Mohawk Valley Astronomical Society

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Alien Skies, Part 1 of 2: The Inner Planets & Pluto

by Perry Pezzolanella

We are familiar with the blue sky with white or gray clouds here on Earth, and a Sun and Moon that hardly vary in size. Stars in the night sky are always familiar as they change predictably throughout the seasons. The planets come and go, sometime brighter than other times, but those of us who observe regularly will never confuse Venus for an airplane. Our Moon goes through regular phases with an average diameter of 31 arcminutes and magnitude of -12 at full phase, varying little from month to month, even when the media calls it super. The views of the sky from other worlds are different from what we are familiar with here on Earth. Some are downright alien with different color sunlight, rings arching high, and multiple moons shining at once! The sky itself can appear different due to a number of factors. The density and chemical composition of the atmosphere can contribute to a difference in color, clarity, haze, and cloud type. Also visible are planets and moons of different brightness and phases than seen on Earth that will truly make it feel like one is not in Kansas anymore.

The sky and scenery have been observed by landers and rovers on Mars, Venus, Titan, and the Moon, but it can be simulated for other worlds taking into consideration the composition and characteristics of an atmosphere, if present, and the Sun, moons, rings, and other objects using computer software. Here is a tour of what it is like to view the sky from other worlds:

Mercury has no atmosphere so the sky is starlit even during the daytime. Mercury has an eccentric orbit so the Sun on average is 2.5 times larger than as seen from Earth but can be from 2.2 to 3.2 times as large and as much as 10.2 times brighter. The planets still look like stars with Venus shining as bright as magnitude -7.7 and the Earth appearing as a double star shining at magnitude -5.0 with the Moon at magnitude -1.7 near it (slightly brighter than Sirius is in our sky). The other planets will appear the same as we see them from Earth but somewhat less bright at opposition. Since Mercury orbits the Sun faster than it can rotate on its axis, the Sun will appear to briefly travel backward in the sky near perihelion before continuing its regular motion after Mercury rounds the Sun and slows down. This means that some locations on Mercury can witness a double sunset or sunrise. This could be very confusing indeed to someone on the surface who did not understand the orbital mechanics behind it.

The dense atmosphere of Venus bathes the surface in an eerie orange light and the Sun is almost always hidden behind the clouds 30 miles overhead. Thin patches might briefly reveal a hazy Sun that is about 1.5 times as large as seen from Earth and about 3 times brighter. A very fine heat haze from the blistering 870ºF surface or a mist of sulfuric acid gives the distant hills a hazy appearance in the photos taken from the Soviet Venera landers where the yellow-orange light is obvious. Venus rotates very slowly and in the opposite direction, so the Sun could be seen rising in the west and then setting in the east 58.5 days later. Because of the thick atmosphere and slow rotation, twilight lasts nearly two weeks and the dim light would be deep brownish-orange to dull red. The Earth is an obvious double star with the Moon next to it and both are brilliant as seen from Venus. Earth shines as bright as magnitude -6.6 and the Moon at magnitude -2.7, as bright as we see Jupiter at most oppositions here on Earth! Mercury is easier to see because Venus is much closer to it and would appear higher in the sky, about 41º above the horizon at its best and shining as bright as magnitude -2.7, an awesome sight if only the sky were clear.

The sky from the surface of the Moon at night appears no different than from Earth, but the lack of an atmosphere would make the stars and planets appear brighter, especially the Sun. The Earth is the big difference in the sky as it looms four times larger than the Moon does from earth and is 50 times brighter than the Full Moon. An eclipse of the Sun by the Earth would color the lunar landscape varying shades of orange, red, and brown. The sky is black and full of stars even during the daytime.

Mars has a thin, dusty atmosphere so daylight is quite bright and the stars are not visible. There is so much dust that twilight lasts a long time, although not weeks like Venus because Mars rotates once in 24 hours and 37 minutes and the Sun travels across the sky almost as fast as seen on Earth. At sunrise and sunset, the sky closest to the Sun appears bluish from the fine dust and water ice crystals. The Sun appears about 5/8 as large as seen from Earth and about 40% as bright, much like a slightly cloudy day here. Mars has two tiny moons, Phobos and Deimos, with Phobos appearing 1/3 as large as our Moon in the sky and Deimos barely more than a star at 2 arcminutes across and about as bright as we see Venus on Earth. Both moons eclipse the Sun but are actually transits as neither completely blocks the Sun. A Phobos transit would dim the scenery very swiftly like an approaching storm but briefly, while a Deimos transit would hardly be noticed. Neither moon is visible from the polar regions because they orbit so close to Mars. From Phobos Mars appears 6400 times larger and 2500 times brighter than the full Moon as seen from Earth and appears as a sphere 42º across taking up 1/4 of the sky! From Deimos Mars appears 1000 times larger and 400 times brighter than the Full Moon as seen from Earth and appears as a sphere 16.5º across taking up 1/11 of the sky. Earth is a double star shining at magnitude -2.5 with the Moon at magnitude +0.9 nearby. Venus shines at magnitude -3.2 and Mercury about magnitude 0 but quite low. The outer planets are slightly brighter than as seen from Earth.

Pluto has a thin atmosphere and the sky is still transparent but there could be haziness along the horizon, which would dim the stars and Sun. The Sun is star-like but still very bright shining at magnitude -18 and about 150 to 450 times the light of the Full Moon. The variability is due to Pluto’s highly eccentric orbit of 2.7 to 4.5 billion miles from the Sun. The change in the light level would be noticeable if one could live at least 100 years, but the Sun is still too dangerous to stare at. From Pluto all of the planets would appear as stars closer to the Sun and dimmer than as seen from Earth with the inner planets nearly impossible to see near the glare of the Sun. Charon is the only one of the five moons to show a disc not quite as large as the Full Moon but bright enough to illuminate Pluto’s night side. From Charon Pluto would appear twice as large and four times brighter, but still dim due to the vast distance from the Sun.

These worlds are different from anything we experience on Earth, but there are stranger places of even greater curiosity, beauty and awe. Next month the giant outer planets will be visited where rings and countless moons rule and the surface of one moon can trick someone into believing it is almost like Earth on a smoggy day.