Astronomy

Why doesn't the moon twinkle?

Why doesn't the moon twinkle?


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Stars twinkle because their light has to squeeze through several different layers of the Earth's atmosphere. So why doesn't the moon twinkle as well?


The first handful of hits on Google actually return incomplete and even wrong answers (e.g. "Because the Moon is much brighter" which is plain wrong, and "Because the Moon is closer" which is incomplete [see below]). So here's the answer:

As you mention, when light enters our atmosphere, it goes through several parcels of gas with varying density, temperature, pressure, and humidity. These differences make the refractive index of the parcels different, and since they move around (the scientific term for air moving around is "wind"), the light rays take slightly different paths through the atmosphere.

Stars are point sources

Stars are immensely far away, effectively making them point sources. When you look at a point source through the atmosphere, the different paths taken from one moment to another makes it "jump around" - i.e. it twinkles (or scintillates).

The region in which the point source jumps around spans an angle of the order of an arcsecond. If you take a picture of a star, then during the exposure time, the star has jumped around everywhere inside this region, and thus it's no longer a point, but a "disk".

… the Moon is not

The same is true for the Moon, but since the Moon (as seen from Earth) is much larger (roughly 2000 times larger, to be specific) than this "seeing disk" as it's called, you simply don't notice it. However, if you are observing details on the Moon through a telescope, then the seeing puts a limit on how fine details you can see.

The same is even true for planets. The planets you can see with the naked eye span from several arcsec up to almost an arcmin. Although they look like point sources (because the resolution of the human eye is roughly 1 arcmin), they aren't, and you will notice that they don't twinkle (unless they're near the horizon where their light goes through a thicker layer of atmosphere).

The image below may help understanding why you see the twinkling of a star, but not of the Moon (greatly exaggerated):


EDIT: Due to the comments below, I added the following paragraph:

Neither absolute size, nor distance is important in itself. Only the ratio is.

As described above, what makes a light source twinkle depends on its apparent size compared to the seeing $s$, i.e. its angular diameter $delta$ defined by the ratio between its absolute diameter $d$ and its distance $D$ from Earth: $$ delta = 2 arctan left( frac{d}{2D} ight) simeq frac{d}{D},,,mathrm{for,small,angles} $$

If $delta lesssim s$, the object twinkles. If it's larger, it doesn't.

Hence, saying that the Moon doesn't twinkle because it's close is an incomplete answer, since for instance a powerful laser 400 km from Earth - i.e. 1000 times closer than the Moon - would still twinkle because it's small. Or vice versa, the Moon would twinkle even at the distance it is, if it were just 2000 times smaller.


Finally, to achieve good images with a telescope you not only want to put it at a remote site (to avoid light pollution), but also - to minimize the seeing - at high altitudes (to have less air) and at particularly dry regions (to have less humidity). Alternatively you can just put it in space.


The wikipedia page on twinkling, aka scintillation, covers it quite succinctly; it boils down to the fact that distant stars are sufficiently distant to be a point source of coherent light. Solar planets and Luna are close enough to have a resolvable diameter while being visible, which means that their light is not coherent like a point source's might be.

Mathematically, the threshold at which a distant source of light becomes an effective point source is going to be a function of its size and distance, relative to the aperture size of the viewing device (in this case, the human eye). You could effectively think of it as a cylinder between the aperture and the perimeter of the light source: when that cylinder is sufficiently narrow when passing through the atmosphere, you get visible twinkling.

It's important to note that scintillation is not the mirage effect, which is caused by temperature gradients in the atmosphere and causes the 'swimming' effect. Scintillation does not displace the apparent position of the light source, instead resulting in variations of brightness and colour. The actual mechanism of scintillation results from plane-wave light and atmospheric turbulence causing interference in that light's wavefront. This is clearly demonstrated by this image from NASA.


Why doesn't the moon twinkle? - Astronomy

The songline goes "Twinkle twinkle little star". What is the cause of the "twinkling" of stars? Does light from planets "twinkle" as does light from stars?

A young person of my acquaintance asked me this question, and I didn't have a good answer.

Stars twinkle because of turbulence in the atmosphere of the Earth. As the atmosphere churns, the light from the star is refracted in different directions. This causes the star's image to change slightly in brightness and position, hence "twinkle." This is one of the reasons the Hubble telescope is so successful: in space, there is no atmosphere to make the stars twinkle, allowing a much better image to be obtained.

Planets do not twinkle the way stars do. In fact, this is a good way of figuring out if a particular object you see in the sky is a planet or a star. The reason is that stars are so far away that they are essentially points of light on the sky, while planets actually have finite size. The size of a planet on the sky in a sense "averages out" the turbulent effects of the atmosphere, presenting a relatively stable image to the eye.

This page updated on June 27, 2015

About the Author

Dave Kornreich

Dave was the founder of Ask an Astronomer. He got his PhD from Cornell in 2001 and is now an assistant professor in the Department of Physics and Physical Science at Humboldt State University in California. There he runs his own version of Ask the Astronomer. He also helps us out with the odd cosmology question.


What Causes The Phases of the Moon?

  1. The Earth Blocks the Light that comes from the Sun
  2. The Crescents are caused by shadows of the Earth from the Sun onto the Moon
  3. If the Earth is between the Moon and the Sun, we will not see the Moon.

None of these are actually scientifically correct, lets actually look at what is happening.

Before we can actually understand the phases, we need to know a couple of things.

Firstly, there is only one source of light in the solar system and that is the Sun (which is at the center of the Solar System). The Sun produces all of the light. So both the Earth and the Moon are half illuminated by that one source of light. As the Moon moves around the Earth, our perspective of it changes.

It is therefore our perception of the Moon that provides the various faces.

In total, there are 8 distinct phases of the moon, which occur at different times when the Moon moves around the Earth. Here’s a closer look at them.


Do planets twinkle?

Planets tend to be less susceptible to refraction than stars.

While stars and planets appear as similar sizes to the naked eye, a planet is actually a shrunk disc and is generating more light to your eye than a much more distant star, which is really just a pinpoint.

Because there is more light, you don’t notice so much if a small number of light waves get refracted away from the center of the light source. In other words, you don’t notice the fuzziness and resulting dimness as much because there is enough light in the center of the object to maintain the brightness.

There are conditions, though, where planets do twinkle:

When planets are closer to the horizon the column of air that the light needs to pass through is greater than when the planet is higher in the sky:

With more air for the light to pass through, there is a greater chance of the light path meeting turbulence, and a greater chance of twinkling.

Both Venus and Mars can be seen twinkling if low in the sky.

Saturn and Jupiter, because of their larger size (Jupiter is over 20 times the diameter of Mars), don’t tend to receive the twinkling effect.


Astronomy Jokes And Humour

Does anyone out there have any funny astronomy jokes? The best I’ve heard so far are:

SPACE

* Einstein developed a theory about space, and it was about time too!

* I’m reading a book about anti-gravity.. it’s impossible to put down!

Q: How many ears does Captain Kirk have?
A: Three. A left ear, a right ear, and a final frontier!

Q: What’s a light-year?
A: The same as a regular year, but with less calories.

STARS

Q: What does a star win in a competition?
A: A constellation prize

Q: What kind of stars wear sunglasses?
A: Movie stars

Q: Why did the star get arrested?
A: Because it was a shooting star!

Q: Why didn’t the Dog Star laugh at the joke?
A: It was too Sirius.

Q: Who here can tell me the distance from Betelgeuse to Procyon
using a standard chart?”
A: About an inch and a half.

* Sirius, the dog star, is moving closer to Earth at a rate of nine miles per
second. This means someday we could be in Sirius trouble.

ASTRONAUTS

Q: Where does an astronaut dock his spacecraft?
A: At a parking meteor.

Q: What happened to the astronaut who stepped on chewing gum?
A: He got stuck in Orbit!

Q: How does one astronaut on the moon tell another astronaut that he is sorry?
A: He Apollo-gises.

Q: How do you get a baby astronaut to sleep?
A: You rocket!

Q: What do you call a crazy spaceman?
A: An astro-nut

Q: What do astronauts wear to keep warm?
A: Apollo-neck sweaters!

Q: How do spacemen pass the time on long trips?
A: They play astronauts and crosses!

ALIENS

Q: What do you call an alien with three eyes?
A: An aliiien!

Q. What do you do if you see an an aggressive alien?
A. Give it some space!

Q. What should you do if you see a green alien?
A. Wait until it’s ripe!

Q: What come from another world and are really slow?
A: Snailiens!

Q: What did the alien say to the garden?
A: Take me to your weeder!

PLANETS

* Scientists have found that the center of Jupiter contains the letter “i”.

*Jupiter came down to Earth one day and helped these two criminals plan a bank robbery. Anyway, they both got caught and after the judge sentenced the two earthlings to fifteen years behind bars, Jupiter was a bit shocked to get arrested and handed a ten year stretch himself. “But your honour. I didn’t even take part in the robbery!” said Jupiter. “Yes” replied the judge. “But you did help them Planet!”

Q: How does Jupiter hold up his trousers?
A: With an asteroid belt.

Q: What type of songs do the planets sing?
A: Nep-tunes!

Q: What did Mars say to Saturn?
A: Give me a ring sometime!

*The density of Saturn is so low that the whole planet would float on the water in your bath? However, you wouldn’t want to try this experiment at home as it would leave a massive ring around the tub.

MOON

*One kid asks the other, “Which is closer, Florida or the Moon? the second answers: “Duh! The Moon! You can’t see Florida from here!

Q: what do you call a tick on the moon?
A: A luna-tick.

Q: How do you know when the moon is going broke?
A: When it’s down to its last quarter.

Q: What was the name of the first satellite to orbit the Earth?
A: The Moon.

Q: How does the Man in the Moon cut his hair?
A: Eclipse it.

Q: Did you hear about the bones they found on the moon?
A: It seems like the cow didn’t make it after all. (hey diddle diddle)

Q: What do moon people do when they get married?
A: They go off on their honeyearth!

Q: Why does a moon rock taste better than an Earth rock?
A: It’s a little meteor.

Q: Why couldn’t the astronaut book a room on the moon?
A: Because it was full.

Q: How is the moon like a dollar?
A: It has four quarters.

Q: Why is the Moon bald?
A: He has no ‘air

SUN

Q: Why has Ms. Moon left Mr. Sun?
A: Because he never wants to go out with her at night.

*I was up all night wondering where the Sun had gone… then it dawned on me.

*Living on Earth might be expensive but at least you get a free trip around the Sun every year.

* Copernicus’ parents might deserve some of the credit for his great discovery. Apparently at the age of twelve they said to him: “Copernicus, young man, when are you going to realize that the world does NOT revolve around you.”


Q: Why can’t we see the lunar landers from the Apollo missions with the Hubble (or any other) telescope?

Physicist: About why you’d expect: they’re just too damn small and too damn far away. Nothing fancy. That’s not to say that we can never get images, just that you need to be a lot closer. The lunar landers are each about 4 meters across and about 384,400,000 meters away, which makes them about as hard to see as a single coin from a thousand miles away. You gotta squint.

A picture of the Apollo 17 landing site taken by the Lunar Reconnaissance Orbiter which, as the name implies, was in orbit around the Moon when it took this presumably reconnaissance-related picture. Those meandering lines are tracks left by a lunar rover. Click to enlarge.

In fact, a big part of why we (humans) bother to go to the Moon, other planets, and space in general is that photographs from Earth leave a lot to be desired. In addition to being far from everything else, here on the surface of Earth we’re stuck at the bottom of an ever-moving sea of air. In exactly the same way that the surface of water scatters light, air makes it difficult for astronomers to practice their dread craft.

Also, not for nothing, telescopes are terrible at retrieving material samples.

The Apollo 17 landing site from even closer.

You and every telescope on Earth (and the Hubble Telescope in low Earth orbit) are all about a quarter million miles from the Moon and the landing sites thereon. If we ever get around to building something bigger on the Moon, like mines or cities or president’s heads, then we shouldn’t have nearly as much trouble seeing it from Earth.

Answer Gravy: It turns out that the best/biggest telescopes we use today on Earth can’t detect things the size and distance of the lunar landers using visible light. This isn’t due to poor design the devices we’re using now are, in a word, perfect. They literally cannot be made appreciably better (at detecting visible light). The roadblock is more fundamental.

The “resolving power” of a telescope, is described in terms of whether or not you can tell the difference between a pair of adjacent points. If the two points are too close together, then you’ll see them blurred together as one point and they are “not resolved”. If they’re far enough apart, then you see both points independently.

Whether it uses mirrors or lenses, the resolving power of every telescope is limited by some fundamental constraints determined by the wavelength of the light that’s being observed and by the size of the aperture.

Every point in every image is surrounded by a rapidly diminishing “Airy disk” which are a symptom of light being wave-like. This is only a problem really close to the diffraction limit. You don’t see these when you take a picture on a regular camera because these rings are smaller than the individual pixels in the camera’s CCD (by design).

Because light is a wave it experiences “diffraction” which makes it “ooze around corners” and generally end up going in the wrong directions. But the larger a telescope’s opening, the more the light waves have a chance to interfere in such a way that they propagate in straight lines, which makes for cleaner images where the light ends up more-or-less where it’s supposed to be when it gets to the film or CCD or your retina or whatever.

It turns out that the relationship between the smallest resolvable angle, θ , the wavelength, λ , and the diameter, D, of the aperture is remarkably simple:

Visible light has a wavelength of around 0.5 micrometers (about 2,000,000 per meter) and the largest visible-spectrum telescopes on Earth are about 10 meters across (Hubble is a more humble 2.4m across). That means that the absolute best resolution that any of our telescopes can hope to achieve, under absolutely ideal circumstances, is about . Or, for the angle buffs out there, about 0.01 arcseconds. This doesn’t take into account the scattering due to the atmosphere we can do a little to combat that from the ground, but our techniques aren’t perfect.

By carefully looking at how the atmosphere distorts a laser beam shot upwards from a telescope on the ground, we can take into account how the atmosphere affects light coming into the telescope from space.

The lunar landers are a little over 4 meters across (seen from above) and are about 384,403,000 meters away. That means that the landers subtend an angle of about 0.002 arcseconds. In order to see this from Earth, we’d need a telescope that is, at absolute minimum, about 200 meters across. If we wanted the image to be a more than a single pixel, then we’d need a mirror that’s a few miles across.


Why does Venus sparkle but the moon doesn't?

I mean I am looking outside right now and Venus is absolutely memorizing to look at. I love the moon and could stare it it all night - but it doesn't have this quality to it.

I was all ready with an explanation of atmospheric seeing, but I get what you are saying.

The brightness of objects such as asteroids, comets, planets, and moons is determined by the amount of light the object reflects. This amount is the "albedo" of the object. Specifically, it's the measure of the reflectance of the object's surface or atmosphere. The moon's albedo is about 12% due to rock and dust, while Venus' albedo is 75%. A mirror is

99%. So Venus is quite brilliant in the sky.

So even though the moon is smaller than Venus and reflects far less of the light that strikes it, the moon is significantly brighter to us because of its proximity. It's just brighter in a different, non-brilliant way.

DoctorSteve, you said Venus has a shine to it. That's exactly what Iɽ say: the moon is illuminated, but Venus shines.


Twinkling

Twinkling, also called scintillation, is a generic term for variations in apparent brightness, colour, or position of a distant luminous object viewed through a medium. [1] If the object lies outside the Earth's atmosphere, as in the case of stars and planets, the phenomenon is termed astronomical scintillation for objects within the atmosphere, the phenomenon is termed terrestrial scintillation. [2] As one of the three principal factors governing astronomical seeing (the others being light pollution and cloud cover), atmospheric scintillation is defined as variations in illuminance only.

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In simple terms, twinkling of stars is caused by the passing of light through different layers of a turbulent atmosphere. Most scintillation effects are caused by anomalous atmospheric refraction caused by small-scale fluctuations in air density usually related to temperature gradients. [3] [4] Scintillation effects are always much more pronounced near the horizon than near the zenith (directly overhead), [5] since light rays near the horizon must penetrate a denser layer of and have longer paths through the atmosphere before reaching the observer. Atmospheric twinkling is measured quantitatively using a scintillometer. [6] The effects of twinkling are reduced by using a larger receiver aperture this effect is known as aperture averaging. [7] [8]

While light from stars and other astronomical objects are likely to twinkle, [9] twinkling usually does not cause images of planets to flicker appreciably. [10] [11] Stars twinkle because they are so far from Earth that they appear as point sources of light easily disturbed by Earth's atmospheric turbulence, which acts like lenses and prisms diverting the light's path. Large astronomical objects closer to Earth, like the Moon and other planets, encompass many points in space and can be resolved as objects with observable diameters. With multiple observed points of light traversing the atmosphere, their light's deviations average out and the viewer perceives less variation in light coming from them. [12] [13]


Why doesn't the moon twinkle? - Astronomy

The songline goes "Twinkle twinkle little star". What is the cause of the "twinkling" of stars? Does light from planets "twinkle" as does light from stars?

A young person of my acquaintance asked me this question, and I didn't have a good answer.

Stars twinkle because of turbulence in the atmosphere of the Earth. As the atmosphere churns, the light from the star is refracted in different directions. This causes the star's image to change slightly in brightness and position, hence "twinkle." This is one of the reasons the Hubble telescope is so successful: in space, there is no atmosphere to make the stars twinkle, allowing a much better image to be obtained.

Planets do not twinkle the way stars do. In fact, this is a good way of figuring out if a particular object you see in the sky is a planet or a star. The reason is that stars are so far away that they are essentially points of light on the sky, while planets actually have finite size. The size of a planet on the sky in a sense "averages out" the turbulent effects of the atmosphere, presenting a relatively stable image to the eye.

This page updated on June 27, 2015

About the Author

Dave Kornreich

Dave was the founder of Ask an Astronomer. He got his PhD from Cornell in 2001 and is now an assistant professor in the Department of Physics and Physical Science at Humboldt State University in California. There he runs his own version of Ask the Astronomer. He also helps us out with the odd cosmology question.


Why do stars twinkle, but planets don’t?

The more atmosphere you are peering through, the more stars (or planets) appear to twinkle. Illustration by AstroBob, via The Random Science blog.

Stars twinkle, while planets (usually) shine steadily. Why?

Stars twinkle because … they’re so far away from Earth that, even through large telescopes, they appear only as pinpoints. And it’s easy for Earth’s atmosphere to disturb the pinpoint light of a star. As a star’s light pierces our atmosphere, each single stream of starlight is refracted – caused to change direction, slightly – by the various temperature and density layers in Earth’s atmosphere. You might think of it as the light traveling a zig-zag path to our eyes, instead of the straight path the light would travel if Earth didn’t have an atmosphere.

Planets shine more steadily because … they’re closer to Earth and so appear not as pinpoints, but as tiny disks in our sky. You can see planets as disks if you looked through a telescope, while stars remain pinpoints. The light from these little disks is also refracted by Earth’s atmosphere, as it travels toward our eyes. But – while the light from one edge of a planet’s disk might be forced to “zig” one way – light from the opposite edge of the disk might be “zagging” in an opposite way. The zigs and zags of light from a planetary disk cancel each other out, and that’s why planets appear to shine steadily.

Astronomers use the term ‘scintillation’ to describe the twinkling of stars. Illustration via Tom Callen of the Cosmonova theater in Sweden.

You might see planets twinkling if you spot them low in the sky. That’s because, in the direction of any horizon, you’re looking through more atmosphere than when you look overhead.

If you could see stars and planets from outer space, both would shine steadily. There’d be no atmosphere to disturb the steady streaming of their light.

Can you figure out which objects are stars and which are planets just by looking for the twinklers vs the non-twinklers? Experienced observers often can, but, at first, if you can recognize a planet in some other way, you might notice the steadiness of its light by contrasting it to a nearby star.

Bottom line: Explanation of why stars twinkle in the night sky but planets don’t.


Watch the video: Γιατί Φεγγάρι Μου Όμορφο Why My Fair Moon (December 2022).