Astronomy:Astronomy on Mercury

Short description: Sky from planet Mercury

Astronomy on Mercury is the sky as viewed from the planet Mercury. Because Mercury only has a thin Atmosphere, the sky will be black.

Lunar eclipse as viewed from Mercury, captured from the BepiColombo spacecraft. The Moon can be seen falling into the shadow of Earth.

Sun

Due to the proximity of Mercury to the Sun, it will give 11 times more light. The Sun will have an Angular size of 3.75 to 2°. From Perihelion to Aphelion, the size of the Sun increases almost 53%. This is due to the high eccentricity of the Sun's orbit.

Apparent size of the Sun as seen from the Solar System's planets

Due to tidal locking, three rotations of Mercury, is equal to two revolutions around the Sun. Because of this, the method of plotting the Sun's position at the same time each day would yield only a single point. However, the equation of time can still be calculated for any time of the year, so an analemma can be graphed with this information. The resulting curve is a nearly straight east–west line.

During a Mercurian day, the Sun would be seen rising in the east, move up for a while, stop in the sky, head backwards for a while, and then resume going forwards again. This peculiar movement is due to the orbit of Mercury. If the retrograde motion happens within few hours from sunrise, an observer would see two sunsets and two sunrises, in the same day. The Sun would be seen rising, stopping midway, going back down and setting, rise again, and continue its normal journey. Approximately four (Earth) days before perihelion, the angular speed of Mercury's orbit exactly matches its rotational velocity, so that the Sun's apparent motion stops. At perihelion, Mercury's orbital angular velocity slightly exceeds the rotational velocity, making the Sun appear to go retrograde. Four days after perihelion, the Sun's normal apparent motion resumes. From the moment the top of the Sun can be seen to the moment someone can see a full circle, a sunrise would take almost 6 hours.

Planets and Stars

Apart from the Sun, Venus would be the brightest celestial body. Venus will be brighter from Mercury, than from Earth. The reason for this is that when Venus is closest to Earth, it is between the Earth and the Sun, so only its night side is seen. Even when Venus is brightest in the Earth's sky, humans see only a narrow crescent. For a Mercurian observer, on the other hand, Venus is closest when it is in opposition to the Sun and is showing its full disk. The apparent magnitude of Venus is as bright as −7.7. The Earth and the Moon also will be very bright, their apparent magnitudes being about −5 and −1.2, respectively. The maximum apparent distance between the Earth and the Moon is about 15′. Which means that an observer on Mercury could differentiate between Earth and Moon as two separate dots in the sky. The Moon will come closer and closer towards Earth, eventually transiting Earth and moving over to the other side. This movement is because of the revolution of Moon around Earth.

Overexposed picture Moon and Earth as seen from Mercury. Taken by the MESSENGER spaceprobe. Down below are Computer Generated images of what Earth and the Moon might have looked like. Apollo program landing sites are marked. Pluto is also in the field but is too dim to be seen.

The zodiacal light will be more prominent than it is from Earth. Mercury has a southern pole star, α Pictoris, a magnitude 3.2 star. It is fainter than Earth's Polaris. Omicron Draconis is its north star.