Skies of other planets

This article deals with what an observer placed on the surface of a planet will see overhead.

Table of contents

1 Moon
2 Mars
3 Mercury
4 Asteroid belt
5 Jupiter's moons
6 Extrasolar planets

Moon

Moon has no atmosphere, so its sky is always black. However, in the daytime it will be impossible to see stars unless the observer is well shielded from the sunlight, direct or reflected from the ground.

The Sun for the lunar observer is the same as for the terrestrial one, only it is somewhat brighter due to lack of atmospheric scattering and absorbtion. The lunar day lasts a month. The invisible side of the Moon in not "dark" — it is sunlit in the same way as the visible one.

The most interesting feature of the Moon's sky is, of course, the Earth. Its visible diameter is four times the diameter of the Moon as seen from the Earth. The Earth will show phases just like the Moon for the terrestrial observer, but they will be opposite: when the terrestrial observer sees the full Moon, the lunar observer will see "new Earth", and vice versa. The Earth's albedo is much higher than that of the Moon, so the full Earth will glow 130 times brighter that the full Moon does for the terrestrial observer.

The Earth can be seen only on the visible hemisphere of the Moon, still the Earth is not fixed on the lunar sky: due to librations, it performs a complex wobbling movement. Near the boundary of the visible and invisible hemispheres of the Moon, the Earth will be sometimes below the horizon, and sometimes above it.

The Earth and the Sun sometimes meet on the lunar sky, which causes an eclipse. On the Earth, one sees a lunar eclipse, where the Sun disappears behind the Moon; but on the Moon, one sees the Sun goes behind the Earth. As visible diameter of the Earth is four times more than that of the Sun, the Sun can hide behind the Earth for hours. The Earth's atmosphere then becomes visible as a reddish ring.

The terrestrial solar eclipses, on the other hand, are difficult to notice for lunar observers. All one can see is a small darkened spot on the full Earth's disk.

Mars

Mars has only a thin atmosphere, so its sky is perhaps sufficiently dark to allow the brightest stars to be visible in the daytime.

The Sun on the Mars is seen 1.6 times smaller as on Earth, so it sends 2.5 times less light.

Mars has two natural satellites: Phobos and Deimos, both will be visible as more than just dots (especially Phobos). The apparent motion of Phobos will be in reverse, due to its fast orbital motion: it will rise on the west and set on the east.

The Earth is well seen from Mars as a double star (the Moon seen as a fainter companion). The maximum visible distance between the Earth and the Moon will be about 25′ (at conjunction of the Earth and the Sun — for the terrestrial observer it will be opposition of Mars and the Sun).

Mercury

Mercury has no atmosphere, like the Moon, so its sky is always black. The day here is 176 terrestrial days (exactly 2 mercurian years).

The visible diameter of the Sun on Mercury is 2.5 larger than on Earth, so it is more than 6 times brighter.

The second brightest object on the mercurian sky is Venus. It is here much brigher than for terrestrial observers. The cause for this is that when Venus is closest to Earth, it is between the Earth and the Sun, so we see its dark side. In fact, when Venus is brightest in the Earth's sky, we see only a narrow crescent of it.

For a Mercurial observer, on the other hand, Venus is closest when it is in opposition to the Sun and shows its full disk. The apparent magnitude of Venus should then be approximately ~−8.

The Earth and the Moon are also well visible, their apparent magnitude being about −6 and −1 respectively. The maximum apparent distance between the Earth and the Moon will be about 15′.

All other planets will be visible just like they do on Earth, being only somewhat less bright in opposition.

The zodiacal light for Mercurial observers should be much more prominent than for terrestrial ones.