth's orbit. The direction it goes is clockwise seen looking down on the orbit.
  • The direction of the semi-major axis of the ellipse that defines Moon's orbit takes about 8.9 years to complete a cycle. the direction it goes is counterclockwise, also seen looking down on the orbit.

If you want to see the dates and positions of the Moon, thanks to Newton and precision measurements of where it has been, we can predict it very accurately from the laws of motion and gravity, plus some complex geometry. Fortunately, there's a program that will do that for us on-line.

Visualizing the Motions

Recently, each year NASA has produced an animation that shows the appearance of the Moon through the year, and let's you see how the complexity of the orbits of Earth and Moon, and their rotations, combine to alter how the Moon appears to us. This is the one for 2020. If that is not the current year or the year you are interested in, try searching "YouTube" for "NASA moon 2020" or another year of interest.

For a better view, click the "full screen" icon on the lower right, or use the link on the resource page to get the animation in the full sized browser window. Running full screen you can see the crater names appear as they are highlighted. The animation covers a full year.

This animation does not show you how the Moon's rising and setting on the horizon vary during the month, month to month, or even year to year. That is indeed very complex, but it was known to the the architects of Stonehenge who placed sight lines in the structure marking the extremes of the lunar progression on the horizon. It is speculated that those lines enabled predicting the occurrence of lunar and solar eclipses. There's a short video lecture about this if you are interested in the archaeology of Stonehenge and how this works.

First Questions about Observing the Moon

1. What is the phase of the Moon now? In responding to this answer you will need to tell us the date too so that we can check it. Identify the phase as new, waxing (increasing) crescent, first quarter, waxing gibbous, full, waning (decreasing) gibbous, last quarter waning crescent. The terms "waxing" and "waning" are commonly used to describe the Moon's appearance, but may be new to you. You will need a clear night and a few minutes to go out and look for yourself.

2. When will the Moon next be last quarter, and at what time of night local time will it rise? (If it is now last quarter, give the one next month.) For this, use online resources linked from our Moon resource page .

3. Measure the apparent angular size of the Moon in the night sky yourself. What do you find? Here's how to do it.

How to measure the angular size of the Moon

You will need something to measure with a small ruler, yardstick, or tape measure will do. Also, you will need a night when you can see the Moon. Just wait for the opportunity to finish this one. Extend your arm fully, and measure how far the ends of your fingers are from your eye. For most people it will be about 1 yard, a little less than 1 meter. If you are using metric units, measure to the nearest centimeter. In Imperial units (not recommended for science), measure to the nearest inch.

When the Moon is visible, hold something appropriately small at arms length. You can try the eraser on the end of pencil, but may need something a little smaller than that. Pick an object just large enough to block out a diameter of the Moon. If the diameter of the object you chose is (d) and the length of your arm is (ell) , the angle covered by the Moon is about

( heta = 180/pi imes d/ell = 57.3 imes d/ell )

in degrees. This works as long as the angle is small, and is a common approximation for astronomy. To give an example, suppose you found that a object 6 mm across covered the Moon when it was 750 mm from your eye. The angle would be

( heta = 57.3 imes 6/750 = 0.46^circ )

In responding to this question you will be asked to describe the details of how you made the measurement. It helps to take notes, then fill in all the answers on this site at one time later.

Full Moon

From the resource page, click on "Full Moon" or go directly to this link

to view a telescopic image of the full Moon. This one was take a couple of days before it was completely full, but it shows most of the surface with the direct illumination of sunlight that is characteristic of when the the Moon is in the opposite direction in the sky from the Sun. We are seeing the Moon with the Sun behind us, and there are no shadows on the center of the Moon. You will notice some along the left side (the east as the Moon is in the sky).

Notice the bright features with "rays" that extend far across the disk. These "rayed" craters were created by recent (on a lunar time scale) impacts that splattered debris over the darker surface, and the ray material crosses older features such as the dark basalt that highlights the big impact basins called lunar mare (or seas). Answer these questions by looking at this image, and with the help of lunar maps and links on the resource page to identify names.

Just a quick word about directions. It is common to label planetary objects by directions analogous to compass directions on Earth. Just as when you look at a globe of Earth with north up, the west side would be on your left and the east side would be on your right, when you look at the Moon in the sky, its "west" is on your left and its "east" is on your right. Those are directions a person walking on the Moon would use. However, for us, here on Earth, east is to our left and west is to our right. In the sky the directions are reversed left-right for us from what they would be for a person mapping the Moon on the Moon. Here we will refer to compass directions from our point of view so that with north up, east in our sky is to our left. However if you look at a lunar map they may be reversed. The images on our resource page show the Moon as you would see it with your own eyes in our northern hemisphere night sky.

4. At the bottom, or south polar end, there is a major rayed crater. Who is it named for, and what was he famous for?

5. About midway top to bottom, but toward the left, there is another bright big rayed crater. Next to it more toward the east (left) there is a smaller one. What are their names?

6. Mare Imbrium, the sea of "rains", is prominent in the full Moon image. It is a large round basin filled with lava, bounded by a mountain range on 3/4ths of its outline that is the remaining edge of the crater now filled up with basalt. Some rays cross it from below. Identify Mare Imbrium on the image. Also identify the "Appenines". If you need help, try links in Wikipedia which will guide you to these regions. In answering, you will identify them the image but for now just make a note so you can fill this in later.

Apollo 15 landed in Mare Imbrium.

7. Using the screen image of the full Moon and a ruler, measure the diameter of the full disk and approximately measure the diameter of the circular Mare Imbrium. How far across is Mare Imbrium in kilometers? (There are 0.62 miles in 1.0 km.)

Here's how to figure this out. The Moon's full diameter is 3474 km. Measure the diameter of the image and call that (D_) . Measure the diameter of the Mare using the same scale (mm recommended) and call that (d_) . Then simple proportion gives the size of the Mare in kilometers

In supplying your answer, think about the effect of the Moon's curved surface on how this measurement is made. If you wanted to make a really accurate measurement, what would be required? For comparison, the continental US is about 4300 km across, that is it is bigger than the entire Moon!

First Quarter Moon

Now select the first quarter Moon image from the resource page, or go directly here

In this case the angle from the Moon to the Earth is nearly 90 degrees from the angle to the Sun and the Moon appears half illuminated. We call it a "quarter" rather than a "half" Moon, undertanding that we are seeing only 25% of its surface.

8. The flat looking oval mare that is on the far right center (western side in our sky) side of the image is also visible in other images that are on the resource page. What is its name, and why does it look oval rather than round? For a hint, look at the appearance of craters toward the Moon's south pole too.

Magnify the image by clicking the "+" button so that you can see as much detail as possible. Look along the terminator, the line separating the light and dark sides down the center. If you were on the terminator the Sun would be on the horizon and shadows would be very long. Near the top (north), bordering on Mare Imbrium, there is a round crater with the mountains that define its rim just sticking up into sunlight. This is the crater Plato which we will look at carefully next. Before getting to that though, immediately to its right is a gash through the rim of Mare Imbrium often called the Alpine Valley or Vallis Alpes. While it appears as if something swept this area clear in a impact (Mare Imbrium is over 3.8 billion years old) this valley floor is also filled with lava.

9. What caused this feature? (Hint: Read more here, or elsewhere, to find the answer.) What is the name of the other irregularly shaped Mare that is "north" of Mare Imbrium and to which this valley extends? Understanding these landmarks makes it easier to find your way around the Moon at any phase, and to place its rugged features into a context of how they formed.

Plato

The crater Plato which you found in the image of the first quarter Moon is best seen a day or two later when sunlight reaches into the crater. We have a more detailed image of it

and while a few better pictures have been taken with Earth-based telescopes, this one shows almost all the detail that can be captured unless the sky is exceptionally stable. Notice how the jagged shadows of the mountains forming its rim extend well into the crater across lava flow that has filled its interior. There are a few small "craterlets" that resulted from impacts on the Moon after the event that made Plato. The question for you is, "How high are the mountains above the crater floor?"

To answer this, look at the full, first quarter, and last quarter images. They all show Plato but only in this detailed image can you see the shadows well. We need to know the angle of the Sun as seen above the horizon on Plato. If you know that angle, then you can calculate the height of the mountains.

Here's how to do this, step by step.

We will use the diameter of Plato as a "scale" to measure the mountains, so find its diameter by looking at a a Moon image where you can see the full diameter of the Moon as well as a nicely defined crater. Use the same method you used for finding the diameter of Mare Imbrium to find the diameter of Plato. Of course, Plato is much smaller than the big impact basin below it, but the idea of the measurement is the same.

Measure the diameter of the Moon on your screen, the diameter of Plato with the same ruler, and then from the ratio find the diameter of Plato in km.

10. What is the diameter of Plato in km?

In the detailed Plato image linked above, how far are the mountains that are casting their shadow on the crater floor from the "terminator" defining the sunset line on the Moon. Use Plato's diameter as your ruler for this, and come up with an estimate as best you can. It will not be exact because the terminator line varies as the height of the lunar terrain, but you can get an estimate that is good to maybe 20% if you are careful. You will use this distance to find the angle of the Sun above the horizon. Let's day this distance is (X) given in km. The circumference of the Moon is ( pi D ) where (D) is the Moon's diameter. Since it takes 360 degrees to go the full circumference, if these mountains are (X) from the terminator, they are an angle seen from the center of the Moon away from the equator to the mountains in degrees

( heta = 360 imes (X/C) = 360 imes ( X / (pi imes 3474) ) )

( heta = 0.033 imes X ) degrees

if you measure the distance (X) of the mountains from the terminator in km. Keep in mind that Plato is not a very big crater, and this angle will be rather small. Now we also know how high the Sun is in Plato's sky at this moment. Think about it.

The Sun is on the horizon to a viewer on the terminator. As the viewpoint moves over toward the Sun, the Sun gets higher in the sky. That means that for every degree the viewpoint moves in that direction, the Sun goes up a degree. If the viewpoint moved a full 90 degrees, then the Sun would be overhead. It is really simple to picture.

11. How high is the Sun above the lunar horizon seen from the center of Plato? Give your answer in degrees. You can tell if your answer is reasonable by considering that the Sun will appear to go around the Moon in one lunar month, if you were on the Moon. That means it goes 360 degrees in a month, or about 12 degrees a day. If Plato were exactly on the terminator today the Sun would be on the horizon seen from Plato. The next day the Sun would be up in the sky 12 degrees above the horizon.

Again using the diameter of Plato as a measuring stick, how long is the shadow of the mountains on the crater floor in km? If the shadow has a length (S) for the Sun at an angle ( heta) in degrees, as long as the angle is small the height of the mountains is

Here "H" is in km if you measure S in km. Be sure you use ( heta) in degrees. The 57.3 converts from degrees to radians.

12. How high is this mountainous rim above the crater floor of Plato?

You could use this method cautiously to measure the depths of craters anywhere on the Moon by finding how long it takes for the sunrise (terminator) line to move out of the crater, and then at some time later find the length of the shadow from the rim. Of course more precisely now we have surveyed the Moon from orbit around it using radar and precision measurements of altitude so the terrain is very well studied.

Libration

We will finish up by revisiting the idea of "libration", which is the apparent nodding of the Moon that lets us see more than half its surface. Libration in latitude is north-south and is from the tip of the rotation axis of the Moon seen from Earth. Libration in longitude is east-west, and is from the variations in the Moon's orbital speed because its orbit is elliptical rather than perfectly circular.

13. Of the images on the resource web page from our telescope, look for libration by comparing the images of different phases and dates. What did you find? If you are having trouble seeing the effect, it is more dramatic in the NASA video. After you look at the video again, come back to the images and see if you can find it.


So Why Did We Call the Image “Shakesperean?”

Shakespeare, the Moon, and human trials and tribulations are all wrapped up together. As Andrew McCarthy says about one of his images, “In uncertain times I look to the sky, and am comforted by the moon and stars.” If you’ve never felt some similar sentiment, get your DNA tested. You may not be human.

Shakespeare was fond of the Moon, too. It came up often in his work. For example, “Arise, fair sun, and kill the envious moon, Who is already sick and pale with grief That thou, her maid, art far more fair than she. . .” That’s from Romeo and Juliet.

The Bard also took a swipe at astrology in King Lear, and pointed out the folly in blaming the Moon and other bodies for their own ill-fortune:

“This is the excellent foppery of the world, that, when we are sick in fortune,–often the surfeit of our own behavior,–we make guilty of our disasters the sun, the moon, and the stars: as if we were villains by necessity fools by heavenly compulsion knaves, thieves, and treachers, by spherical predominance drunkards, liars, and adulterers, by an enforced obedience of planetary influence and all that we are evil in, by a divine thrusting on: an admirable evasion of whoremaster man, to lay his goatish disposition to the charge of a star.”

William Shakespeare, 1564-1616. Image Credit: By John Taylor – Official gallery link, Public Domain, https://commons.wikimedia.org/w/index.php?curid=5442977

Greats like Shakespeare make us think twice about things: life, love, nature, the Moon, our fate. They make us see it with new eyes.

And if it’s not too grand a statement, I think McCarthy’s image of the Moon does the same. Even in an age where our super-telescopes give us splendorous images of the cosmos and all the objects that populate it, a simple, yet unattainable image of the Moon is somehow more humanistic.


I haven't checked those figures, but regardless, they could not possibly account for the illusion--no one would ever be able to reliably discern that small difference.

And the diagrams mostly deal with the sun below the horizon, or at the horizon, which would make that exercise a little more difficult.

If the earth were transparent though, and you imagined a straight line from the sun (even below the horizon) to the moon, that line would be perpendicular to the terminator to the degree necessary for discussion of this problem. That's my objection to the diagram--the line between the sun and moon is not perpendicular in the diagram, but we know that it is.

I agree that it would be difficult to mentally draw that line between a nearly full moon and the sun--because we as observers would be between them--but it's not like we look at the full moon and infer that the sun is shining behind us no matter which way we turn.

Here's a walk-through in a toy model that might help with the visualization. We assume the Earth's orbit, the moon's orbit and the Earth's equator all lie in the same plane. This simplifies the visualization without doing any violence to the real situation.
Now let's deal with a waxing gibbous moon, halfway from first quarter to full.
We go to the north pole, and look at the moon and sun. Because of our simplifying assumptions, they're both lying on the horizon, with the moon 135 degrees away from the sun. Its fully illuminated side is facing the sun, terminator orthogonal to the horizon. We can draw a horizontal line on the sky, parallel to the horizon, connecting the sun and moon, and it, too, is orthogonal to the moon's terminator. This is the path light rays are travelling from sun to moon, and all seems consistent and normal.
Now go the equator. Find a point on the equator where the sun is sitting on the the western horizon. The gibbous moon is 45 degrees above the eastern horizon (ie, still 135 degrees from the sun). Its illuminated side is pointing towards the zenith: that is, it is facing upwards, despite the fact that the sun is on the horizon. The line we drew on the sky connecting the sun and moon now climbs vertically from the western horizon, crosses the zenith, and descends to the east until it hits the moon, still orthogonal to the moon's (now horizontal) terminator. After a bit of thought we can see that this is correct: the sun's rays are shining far over our heads to illuminate the moon, so we can see a little way "under" the illuminated face to the shadowed side.
Now, picture a line of observers strung out along the line of longitude that connects our equatorial observing point to the north pole. For each of them the sun is going to be on the horizon. For each of them, the moon's illuminated face is going to be pointing in some direction intermediate between straight up (the equatorial situation) and horizontally (the polar situation). So every single observer in those mid-latitudes will see the illuminated side of the moon tilted upwards to some degree, despite the fact that all of them see the sun on the western horizon. Wait a few moments for the sun to set, and all will have a view similar to the one described in the OP.
For these observers, the line we drew in the sky now appears to curve upwards from the sun's position, reach a high point due south, and then curve down to meet the moon orthogonal to its terminator.
But it's the same line. We thought it was straight when it was parallel to the horizon, and we convinced ourselves it was straight when it ran vertically. But our brain is unhappy with the convergence that is an inevitable part of seeing long parallel lines in a perspective that covers a big angular arc. A long line that converges on both horizons (as sun rays at sunset do), seems like it must have a curve in it somewhere. So in these intermediate latitudes, it seems like we ought to be able to draw a straighter line between moon and sun, undercutting the line in the sky we previously agreed to be straight!
But imagine (for the moment) you could do that. So draw a new line that undercuts our original line and looks straighter. Now head back to the pole, so that our original line is running along the horizon, connecting the sun and moon. Where is our new line? It must be leaving the sun, curving below the horizon and then up again to hit the moon! So it can't be straighter.

So it's all just a trick of perspective. If, at some intermediate latitude, you could do an experiment in which you held your head dead steady, and stretched a piece of string from your right thumb (covering the sun on the horizon) to your left thumb (covering the gibbous moon high in the sky), you'd find the string did seem to pass higher than your head and then descend towards your view of the moon, meeting it at right angles to its terminator.

Thank you Grant, I respect your knowledge and explanation but I'm still confused. I'm sure this apparent stupidity on my part is simply a lack of intelligence or understanding but so be it.

From your explanation I have concluded the following

1) that from the earth's north pole, the moon's terminator appears vertical and perpendicular to the sun
2) that from the earth's equator, the moon's terminator appears horizontal and perpendicular to the sun

but I'm having trouble with the part that says

3) that between the earth's equator and the earth's north pole the moon's terminator, rather than being perpendicular to the sun, should be perpendicular to a point higher than the sun.

Imagine I'm at the earth's north pole and the moon's terminator is vertical, the eastern (left) side of the moon is dark and the western (right) side of the moon is lit. If we freeze time and I walk southwards towards the moon, the sun will stay on the horizon and the moon will rise in the sky. When I reach the equator the moon will be high overhead in the eastern sky and the eastern side will be dark and the western side will be lit, and the terminator will appear horizontal. But if you analyse the transition of the terminator from vertical at the pole (dark side east, or left) to horizontal at the equator (dark side east) the terminator has simply rotated clockwise by 90 degrees. At no point does it rotate anti-clockwise. And what I am seeing out of my window implies an anti-clockwise rotation of the terminator.


Waning Moon, with panoramic views along the sunset terminator

These images represent data that has remained unfinished since last August 24, 2019. I have previously posted images of Copernicus and Mare Orientale, but have not completed the set of data for the entire Moon until now. Aside from the usual time constraints when dealing with large amounts of data, this data also proved to be somewhat difficult to deal with for a variety of reasons, owing in part to the changing sky brightness of each panel as captured in sequence near dawn, but also the use of a 1.4x barlow did not help in any way. In fact, the final image (available by link) was downsized to 50%, in part because the original image scale was too large, but also, I've come to realize that most web browsers display images much larger than I would prefer (above 100% pixel scale), and in most cases this is unflattering.

The image was captured with a C9.25 Edge HD using the ASI183mm camera with a green filter (Baader, bandpass 500-575nm). I also used a 1.4x barlow from Siebert Optics (sold as 1.3x but functions as 1.4x). As mentioned above, the barlow added nothing useful, and in fact made processing more painful, and the final image was downsized. The final image is only 38 megapixels, but I think it manages to pack a lot of detail into that size. You can access the image by following the appropriate links for download at the following link:

I am also providing a few cropped images below, corresponding to regions along the sunset terminator, moving from North to South. These needed to be compressed, so they may or may not be equivalent to the same regions of the original image, but they should be pretty similar. The main feature along the terminator is Copernicus, acting as a bullseye near the apparent center of the Moon, but there are many other interesting features as well. I particularly like the crater Bullialdus, which is located along the terminator slightly to the south of Copernicus, and also the craters Wilhelm and Longomontanus, which are prominent along the southern terminator. Owing to a favorable libration, Mare Orientale is visible along the western limb, which I have posted about previously.

In all cases, please click to view the larger images.

Edited by Tom Glenn, 27 December 2019 - 05:00 AM.

#2 Tom Glenn

#3 Tom Glenn

#4 Tom Glenn

#5 Tom Glenn

#6 Tom Glenn

#7 gfstallin

These images represent data that has remained unfinished since last August 24, 2019. I have previously posted images of Copernicus and Mare Orientale, but have not completed the set of data for the entire Moon until now. Aside from the usual time constraints when dealing with large amounts of data, this data also proved to be somewhat difficult to deal with for a variety of reasons, owing in part to the changing sky brightness of each panel as captured in sequence near dawn, but also the use of a 1.4x barlow did not help in any way. In fact, the final image (available by link) was downsized to 50%, in part because the original image scale was too large, but also, I've come to realize that most web browsers display images much larger than I would prefer (above 100% pixel scale), and in most cases this is unflattering.

The image was captured with a C9.25 Edge HD using the ASI183mm camera with a green filter (Baader, bandpass 500-575nm). I also used a 1.4x barlow from Siebert Optics (sold as 1.3x but functions as 1.4x). As mentioned above, the barlow added nothing useful, and in fact made processing more painful, and the final image was downsized. The final image is only 38 megapixels, but I think it manages to pack a lot of detail into that size. You can access the image by following the appropriate links for download at the following link:

August_24_2019_TG.jpg

Excellent work - per usual. Your craters are so. dark. I've been been playing around with various settings attempting to improve performance around dark edges and the limb of the moon, but I cannot avoid rebounds. Any minimal amount of processing of the final image appears to create them, so I'm at a loss on that front.

I've got a couple questions for you. I received the Baader Q-barlow for Christmas, which is advertised as 1.3x when screwed into the eyepiece. I'll see what how that works when screwed into the nosepiece of the camera. I'm also using a 183mm these days. You noted that it made processing more painful. Can you elaborate on how/why?

#8 BillHarris

#9 james7ca

These are all very nice, but I think my favorite may be Sinus Iridum.

#10 Tom Glenn

George, Bill, and James, thanks for the comments.

George, your questions all touch on interesting topics. Sorry if this strays off topic, but in an attempt to address to your questions, I will briefly discuss the issue with the barlow, and then a separate issue about artifacts. The problem with the barlow is twofold. Keep in mind this is all very specific to this particular imaging system (C9.25 Edge and ASI183mm) as it relates to capturing the entire Moon with as much detail as possible in one session.

First, and most importantly, is the loss of field of view (FOV). A 1.4x barlow decreases the area of the FOV by 2x, so it takes about twice as many panels to cover the Moon. This would be trivial if the camera were using a fast frame rate, but it isn’t, and so each image panel takes about 4-5 minutes to capture. Total capture time for these mosaics is generally 20-30 minutes, depending on the lunar phase. But an increase in time to cover the Moon when using the barlow greatly increases the probability that seeing and/or transparency do not remain consistent throughout all recordings. Also, more image panels increase the workload required to compose the mosaic, and also increases the total number of capture files, which can easily fill a hard drive very quickly with this camera.

Second, the use of the 1.4x barlow reduces the amount of light by a full stop, so the exposure has to be adjusted accordingly, usually by increasing the gain. This is not necessarily a deal breaker, but it isn’t ideal, mostly because once again, the number of frames you can capture is limited by slow frame rates and file size.

Both of these issues would be completely inconsequential if the final image outcome was improved in some way. But it isn’t. I have noticed zero improvement in resolution imaging with the barlow versus imaging at f/10 and then drizzling and resizing the output so they are the same scale. The increased scale can sometimes trick you into thinking there is improvement, but comparing the barlow to the drizzle output, they are identical. So, the actual benefit to the barlow is zero, but there are negatives, so there is really no reason to use it. Results can be different on planets, in which you can take many recordings with far more frames, and higher gain is largely irrelevant. But for the Moon, I don’t see much reason to try this again. In fact, for this very reason, in the lunar imaging I have done after these images were captured, I have reverted back to imaging at f/10. Examples below.

Concerning your other question about dark crater floors and “rebound”, I can only assume that by rebound you are referring to some type of ringing artifact. Nearly two years ago I started a thread about some artifacts in lunar images (link here). It started as a simple question, because at the time I was not very experienced at image processing. There were some interesting discussions, although it largely ended inconclusive, with the consensus being that the artifacts are multifactorial in origin. I should probably update the thread with some additional observations, but suffice it to say that artifacts in lunar images are not all created equal.

It is a common assumption that artifacts are a result of over-processing an image, usually by deconvolution or sharpening. This is not always the case, however. Many ringing artifacts are indeed caused by deconvolution , but some are caused by diffraction. I now believe that the ubiquitous white rings that hover inside many craters and along other sharp edges on the Moon are primarily caused by diffraction. Evidence for this is based on several observations. First, when I go back and look at my raw data, I can easily find examples of the white rings inside craters in the raw stack that has not been sharpened or deconvolved at all. Although deconvolution exaggerates the artifact, the artifact is actually present in the raw data. Second, the white ring artifacts are always hovering at a uniform distance away from crater rims throughout the image, and this distance is exactly the angular separation predicted by calculating the distance between alternating energy maxima and minima of an Airy pattern produced by the aperture of the telescope. And this distance scales perfectly with aperture, such that my 6” scope and 9.25” scope have different measurements to the artifacts, that are predicted by their aperture. Third, it is not possible to recreate the white ring artifact inside the craters by simple deconvolution of a “perfect” image, such as an LRO image. You can introduce other ringing artifacts, but not the diffraction ringing that produces the hovering white ring. (More on LRO images at the end). The other prediction here is that the angular size of the artifact depends upon wavelength, but all of my images are fairly close in that respect (green and red filters, really too close to measure a difference with this setup).

This is not to say that all artifacts are caused by diffraction. Deconvolution itself causes ringing and other distortions along sharp edges. Strong deconvolution can cause a crater to look like it has a double rim, etc. I’m NOT referring to those types of artifacts here, which can be mitigated by simply sharpening less aggressively. But the faint white rings that float inside (and outside) crater rims (and the lunar limb) do appear to have their origins in diffraction, that is then exaggerated by deconvolution. This puts the artifact in the same category as artifacts along the bright limb of Mars and Venus, which are similarly caused by diffraction, and are then strengthened by sharpening. In hindsight, I think this all makes good sense. We are frequently sampling at the diffraction level (on purpose), and so diffraction is exactly what we are recording.

This does, however, mean that these crater artifacts are largely impossible to prevent, although there are several methods to try and reduce their influence. Because deconvoluton exaggerates the strength of the artifact, you can choose to use less deconvolution. However, this can have the consequence of less real detail in the image, and because the artifact is present in the raw data, it will appear with any amount of deconvolution. Another method is to hide the artifacts in shadow. Unfortunately, the very regions of the lunar surface in which the artifact is most noticeable (near the terminator) tend to be the exact regions that often benefit from raising shadows, if the goal is to achieve a realistic looking image that matches natural illumination of the Moon. On rugged, mountainous terrain, you can easily reduce the black level to zero without consequence, but if you do this along regions of the terminator that pass through maria, you will destroy fine detail on the lunar surface and the image won’t actually look like the Moon did at the time. To me, that amounts to sacrificing one artifact for another, namely, an unnatural looking illumination. It’s up to each individual to decide what compromises to make in their images.

The other takeaway here, which is somewhat relevant to the ever-present discussions on this forum about the advantage of large aperture telescopes, is that large scopes have much lower diffraction limits, which means you are less likely to be sampling diffraction. Especially if an image is downsized, the diffraction ringing might actually become so small that it’s inconsequential. In this respect, an image from a large aperture scope that is downsized should be cleaner and higher quality than an equivalently scaled image that was captured with a smaller scope and not rescaled. For an extreme example of this, you can look at NASA’s LRO composite that has been downscaled to a scale of 474m/px, which is approximately the image scale that can be produced with a C8.

What you will notice, however, is that the downsampled LRO image has far more detail than you will ever find in a C8 image, because the image is totally noise free. In fact, despite the image scale being approximately what you can obtain with a C8, the NASA image generally has more fine detail than what you find in the very best examples of C14 images that are presented at higher image scales! Just zoom in and look at the floor of Plato and see how many craters you see. The image is all signal and no noise. And that shows the limitations of imaging through an atmosphere, as well as the benefit to capturing and processing a much higher resolution raw image, and then downsizing. I think many amateur images of the Moon could benefit from some downsampling after processing. For the images in this thread, I downsampled precisely because I did not like how the image was looking at the original scale. There are still obvious artifacts if you look closely, but my goal with these images is almost always the overall composition, rather than how it looks at 200% pixel peeping levels. If the original image was made into a 30 inch print, for example, it would look completely free from artifact.

I guess my advice would be to never stop experimenting with different processing schemes, deconvolution methods, and ways to modify the shadow and black levels in final editing. It is, however, basically impossible to create a perfect image, so you have to pick and choose which compromises to make.


Moon Phase and Libration, 2021

Click on the image to download a high-resolution version with feature labels and additional graphics. Hover over the image to reveal the animation frame number, which can be used to locate and download the corresponding frame from any of the animations on this page, including unlabeled high-resolution Moon images. The data in the table for the entire year can be downloaded as a JSON file or as a text file.

The animation archived on this page shows the geocentric phase, libration, position angle of the axis, and apparent diameter of the Moon throughout the year 2021, at hourly intervals. Until the end of 2021, the initial Dial-A-Moon image will be the frame from this animation for the current hour.

Lunar Reconnaissance Orbiter (LRO) has been in orbit around the Moon since the summer of 2009. Its laser altimeter (LOLA) and camera (LROC) are recording the rugged, airless lunar terrain in exceptional detail, making it possible to visualize the Moon with unprecedented fidelity. This is especially evident in the long shadows cast near the terminator, or day-night line. The pummeled, craggy landscape thrown into high relief at the terminator would be impossible to recreate in the computer without global terrain maps like those from LRO.

The Moon always keeps the same face to us, but not exactly the same face. Because of the tilt and shape of its orbit, we see the Moon from slightly different angles over the course of a month. When a month is compressed into 24 seconds, as it is in this animation, our changing view of the Moon makes it look like it's wobbling. This wobble is called libration.

The word comes from the Latin for "balance scale" (as does the name of the zodiac constellation Libra) and refers to the way such a scale tips up and down on alternating sides. The sub-Earth point gives the amount of libration in longitude and latitude. The sub-Earth point is also the apparent center of the Moon's disk and the location on the Moon where the Earth is directly overhead.

The Moon is subject to other motions as well. It appears to roll back and forth around the sub-Earth point. The roll angle is given by the position angle of the axis, which is the angle of the Moon's north pole relative to celestial north. The Moon also approaches and recedes from us, appearing to grow and shrink. The two extremes, called perigee (near) and apogee (far), differ by as much as 14%.

The most noticed monthly variation in the Moon's appearance is the cycle of phases, caused by the changing angle of the Sun as the Moon orbits the Earth. The cycle begins with the waxing (growing) crescent Moon visible in the west just after sunset. By first quarter, the Moon is high in the sky at sunset and sets around midnight. The full Moon rises at sunset and is high in the sky at midnight. The third quarter Moon is often surprisingly conspicuous in the daylit western sky long after sunrise.

Celestial north is up in these images, corresponding to the view from the northern hemisphere. The descriptions of the print resolution stills also assume a northern hemisphere orientation. (There is also a south-up version of this page.)


Thread: Why does the moon's terminator not appear orthogonal to the direction of the sun?

The moon on the horizon can be zoomed in, so that the photo actually shows it larger on the horizon than higher in the sky. That's a distortion that is similar to the photo that appears in this thread--because if care is taken to not distort the image, the line can appear straight even in the photo.

I can take a series of pictures of a bus starting with the right front wheel, proceed over the bus to the left rear wheel, and stitch them together. The result would show both wheels visible, but that is only because of the distortion of the photo, and is not something that we can actually see.

I need photography lessons from lek, but here's my equatorial-mount panorama. It's not a true ecliptic mount.
I just shortened 1 leg on my camera tripod until it was pointing at what was my best guess as to where Polaris is.

In the panaroma the Moon is noticable, but too small to tell phase. I've labeled the Moon and the approximate position of the Sun.
I can't tell exactly where the Sun is because a large cloud moved in front of it about a minute earlier.

Panaroma:

And this is the portion of the panorama that contains the Moon, zoomed in so you can see phase.

We've already mentioned refraction effects. It's not a matter of "thinking" in this case--the line between the sun and moon is not curved. It's not a matter of refraction or reflection or optical distortion of any sort. It just looks curved to some people. I'm not denying that there are refraction effects in the atmosphere, there are--but not at the level that we are discussing.

I understand how perspective works--parallel lines appear to converge at infinity. But this is not the same thing. The line can even appear in a photograph to not be curved--if the pan is along the line, which is as it should be if we didn't want to distort the line.

I have said that. I've also shown how it is not actually curved.We've already mentioned refraction effects. It's not a matter of "thinking" in this case--the line between the sun and moon is not curved. It's not a matter of refraction or reflection or optical distortion of any sort. It just looks curved to some people. I'm not denying that there are refraction effects in the atmosphere, there are--but not at the level that we are discussing.

I understand how perspective works--parallel lines appear to converge at infinity. But this is not the same thing. The line can even appear in a photograph to not be curved--if the pan is along the line, which is as it should be if we didn't want to distort the line.

This bab about moving the camera along a curved line and stitching the shots together to produce a final photograph showing a straight line is total crap. Jesus, I could take lots of photographs of the circumference of a circle, turning the camera slightly between each shot, and then stitch them together to make the circle look like a straight line. It's a trivial and worthless claim. Wow I could even make a zig-zag line look straight as long as I turn the camera properly and stitch the shots together just right.

I maintain that if the sun and moon are close enough together in the sky for you to take a single photograph with both of them in the frame, the straight line between them (the ecliptic) would have to be drawn as an arc on the photograph, it would not be a straight line on the photograph, unless you're under the ecliptic.

The sun follows the ecliptic, the moon does not.

But what we are discussing here is the straight line between the sun and moon (which would be a straight line in such a picture, ignoring distortion) and whether it would be perpendicular to the terminator. It would.

But that's my point. Stitching the photos together can distort the view--I mentioned doing it to both sides of a bus.The sun follows the ecliptic, the moon does not.

But what we are discussing here is the straight line between the sun and moon (which would be a straight line in such a picture, ignoring distortion) and whether it would be perpendicular to the terminator. It would.

The moon follows the ecliptic closely enough for us to use the approximation.

If the sun and moon had been closer in the sky (or if he'd had a slightly wider angle lens) he could have taken a single photograph with them both in the frame. No stitching. No distortion. And a straight line drawn between the sun and moon on the photograph would not have been perpendicular to the moon's terminator. Would you like me to draw a line on Lek's photograph for you?

Five degrees then is our allowed error

But what you say below cuts to the heart of the matter and makes the ecliptic issue irrelevant.

lek is wrong there, though, it is not a necessity. For instance, a pinhole camera would do it without the need for the line going through the center of view.

PS: If we are talking about getting both bodies in the same frame. Looking at those posts again, lek is talking about a situation where we cannot get both bodies in the same frame, unless he uses a fisheye lens--which distorts the images. So, lek was not wrong, but he is talking about a situation that doesn't pertain to our basic disagreement there.

I was trying too hard to untangle the thread. I like the single statement, agree or disagree better:

If the sun and moon had been closer in the sky (or if he'd had a slightly wider angle lens) he could have taken a single photograph with them both in the frame. No stitching. No distortion. And a straight line drawn between the sun and moon on the photograph would not have been perpendicular to the moon's terminator. 1

The notion has been suggested that a photograph taken with a
fisheye lens is distorted, while one taken with a normal lens
is not distorted. I've thought about this question and related
questions for many years. (Which is a good reason for me to
feel embarrassed at having given an incorrect explanation of
the illusion early in the thread.)

All representations of three-dimensional objects in 3-D space
on two-dimensional surfaces are distorted. The question is
whether you notice the distortion.

Any image projected on the eye's retina is curved. Some shapes
and figures can seem less curved than others, depending on the
detail they contain, how they are positioned on the retina, and
whether the person is trying to see the curvature. I notice it
if the detail makes it possible and I want to see it. I do not
notice it if I'm not looking for it.

All that is also true of photographs. In addition, changing
focal length or film size or cropping alter the size of the
field. Generally, the larger the field, the greater the
curvature, and the more noticeable the curvature is. Fisheye
lenses have particularly large fields and large curvature from
edge-to edge. Panoramic cameras capture wide fields without
such obvious curvature, but introduce other distortions, which
is evidenced by, for example, the ability of a single person
to show up in more than one place in a single photo!

Pinhole cameras distort as much as any other. If the image is
projected onto a flat surface, it is distorted progressively
more away from the center, like a camera with a lens. If it
is projected onto the inside of a sphere, the resulting image
is essentially distortion-free when viewed from the pinhole
location, but that is the same as an imge made by a camera
with a lens when viewed from the position of the lens. Viewed
from any other position, of course, the image will be highly
distorted.

A straight line which passes through the center of the field
is not distorted from side-to-side, but it is distorted from
end-to end. Imagine a straight line with tick marks on it at
equal intervals. The tick marks appear widely-separated near
the center of the field, and closely-packed near the edges.

That's more or less my point.

Perspective cannot be blamed for this illusion. Lines get mapped to lines.

So how does that work, if you're standing between two parallel railway lines, that stretch from the horizon on your left to the horizon on your right? They meet at the horizon on both sides of you, but if you look down they pass each side of your feet, maybe twenty degrees apart.
Zero degrees apart twenty degrees apart zero degrees apart.
I think you'd be hard pressed to process the converging lines to your left and the converging lines to your right as forming parts of the same pair of straight lines inside your head. It certainly doesn't work for me.

That's my point. It is inside your head.

That's what makes it an illusion, rather than something physical. In the case of either line, if you draw a straight line along it, it stays along it. Some people can look at rail lines and not be convinced that they actually converge.

That's my point. It is inside your head.

That's what makes it an illusion, rather than something physical.

Which is my point, referring to your remark "Perspective cannot be blamed for this illusion."
Surely perspective is inside your head, rather than something physical? And the apparent curvature is part of the process of perceiving perspective, just as the apparent convergence is.

Then you're in something of a minority, I think, since the illusion as described by the OP is well reported. It's certainly very striking to me.
Do you do a lot of sky-watching? (I'm wondering if the habit of orientating yourself along great circles might make you better at seeing straight lines as running straight over arcs wider than your central visual field.)

All of us here do a lot of skywatching, no?

But I'd say from my experience, it's engineers that have a developed three dimensional sense. Astronomers too maybe. And mathematicians. Taxi drivers. And artists. Hunters. Climbers. I suppose I could fit in any of those groups. Also pilots I'd imagine. But I'm not convinced it's necessary.

Yes, okay, but do you do a lot?

Originally Posted by [url=http://www.bautforum.com/showthread.php?p=698829#post698829]hhEb09'1[/url]

Can somebody please take a photograph of the moon and sun in the same frame and show hhEb09'1 that he's sadly wrong. The moon is more or less full now but should be a nice quarter crescent and only 45 degrees from the sun in about 10 days time. That should allow them both to appear in the frame using a 50mm lens, which closely represents the magnification of the eye, so there shouldn't be any distortion due to wide-angle or telephoto lensing.

The clock is ticking hhEb09'1. Maybe you'd like to place a wager?

A 50 mm lens on a 35 mm camera doesn't have a wide enough field
to capture Sun and Moon in one frame when 45 degrees apart.

A 50 mm lens on a 35 mm camera doesn't guarantee elimination
of distortion. It does minimize distortion to a large extent,
but it is only an approximation. The human mind accomodates
a considerable amount of distortion without noticing it. So
the distortion in a photograph has to be quite large before
you think, "That doesn't look quite right."

As I said previously, a straight line through the center of any
lens should be undistorted from side-to-side. A straight line
drawn in the sky from Sun to Moon, and photographed so that the
image of the line passes through the center of the lens, should
be perpendicular to the Moon's terminator in the photo.

A 50 mm lens on a 35 mm camera doesn't have a wide enough field
to capture Sun and Moon in one frame when 45 degrees apart.

A 50 mm lens on a 35 mm camera doesn't guarantee elimination
of distortion. It does minimize distortion to a large extent,
but it is only an approximation. The human mind accomodates
a considerable amount of distortion without noticing it. So
the distortion in a photograph has to be quite large before
you think, "That doesn't look quite right."

As I said previously, a straight line through the center of any
lens should be undistorted from side-to-side. A straight line
drawn in the sky from Sun to Moon, and photographed so that the
image of the line passes through the center of the lens, should
be perpendicular to the Moon's terminator in the photo.

Well then we'll just have to wait and see won't we. No matter how hard I try I can't imagine a photograph of the ecliptic projected in a planetarium showing a straight line, even if the image of the line passes through the centre of the lens (which is a pretty pointless requirement in my opinion - good quality camera lenses around 50mm do not distort straight lines very much, even at the edge of the frame), unless you have the camera in the plane of the ecliptic.

Nice picture.. if I had a landscape monitor i'd make it my desktop background,
might be worth submitting it to APOD/EPOD

I quite by coincidence took an image on 8 March that illustrates the illusion.

That isn't relevant. A straight line between the Sun and Moon
is a straight line nomatter where you view it from.

I'm not sure why you talk about lines on a planetarium dome.
A planetarium dome is a poor simulation of the sky, because
the sky is not a dome! It resembles a dome in some ways,
but only some.

I agree with Grant's analysis, based in part on your excellent
diagrams. A straight line between the Sun and Moon can be made
by, for example, a yardstick or a piece of string stretched
between your hands. That straight line is the thick, gray line
in your diagrams. It is curved in your diagrams because of the
way it is projected onto the diagrams. And it actually looks
curved in exactly the same way as your diagrams. (If they were
drawn to accurate scale and so forth. You just drew them by
eye, which was accurate enough for the purpose.)

If you want to accurately draw a straight line across the sky,
you need to hold up a yardstick, or a piece of string, or hire
a skywriter to make a trail on a windless day, or get lucky
and see crepuscular rays which stretch all the way from the
Sun to a point on the far side of the sky. Such a line will
look straight or curved depending on how you look at it.

I think you posted the wrong link. I'd like to see your photo
when you get the link straightened out.

I think you posted the wrong link. I'd like to see your photo
when you get the link straightened out.


Photographer Creates 'Impossible' Image Of The Moon's Surface

A photographer has created an image of the moon that has never been seen before.

Using thousands of photographs taken over a couple of weeks, astrophotographer Andrew McCarthy built a composite picture, showing the incredible depth of the Moon's surface.

The California-based snapper posted the super-clear pic to his Instagram account. Titled 'All Terminator', Andrew described it as an 'impossible scene'.

He wrote: "This moon might look a little funny to you, and that's because it is an impossible scene.

"From two weeks of images of the waxing moon, I took the section of the picture that has the most contrast (right before the lunar terminator where shadows are the longest), aligned and blended them to show the rich texture across the entire surface.

Andrew spent two weeks creating this incredible image. Credit: SWNS

"This was exhausting to say the least, namely because the moon doesn't line up day over day, so each image had to be mapped to a 3D sphere and adjusted to make sure each image aligned."

'Lunar terminator' is the term used to describe the line between the light and dark side of the moon.

The sun creates larger and longer shadows in the terminator, which help give the image a three-dimensional appearance.

They also make the moon's surface much clearer, giving the craters much more focus and making them more prominent in the photo.

The California-based snapper used thousands of photos to create one detailed image. Credit: SWNS

Sadly, Andrew's photo couldn't shed on any more light on claims made last week that there were cities on the moon.

Self-styled UFO and alien hunter Scott C Waring claims he's seen proof that aliens not only exist but also that they're on the moon.

On his website, The ET Database, he shared what he calls ' 100% Indisputable Proof ' of 'alien cities'.

Waring says he was looking at some detailed images that 'partially reveal the dark side of the moon, or almost dark side because Earth cannot view this part of the moon, but a little sun light does hit part of it'.

He continued: "The white dots, which so many inexperienced people have called photo flaws, are real. My proof is the shadow. Look at the shadow covering them."

He then added: " You will notice like I did that there is some unusual formations of white dots. These are evenly spaced apart and stay close to one another. That is because you are looking at cities on the moon."


Why Is It Called a Quarter Moon (Not a Half Moon)?

When you’re looking at a Moon that’s half-illuminated—like half a pie—why is it called a “Quarter Moon” instead of a “Half Moon”? Seems confusing, right? Bob Berman defines the Quarter Moon—and explains why it’s the most interesting Moon phase in his eyes. Let’s take a closer look at the beautiful Quarter Moon.

Why Do We Call It the Quarter Moon?

We’ve all looked up at the night sky and seen half of the Moon’s disk illuminated. If you had two half Moons and fit them together, you’d get a full Moon. But when you’re looking at a Half Moon, the official name is “Quarter Moon.” There’s no half-moon phase, at least not in any official way. But it appears as half-illuminated. This may seem odd, but let me explain.

Think of the Moon going around the Earth as a runner going around baseball plates (first base, etc.).

  • Earth is the pitcher. When the runner hits the ball, it goes to first base (one quarter of the way around). Similarly, at the Quarter Moon, the Moon is one quarter of the way through its orbit.
  • Then, the runner goes to second base (half way around), then to third base (three quarters around). The Moon is three-quarters of the way through it’s orbital cycle and, therefore, is called the Third Quarter Moon.

At first base or third base, you get the quarter Moons.

First Quarter vs. Third Quarter

With the Quarter Moon—which looks like half the Moon—we can see exactly 50% of the Moon’s face illuminated from Earth.

Sometimes it also gets confusing to remember which “Quarter” we are seeing:

  • The Moon appears lit on the right half of the Moon during the First Quarter.
  • The Moon appears lit on the left side during the Third Quarter because the Moon is on the other side of Earth.

Again, if you think of the baseball analogy, and you’re standing at home plate, the first base or First Quarter is on your right side. The third base or Third Quarter is on your side.

The First Quarter happens around day 7 of a Moon’s cycle (one week after the New Moon) and the Third Quarter usually happens around day 22 (three weeks after the New Moon).

Why the Quarter Moons Are Special

To me, the Quarter Moon is much more interesting than the Full Moon. This is the Moon that’s at its highest at sunset just around dinner time.

While the Full Moon provides a lot of light on Earth, if you’re observing the Moon’s surface, most beginning astronomers can’t see much beyond the blinding orb. The Sun then shines straight down like a flash camera to erase all shadows and highlights.

First Quarter Moon

But take a look at the Quarter Moon. The First Quarter Moon is the “Half Moon” that we see most.

The shadowing is perfect. You see all the mountains and craters. It’s fascinating to look at. The First Quarter Moon explodes with breathtaking detail for anyone with binoculars, spotting scope, or even the smallest telescope.

Last Quarter Moon

With the Last Quarter Moon, the left half appears to be lit up by sunshine and the rest immersed in shadow.

It doesn’t even rise until midnight and it’s not at its highest until around dawn. Who’s up then? Nobody! Most of us don’t want to haul our telescopes out at 5 A.M. or 6 A.M. to look at the Half Moon when you could look at the “other Half Moon” (the First Quarter Moon) at six in the evening when it’s convenient. Everyone’s used to the First Quarter Moon.

More Cool Quarter Moon Facts

The Quarter Moon aims its terminator, the day-night line that is home to all the juicy detail, straight at us. It’s lies directly ahead of us as Earth is zooming through the universe. This means highlighted craters then face you like actors hamming it up, instead of pointing, foreshortened, in other directions the way the rest of the lunar phases do.

You’d think a Half Moon would be half as bright as a Full Moon, right? Oddly enough, a Half Moon is only one tenth as bright as a Full Moon. Yet why does it seem so bright? This is because the Full Moon throws sunlight straight back at us like a movie screen, while the First Quarter’s sideways illumination creates innumerable unseen shadows in the Moon’s powdery surface.

How to Best View the Quarter Moon

Point the cheapest telescope towards the Quarter Moon. Stay below 60 power and the entire Moon will fill the field like a scene from 2001.

Even ordinary binoculars reveal the lunar Apennines, that mountain range just above dead center, whose jagged Himalaya-sized peaks tower straight up at you like skyscrapers.

Then there’s the badlands, the southern region, crazily pockmarked with a generous sampling of the 30,000 craters visible from Earth.

The scene changes dramatically each night as the terminator slithers over the Moon’s surface at 10 miles per hour. (A lunar jogger with enough stamina could keep nightfall at bay!)

Yes, this is a Moon phase packed with misconception. Even its name is misleading: how many realize that the Quarter Moon is the same thing as a Half Moon?


Moon's Phases Are a Lunar Delight for Stargazers

If you have recently received a telescope as a holiday gift, it is likely that your very first target will be our nearest neighbor in space: the moon.

When is the best time to observe the moon with a telescope? Most astronomy neophytes might say it is when it's at full phase, but that's probably the worst time to look at it! When the moon is full it tends to be dazzlingly bright as well as flat and one-dimensional in appearance.

In contrast, the interval when the moon is at or just past first quarter phase, or at or just before last quarter phase, is when we get the best views of the lunar landscape right along the sunrise-sunset line or terminator. The terminator can also be defined as that variable line between the illuminated portion and the part of the moon in shadow.

Along with the fact that a half moon offers more viewing comfort to the eye as opposed to a full moon, using a telescope with just small optical power (magnifications of 20- to 40-power), or even with good binoculars, we can then see a wealth of detail on its surface. Around those times when the moon is half-lit or gibbous phase, those features lying close to the terminator stand out in sharp, clear relief. [The Moon: 10 Surprising Lunar Facts]

In contrast to a half moon, a full moonis almost completely illuminated, especially right around its center the sun shines straight down even into all the microscopic crevices and except for perhaps around its immediate edges, you will find no visible shadows at all.

The moon will arrive at last quarter phase on Thursday, Feb 11, at 10:50 p.m. EST (0350 Feb. 12 GMT). That will be the moment when the moon's disk is exactly 50-percent illuminated. Lunar mountains will be visible as the sun lights them from the right.

How does a last quarter moon's brightness compare with a full moon? You might think it would be half as bright as a full moon, but in reality astronomers tell us that the last quarter (or first quarter) moon is only 1/11th as bright as full. This is due to the fact that, a half moon is heavily shadowed, even on its illuminated half. And believe or not, it isn't until just 2.4 days before full that the moon actually becomes half as bright as full!

Finally, have you ever noticed that when artists portray the moon, they invariably seem to show it as either a slender crescent or full?

Half-moons are shown far less frequently, while gibbous moons are rarely depicted at all. The word gibbous is derived from the Latin word "gibbus" meaning, "hump." An unusual word to be sure, but in describing the moon between half and full, it's the correct term.

Yet interestingly, the gibbous moon, the phase of the moon that we are now seeing in our current evening sky is the most-seen phase. It occurs for the half month between first and last quarter (although for many it looks "full" for two or even three nights around the time of full moon).

Because it is in the sky for more than half the night we're more apt to see the gibbous moon. In fact, it is even visible during the daytime hours, as will be the case during this upcoming week in mid or late afternoon. In contrast, the oft-pictured crescent moon is visible only during the early evening or early morning hours, and sometimes only briefly.


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