Astronomy

At Mars' equator how long does twilight last?

At Mars' equator how long does twilight last?


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With Mars' thinner atmosphere I'm assuming twilight would not last as long compared to the duration on Earth. Has there been any determination of the length of twilight on Mars at the equator or any other latitude?


According to https://mars.jpl.nasa.gov/MPF/science/clouds.html twilight lasts up to two hours because of dust high up in the atmosphere scattering the sunlight. Mars Pathfinder is in Ares Vallis, 3°North.


At Mars' equator how long does twilight last? - Astronomy

I have now read in several different books which relate the adventures of world travelers the same observation about the sunset at the equator. They state that at the equator "darkness falls almost instantly after sunset, there is no twilight". I ignored it the first few times I saw it, but after reading it again, and again, I began to wonder if it is true, and if so why? I can't figure out why twilight should shorter or longer anywhere in particular. Is is true? Why?

This is a very good question. The reason is a little complicated to understand, though. The simple answer is that at low latitudes, the sun sets perpendicular to the horizon, while at higher latitudes, the sun can set at a more oblique angle, allowing it to remain close to the horizon after sunset for a longer period of time.

The sun rises and sets because of the Earth's rotation. It's easy to understand how things move in the sky due to the rotation by looking at a long-exposure photo taken at night. Each of the circular streaks there is a star, and over the hour or so that the photo was taken, each star appeared to circulate around the North Star, Polaris, seen in the center of the pattern.

You can see from the picture that stars closer to Polaris than the horizon never rise nor set, but continually circle the pole star in the sky. Stars farther from the pole star do rise and set, as the circles on which they travel cut below the horizon. The same idea goes for any object in the sky, including the sun and the moon.

One more thing you need to know about these rotation patterns is this: Polaris sits directly above the North Pole. If you were standing at the North Pole, you would see Polaris directly overhead. All the objects in the sky would simply circulate around that point, and nothing stationary in the sky would rise nor set. This makes sense if you think about standing at the pole, while the Earth's rotation simply spins you around in place. You end up looking in different directions, but nothing enters or exits the sky above you. The closer to the north pole you are, the higher Polaris is in the sky. At the Equator, Polaris is on the horizon.

This means that at the north pole, the sun doesn't rise or set at all due to the Earth's rotation. The sun does move through the sky (from one of those circles on the film to another) on a yearly cycle, though, and this is why there is light at the north pole for 6 months and night for 6 months. Thus, above the arctic circle (or below the antarctic circle), at the right time of the year, you can see the midnight sun-- a sun which doesn't set all day. At that time of year, the sun is a circumpolar object, sitting on one of those star-trails in the picture that never descends below the horizon.

Now imagine that you are pretty far north, but not above the arctic circle. Polaris is very high in the sky, and as the Earth turns, the sun may be on one of those circles you see in the photo which just grazes the horizon. The sun sets, but sets at an angle. It stays close to the horizon for a long time, making twilight linger.

The further south you travel, the lower Polaris will appear in the sky, and the further and quicker the sun will dip below the horizon at night. Eventually, once you get to the tropics, Polaris is on the horizon, and the sun sets directly down below the horizon. Since it has very little movement horizontally with respect to the horizon, it quickly leaves the horizon behind after sunset, making for a very quick twilight. Then, as you proceed southward from the equator, the effect is reversed. Twilights become longer and longer, you see the South Celestial Pole higher and higher in the sky, more objects become circumpolar (i.e. never rising or setting), until eventually you're at the south pole, and the South Celestial Pole is directly over your head, and once again everything is circumpolar, and nothing rises or sets due to Earth's rotation.

I should say one more thing here: there is nothing inherently special about the star Polaris. It just happens to be very near the North Celestial Pole, the geometrical point directly above the Earth's north pole. There is also no corresponding bright south pole star.

I hope this gives you a better understanding of sunrises and sunsets!

This page was last updated on Jan 28, 2019.

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.


At Mars' equator how long does twilight last? - Astronomy

I have now read in several different books which relate the adventures of world travelers the same observation about the sunset at the equator. They state that at the equator "darkness falls almost instantly after sunset, there is no twilight". I ignored it the first few times I saw it, but after reading it again, and again, I began to wonder if it is true, and if so why? I can't figure out why twilight should shorter or longer anywhere in particular. Is is true? Why?

This is a very good question. The reason is a little complicated to understand, though. The simple answer is that at low latitudes, the sun sets perpendicular to the horizon, while at higher latitudes, the sun can set at a more oblique angle, allowing it to remain close to the horizon after sunset for a longer period of time.

The sun rises and sets because of the Earth's rotation. It's easy to understand how things move in the sky due to the rotation by looking at a long-exposure photo taken at night. Each of the circular streaks there is a star, and over the hour or so that the photo was taken, each star appeared to circulate around the North Star, Polaris, seen in the center of the pattern.

You can see from the picture that stars closer to Polaris than the horizon never rise nor set, but continually circle the pole star in the sky. Stars farther from the pole star do rise and set, as the circles on which they travel cut below the horizon. The same idea goes for any object in the sky, including the sun and the moon.

One more thing you need to know about these rotation patterns is this: Polaris sits directly above the North Pole. If you were standing at the North Pole, you would see Polaris directly overhead. All the objects in the sky would simply circulate around that point, and nothing stationary in the sky would rise nor set. This makes sense if you think about standing at the pole, while the Earth's rotation simply spins you around in place. You end up looking in different directions, but nothing enters or exits the sky above you. The closer to the north pole you are, the higher Polaris is in the sky. At the Equator, Polaris is on the horizon.

This means that at the north pole, the sun doesn't rise or set at all due to the Earth's rotation. The sun does move through the sky (from one of those circles on the film to another) on a yearly cycle, though, and this is why there is light at the north pole for 6 months and night for 6 months. Thus, above the arctic circle (or below the antarctic circle), at the right time of the year, you can see the midnight sun-- a sun which doesn't set all day. At that time of year, the sun is a circumpolar object, sitting on one of those star-trails in the picture that never descends below the horizon.

Now imagine that you are pretty far north, but not above the arctic circle. Polaris is very high in the sky, and as the Earth turns, the sun may be on one of those circles you see in the photo which just grazes the horizon. The sun sets, but sets at an angle. It stays close to the horizon for a long time, making twilight linger.

The further south you travel, the lower Polaris will appear in the sky, and the further and quicker the sun will dip below the horizon at night. Eventually, once you get to the tropics, Polaris is on the horizon, and the sun sets directly down below the horizon. Since it has very little movement horizontally with respect to the horizon, it quickly leaves the horizon behind after sunset, making for a very quick twilight. Then, as you proceed southward from the equator, the effect is reversed. Twilights become longer and longer, you see the South Celestial Pole higher and higher in the sky, more objects become circumpolar (i.e. never rising or setting), until eventually you're at the south pole, and the South Celestial Pole is directly over your head, and once again everything is circumpolar, and nothing rises or sets due to Earth's rotation.

I should say one more thing here: there is nothing inherently special about the star Polaris. It just happens to be very near the North Celestial Pole, the geometrical point directly above the Earth's north pole. There is also no corresponding bright south pole star.

I hope this gives you a better understanding of sunrises and sunsets!

This page was last updated on Jan 28, 2019.

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.


At Mars' equator how long does twilight last? - Astronomy

No. Mars rotates in the same direction as Earth. The rotational direction is defined as the direction in which the thumb would point if the fingers of the right hand were curled in the direction of Mars's rotation. That direction is the same for both Mars and Earth. Interestingly, the direction of revolution is also the same for both planets.

This common direction (in which the right-hand thumb would point) identifies North of the ecliptic plane (the plane defined by the Earth's orbit). Most planets (both rotation and revolution) of our solar system as well as the Sun (rotation) share this direction due to their common origin from a rotating mass of particles and gas.

So Earth and Mars both rotate in the same sense (counterclockwise if you look down on the solar system from North of the ecliptic). The Sun will always rise on Mars in the East, with East in the same sense as East on Earth.

This page was last updated on January 31, 2016.

About the Author

Suniti Karunatillake

After learning the ropes in physics at Wabash College, IN, Suniti Karunatillake enrolled in the Department of Physics as a doctoral candidate in Aug, 2001. However, the call of the planets, instilled in childhood by Carl Sagan's documentaries and Arthur C. Clarke's novels, was too strong to keep him anchored there. Suniti was apprenticed with Steve Squyres to become a planetary explorer. He mostly plays with data from the Mars Odyssey Gamma Ray Spectrometer and the Mars Exploration Rovers for his thesis project on Martian surface geochemistry, but often relies on the synergy of numerous remote sensing and surface missions to realize the story of Mars. He now works at Stonybrook.


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See Mars before it gets too distant, lost in the dusk twilight

Mars lies highest in the sky to the south soon after sunset at the beginning of July for observers in the UK, so you should not waste any opportunities to view the Red Planet while it is relatively close and still respectably sized. Tharsis, the great Martian volcanic plateau that is home to the largest volcanoes in the solar system, is turned toward Earth in the first week of the month. Click on the graphic to launch our interactive Mars Mapper. AN graphic by Ade Ashford. The prolonged wet conditions experienced in the UK have certainly made it a challenge to observe during this early part of summer, but one should take some consolation from the knowledge that twilight still lasts all night for much of the British Isles. Fortunately, deep twilight doesn’t interfere with the observation of bright planets, so don’t miss any opportunities to view Mars that lies on the southern meridian soon after sunset, and Saturn that occupies the same position about 1h 20m later.

At the beginning of July, Mars lies in the constellation Libra and shines at a respectable magnitude -1.4 &mdash comparable to the brightest stars. As seen from the centre of the British Isles, the Red Planet only manages to attain a peak altitude of 15 degrees (some three-quarters of the span of an outstretched hand at arm’s length) in the south around 10pm BST.

At the start of the month, Mars is about 54 million miles (86 million kilometres) distant, yet still exceeds 16 arcseconds in diameter. This means that a telescope magnification of little more than 100x will enlarge its ochre-coloured disc to the same size as the Moon appears to the unaided eye (incidentally, the ten-day-old gibbous lunar disc lies near Mars on Thursday, 14 July).

As July dawns, ringed planet Saturn resides in the constellation Ophiuchus about 19 degrees &mdash or the span of an outstretched hand at arm’s length &mdash to the left of Mars. Saturn is currently about magnitude +0.2, or about one-quarter as bright as its planetary sibling nearby. The ringed planet is 849 million miles (1,366 million kilometres) from Earth at the beginning of July, yet its disc still spans 18 arcseconds. Saturn’s north pole and the northern plane of its glorious ring system is tilted by a very favourable 26 degrees in our direction too. The waxing gibbous Moon pays Saturn a visit on Friday, 15 July.


At Mars' equator how long does twilight last? - Astronomy

Mars appears more Earth-like to us than most of the other planets because we can observe its surface, atmospheric clouds and hazes, and its brilliant white polar caps.  The latter are composed of frozen CO2 and underlying water ice, and wax and wane during the Martian year. These aspects, along with the changing seasons and the possibility of life, have made Mars one of the most studied planets in our solar system.

The Red Planet Mars offers both casual and serious observers many challenges and delights, as well as providing astronomers a laboratory to study another planet&rsquos atmosphere and surface. Some Martian features even appear to shift position around the surface over extended periods of time. There are several cooperating international Mars observing programs under way to assist both professional and amateur astronomers. These include the International Mars Patrol (I.M.P.) coordinated by the Mars Section of the Association of Lunar and Planetary Observers (A.L.P.O) and the Terrestrial Planets Section of the British Astronomical Association (B.A.A.). Information for observing Mars during a typical apparition is presented in a separate report titled, “General Information for Apparitions of Mars.”  Also, you can find gobs of information at this site.

With the advent of modern CCD camera technology the amateur can produce useful images of Mars when it is as small as 3.5 arcsec. Early in an apparition, Mars rises in the east or morning sky and sets with the rotation of the Earth in the western or evening sky.  During the past few apparitions (2027-2033), observers began to take CCD images when Mars was only 32 degrees away from the Sun.  Since Mars was only a visual magnitude of

1.8 then the planet would have been difficult to locate bright twilight hours.

In the pre-apparition reports the observer will find the motion of Mars in our sky, the characteristics for that particular apparition, information pertaining to the polar cap(s) and any special events that may be seen during that particular apparition.  As usual a calendar of events will be included with each report that contains cardinal dates for seasonal activity and orbital information of Mars.

MOTION OF MARS IN OUR SKY

As a general rule, an "apparition" begins when a planet emerges from the glare of the Sun shortly after conjunction. Mars will be in conjunction with the Sun on August 19, 2034 (13.9° Ls) however, it will not be safe to observe Mars until after September 24, 2034 when it is at least 12 degrees away from the glare of the Sun.

The apparent declination of Mars begins at -23.6° by the first week in March 2035 in the constellation Sagittarius and will ascend north into the constellation Capricornus by April 11, 2035 and then into Aquarius on May 24th. Mars will be south of the celestial equator until December 05, 2035 when it will then cross north of the celestial equator. This is not good news for those observing in the Northern Hemispheres because Mars will be seen fairly low in their sky during the much of the apparition. Good news for southern hemisphere observers however.

By March 05, 2035, a '1.0' visual magnitude Mars will be seen rising early in the morning sky in the constellation Sagittarius and it will be at western quadrature with the phase defect or terminator of 37.3°.  NOTE: The Solar Elongation for Mars is the angle between the lines of sight from Earth to the Sun and from Earth to Mars.  When these lines of sight form a right triangle then Mars is at quadrature (eastern or western).   For detailed definitions and graphics for the motion of Mars in our sky see these excellent web sites: Planetary Aspects and Elongations and Configurations.


The 2035-2036 apparition of Mars begins retrogression, or retrograde motion against the background stars eleven months after conjunction on August 15, 2035 (247.2° Ls) and continues through October 15, 2035 (285.5° Ls). Each night for this brief period of time before, during and after opposition the Red Planet will appear to move backwards toward the western sky in Pisces and Aquarius. Since the Martian year is about 687 Earth days long -- nearly twice as long as ours, the Martian seasons are similarly extended. While the Earth's seasons are nearly equal in duration, the Martian seasons can vary by as much as 52 days from each other due to that planet's greater orbital eccentricity (see Figure 2).


2035 APPARITION CHARACTERISTICS

Another general rule for predicting oppositions of Mars is from the following: the planet has an approximate 15.8-year periodic opposition cycle, which consists of three or four Aphelic oppositions and three consecutive Perihelic oppositions. Perihelic oppositions are also called "favorable" because the Earth and Mars come closest to each other on those occasions. We sometimes refer to this as the seven Martian synodic periods. This cycle is repeated every 79 years (± 4 to 5 days) and, if one were to live long enough, one would see this cycle nearly replicated in approximately 284 years. The 2035 Mars apparition is considered Perihelic because the orbital longitude at opposition will be 17.3° from the longitude of perihelion (250° Ls).

Closest approach occurs at 1421 UT on September 11, 2035 (264.7° Ls) with an apparent planetary disk diameter of 24.6'' at a distance of 0.3804083 astronomical units (AU) or 36,361,156 mi (56,908,264-km). During closest approach in 2035 the apparent diameter of Mars will be 2.5 arcsec larger than it was at the same period in 2033 however, it will be 20.5 degrees higher in the sky - good for observing the Red Planet for observers in both hemispheres of Earth. It should also be noted that closest approach between Earth and Mars is not necessarily coincident with the time of opposition but varies by as much as two weeks.

Opposition nearly 13 months after conjunction when Mars is on the opposite side of the Earth from the Sun. At that time, the two planets will lie nearly in a straight line with respect to the Sun, or five weeks after retrogression begins. Opposition will occur at 1933 UT on September 15, 2035 (267.3° Ls) with an apparent planetary disk diameter of 24.5 arcsec. Mars will remain visible for around 11 months after opposition and then become lost in the glare of the Sun around August 18, 2036 as it approaches the next conjunction (September 23, 2036). The cycle is complete in 780 Earth days.

The observable disk diameter of Mars will be greater than 6 arcsec from March 05, 2035 [-23.6° &delta ] (151.3° Ls) and will not fall below this value until February 26, 2036 (0.4° Ls), lasting nearly 12 months or 209 degrees Ls. Imaging by CCD devices may begin with a disk diameter of 4.5 arcsec or more, commencing on or about January 01, 2035.

The Sub-Earth ( De ) and Sub-Solar ( Ds ) points are graphically represented in Figures 4 and 5. The 2035-2036 Ephemeris of Mars is tabulated on Internet in this web site. A glossary of Terms appears at the end of this table.


THE SOUTH POLAR REGION

Astronomers will have a view of the Martian South Polar Region (SPR) will be tilted toward the Earth and will remain so throughout the apparition. For more detailed information on the south polar cap click to this web site.

Observers should be aware that during the next apparition of Mars in 2035 a major dust storm may occur to block our view of the clouds and surface of the Red Planet. While it is nearly impossible to predict these events our studies show that the Martian dusty season should begin on or about March 30, 2035 (165° Ls) through February 25,2036 (359° Ls) with the highest probability around the last week in August (255° Ls) and again peaking December 03 (315° Ls). Massive, planet-encircling storms usually occur in the southern hemisphere summer and usually in sensitive areas for the development of dust storms are in northwest Hellas.

Do not be surprised if another early dust storm occurs on or about May 04, 2035 (184 Ls). Observers should be alert for dust clouds in the northeast Hellas Basin, the Serpentis-Noachis region, and the Solis Lacus region For more detailed information on Martian dust storms on this web site.

CALENDAR OF EVENTS -- MARS, 2035 - 2036

Conjunction. Mars is behind the Sun

Ls 151.8°
De -4.8°
Ds 11.6°
RA 18:19
Dec -23.6°
A. Dia 6’’

Apparition begins for observers using 4-inch to 8-inch apertures telescopes and up. Begin low-resolution CCD imaging. Views of surface details not well defined. Mid-summer. Northern clouds frequent. Syrtis Major broad. Are both polar hoods visible?

Ls 175.9°
De -15.9°
Ds 1.8°
RA 20:26
Dec -20.4°
A. Dia 8’’

Late southern winter, SPH present and edge of NPH visible. Hellas frost covered? Are W-clouds present?

Ls 180°
De -17.3°
Ds 0.1°
RA 20:45
Dec -19.5°
A.Dia 8.4’’

Equinox - Northern Autumn/Southern Spring. South Polar Cap (SPC) maximum width. Is the North Polar Hood present? Does SPH or frost cover Hellas ? Hellas should begin to clear and darken. Are W-clouds present? South cap emerges from darkness of Winter. SPH thinning and forms "Life Saver Effect"?

Ls 184.3°
De -18.7°
Ds -1.8°
RA 21:06
Dec -18.3°
A. Dia 8.9’’

Mars at Quadrature. South Polar Cap (SPC) maximum width. Is the North Polar Hood present. Does SPH or frost cover Hellas ? Hellas should begin to clear and darken. Are W-clouds present? South cap emerges from darkness of Winter. SPH thinning and forms "Life Saver Effect?"

Ls 193.0°
De -20.7°
Ds -5.5°
RA 21:43
Dec -15.9°
A.Dia 10’’

SPC should be free of its hood, large and bright. Possible W-clouds in Tharsis- Amazonis . Syrtis Major shrinks or fading on eastern border. NPH bright. White areas brighter? White areas brighter? Within Hellas the features Zea Locus and Alpheus darkening? Mare Hadriacum (265°W, 40°S) and Yaonis Regio (318°W, 43°S) connected the canal Peneus ?

Ls 206.0°
De -22.5°
Ds -10.8°
RA 22:34
Dec -12.1°
A.Dia 12’’

SPC develops dark Magna Depressio at (270°W, 80°S). Syrtis Major narrows rapidly. W-clouds? At 215°Ls Rima Australis (a dark rift) appears connected with Magna Depressio from 20° to 240° longitude and SPC develops bright projection at 10° - 20° longitude in Argenteus Mons (10°-20°W). Dust cloud in Serpentis-Hellaspontus or Noachis -Hellas? Syrtis Major very narrow?

Ls 247.2°
De -20.8°
Ds -23.1°
RA 00:03
Dec -5.9°
A.Dia 21.8’’

Retrogression Begins. SPC in rapid retreat. Novus Mons smaller. Dust clouds expected over Serpentis-Hellaspontus (Ls 250° - 270). Syrtis Major beginning to narrow. Frost in bright deserts? Orographic clouds (W-clouds) possible. Elysium and Arisa Mons bright? Note: Several "planet-encircling dust storms have been reported during this season. High probability for dust clouds at 255° Ls.

Ls 250 °
De -20.7°
Ds -23.5°
RA 00:03
Dec -6.1 °
A.Dia 22.4’’

Mars at Perihelion. SPC in rapid retreat. Novus Mons smaller. Dust clouds expected over Serpentis-Hellaspontus (Ls 250° - 270). Syrtis Major beginning to narrow. Frost in bright deserts? Orographic clouds (W-clouds) possible. Elysium and Arisa Mons bright? Note: Several "planet-encircling dust storms have been reported during this season. High probability for dust clouds at 255° Ls.

Ls 264.3°
De -20.9°
Ds -25.1°
RA 23:47
Dec -7.7°
A.Dia 24.6’’

Mars at Closest Approach. Novus Mons reduced to a few bright patches and soon disappears. Windy season on Mars begins, dust clouds present? Watch for initial dust clouds in south. White patches in bright areas? Hellas bright spots? Numerous bright patches. Syrtis Major beginning to narrow.

Ls 266.8°
De -21.0°
Ds -25.2°
RA 23:43
Dec -8.0°
A.Dia 24.6’’

Mars at Opposition. W-clouds present? NPH extends 50° N? Decreased number of White clouds. "Syrtis Blue Cloud"? White areas in deserts? Dust clouds in south until 270° Ls? Watch for planetary system clouds bands. Orographic cloud over Arsia Mons? Syrtis Major is narrow.

Ls 270°
De -21.3°
Ds -25.2°
RA 23:37
Dec -8.3°
A.Dia 24.6’’

Solstice - Northern Winter/Southern Summer. W-clouds present? NPH extends 50° N? Decreased number of White clouds. "Syrtis Blue Cloud"? White areas in deserts? Dust clouds in south until 270° Ls? Watch for planetary system clouds bands. Orographic cloud over Arsia Mons? Syrtis Major is narrow.

Ls 285.5°
De -23.0°
Ds -24.2°
RA 23:20
Dec -8.1 °
A.Dia 20.4’’

Retrogression Ends. NPH extends 50° N? White clouds rare. Dust storm? Frost patches? Dust storms visible at 285° Ls (Martin & Zurek). Look for orographic clouds over the Tharsis volcanoes. Orographic cloud over Arsia Mons? W-Cloud? SPC small

Ls 314.8°
De -25.9°
Ds -17.6°
RA 00:02
Dec -0.3°
A.Dia 12’’

Edom bright? Is SPC remnant visible in mid-summer? High probability of dusty storm at 315° Ls. Orographic cloud over Arsia Mons? Topographic cloud over Libya ? Wave or frontal cloud activity from NPR? Bright spots in Hellas ? Orographic cloud over Arsia Mons? Topographic cloud over Libya ? Topographic cloud over Edom ?

Ls 325.6 °
De -25.6°
Ds -13.9°
RA 00:36
Dec 4.0°
A.Dia 10’’

Novus Mons reduced to a few bright patches and soon disappears. Windy season on Mars begins, dust clouds present? Watch for initial dust clouds in south. White patches in bright areas? Hellas bright spots? Numerous bright patches. Syrtis Major beginning to narrow.

Ls 339.2 °
De -23.4°
Ds -8.7°
RA 01:27
Dec 9.8°
A.Dia 8’’

NPC large hood present. W-Cloud? Orographic cloud over Arsia Mons? Topographic cloud over Libya ? Topographic cloud over Edom ?

Ls 0 °
De -15.8°
Ds 0.2 °
RA 03:03
Dec 18.4°
A.Dia 6’’’

Equinox - Northern Spring/Southern Autumn. North Polar Hood (NPH) breaking up, North Polar Cap (NPC) should be exposed. (" Areo -" is a prefix often employed when referring to Mars or "Ares.")


All you need to know: 2020’s Harvest Moon

A Harvest Moon via Dan Bush of Missouri Skies.

Here in the Northern Hemisphere, we call the full moon closest to the autumn equinox the Harvest Moon. In 2020, the Northern Hemisphere autumn equinox came on September 22. Depending on where you live worldwide, the first of two October 2020 full moons will fall on October 1 or 2, 2020. Thus, for the Northern Hemisphere, this upcoming full moon in early October – the full moon closest to our autumn equinox – is our Harvest Moon.

For the Southern Hemisphere, the Harvest Moon always comes in March or early April.

Harvest Moon is just a name. In some ways, it’s like any other full moon name. But these autumn full moons do have special characteristics, related to the time of moonrise. Nature is particularly cooperative in giving us full-looking moons near the horizon after sunset, for several evenings in a row, around the time of the Harvest Moon.

What’s more, in 2020, the Harvest Moon will be near a fiery red object in our night sky. People around the world will be looking at the moon and wondering:

It’s not a star. It’s the red planet Mars, now nearly at its best for this two-year period. See the chart below:

In late September and early October 2020, the Northern Hemisphere’s Harvest Moon shines in the vicinity of the brilliant red planet Mars. Mars is super bright now! Read more.

What is a Harvest Moon? On average, the moon rises about 50 minutes later each day. But when a full moon happens close to an autumn equinox, the moon rises closer to the time of sunset. For mid-temperate latitudes, it rises only about 25 to 30 minutes later daily for several days before and after the full Harvest Moon.

For very high northern latitudes, there’s even less time between successive moonrises.

The difference between 50 minutes and 30 minutes might not seem like much. But it means that, in the nights after a full Harvest Moon, you’ll see the moon ascending in the east relatively soon after sunset. The moon will rise during or near twilight on these nights, making it seem as if there are several full moons – for a few nights in a row – around the time of the Harvest Moon.

Why does this happen? Check out the illustrations below:

In autumn, the ecliptic – marking the moon’s approximate path across our sky – makes a narrow angle with the evening horizon. Image via classicalastronomy.com. The narrow angle of the ecliptic means the moon rises noticeably farther north on the horizon from one night to the next. So there is no long period of darkness between sunset and moonrise. Image via classicalastronomy.com. Harvest Moon sunset and moonrise – September 19, 2013 – as seen by EarthSky Facebook friend Andy Somers in Noumea, New Caledonia. One of the characteristics of the Harvest Moon is that it rises around the time of sunset for several evenings in a row.

Because the moon’s orbit around Earth isn’t a perfect circle, the Harvest Moon’s distance from Earth – and apparent size in our sky – is a bit different from year to year. In 2019, the Harvest Moon was actually a micro-moon or mini-moon: the most distant and smallest full moon of the year 2019. This year, in 2020, the Harvest Moon is the second-smallest full moon of 2020. But four years ago – September 28, 2015 – the Harvest Moon was the year’s closest and biggest supermoon.

Still, in any year, you might think the Harvest Moon looks bigger or brighter or more orange. That’s because the Harvest Moon has such a powerful mystique. Many people look for it shortly after sunset around the time of full moon. After sunset around the time of any full moon, the moon will always be near the horizon. It’ll have just risen. It’s the location of the moon near the horizon that causes the Harvest Moon – or any full moon – to look big and orange in color.

The orange color of a moon near the horizon is a true physical effect. It stems from the fact that – when you look toward the horizon – you’re looking through a greater thickness of Earth’s atmosphere than when you gaze up and overhead.

The bigger-than-usual size of a moon seen near the horizon is something else entirely. It’s a trick that your eyes are playing – an illusion – called the Moon Illusion. You can find many lengthy explanations of the Moon Illusion by doing an online search for those words.

Jarred Donkersley caught this photo of 2016’s Harvest Moon at the Vincent Thomas Bridge in San Pedro, California.

When is the Harvest Moon in 2020? The exact time of the full Harvest Moon is October 1 at 21:05 Universal Time. At U.S. time zones, that translates to October 1 at 6:05 p.m. ADT, 5:05 p.m. EDT, 4:05 p.m. CDT, 3:05 p.m. MDT, 2:05 p.m. PDT, 1:05 p.m. Alaskan Time and 11:05 a.m. Hawaiian Time.

So watch for the Harvest Moon in late September and early October … or any of the nights around then.

By the way, more often than not, the September full moon is the Northern Hemisphere’s Harvest Moon. But if the full moon occurs in early October – as it did in 2017 and does in 2020 – the October full moon is that year’s Harvest Moon.

Ed and Bettina Berg in Las Vegas, Nevada, contributed this image of the 2016 Harvest Moon.

How did the Harvest Moon get its name? The shorter-than-usual lag time between moonrises around the full Harvest Moon means no long period of darkness between sunset and moonrise for days in succession.

In the days before tractor lights, the lamp of the Harvest Moon helped farmers to gather their crops, despite the diminishing daylight hours. As the sun’s light faded in the west, the moon would soon rise in the east to illuminate the fields throughout the night.

Who named the Harvest Moon? That name probably sprang to the lips of farmers throughout the Northern Hemisphere, on autumn evenings, as the Harvest Moon aided in bringing in the crops.

The name was popularized in the early 20th century by the song below.

Shine On Harvest Moon
By Nora Bayes and Jack Norworth (1903)

Shine on, shine on harvest moon
Up in the sky,
I ain’t had no lovin’
Since January, February, June or July
Snow time ain’t no time to stay
Outdoors and spoon,
So shine on, shine on harvest moon,
For me and my gal.

And don’t miss this more recent version of the song by Leon Redbone.

Bottom line: According to skylore, the closest full moon to the autumn equinox is the Harvest Moon. In 2020, the autumnal equinox for the Northern Hemisphere comes on September 22. So this hemisphere’s Harvest Moon comes on October 1.


Mars Tilt

Of all the features of Mars, its axial tilt is most similar to Earth. Mars’ tilt is 25 degrees, just a fraction away from the Earth’s 23.5 degrees. And because of this tilt, Mars has seasons, just like the Earth. Of course, since Mars takes twice as long as Earth to orbit the Sun, the seasons are twice as long.

Mars also has a very elliptical orbit. Because of this, the difference between its closest and most distant point along its orbit vary by 19%. This extreme difference makes the planet’s southern winters long and extreme. The northern winters aren’t as long or cold.

Astronomers know that the current tilt of Mars’ axis is just a fluke. Unlike Earth, the planet’s tilt has changed dramatically over long periods of time. In fact, astronomers think that the wobble in the tilt might help explain why vast underground reservoirs of water ice have been found at mid-latitudes, and not just around the planet’s poles. It’s possible that in the distant past, Mars was tilted at a much more extreme angle, and the ice caps were able to grow across the planet. When the tilt was less extreme, the ice remained, and was covered by a layer of dust.

Researchers have developed a model that accounts for the advance and retreat of the subsurface Martian ice sheets over 40 ice ages and 5 million years.

Here’s an article that explains how scientists track the Martian equator in the past. And the lopsided ancient oceans on Mars are explained by its tilt in the past.

Here’s some information about the tilt and seasons on Mars from MSSS. And the Wikipedia article about timekeeping on Mars.

Finally, if you’d like to learn more about Mars in general, we have done several podcast episodes about the Red Planet at Astronomy Cast. Episode 52: Mars, and Episode 91: The Search for Water on Mars.


Mars Dust Storms

Mars dust storms are much different than the dust devils that many people have seen in images sent back from the planet. On Mars a dust storm can develop in a matter of hours and envelope the entire planet within a few days. After developing, it can take weeks for a dust storm on Mars to completely expend itself. Scientists are still trying to determine why the storms become so large and last so long.

All Mars dust storms are powered by sunshine. Solar heating warms the Martian atmosphere and causes the air to move, lifting dust off the ground. The chance for storms is increased when there are great temperature variations like those seen at the equator during the Martian summer. Because the planet’s atmosphere is only about 1% as dense as Earth’s only the smallest dust grains hang in the air.

Surprisingly, many of the dust storms on the planet originate from one impact basin. Hellas Basin is the deepest impact crater in the Solar System. It was formed more than three billion years ago during the Late Bombardment Period when a very large asteroid hit the surface of Mars. The temperatures at the bottom of the crater can be 10 degrees warmer than on the surface and the crater is deeply filled with dust. The difference in temperature fuels wind action that picks up the dust, then storm emerge from the basin.

The dust storms were of great concern when probes were first sent to Mars. Early probes happened to arrive in orbit during large events. The Viking missions of 1976 easily withstood two big dust storms without being damaged. They were not the first missions to survive Martian dust storms. In 1971, Mariner 9 arrived at Mars during the biggest dust storm ever recorded. Mission controllers simply waited a few weeks for the storm to subside, then carried on with the mission. The biggest issue that rovers face during a dust storm is the lack of sunlight. Without the light, the rovers have trouble generating enough power to keep their electronic warm enough to function.

Mars dust storms are of great interest to scientists. Even though several spacecraft have observed the storms first hand, scientists are no closer to a definitive answer. For now, the storms on Mars are going to continue to present challenges to planning a human mission to the planet.

Here’s an article describing how the dust storms threatened the Mars rovers, and another discussing how electrical dust storms could make life on Mars impossible.

Finally, if you’d like to learn more about Mars in general, we have done several podcast episodes about the Red Planet at Astronomy Cast. Episode 52: Mars, and Episode 91: The Search for Water on Mars.