Thursday, February 11, 2021

Eclipses in the solar system

On Earth, solar eclipses are fairly rare events that occur two to five times a year. A solar eclipse occurs when the Moon is near one of the two nodes of its orbit and is perfectly aligned between the Sun and the Earth.

Given these premises, it is therefore obvious that solar eclipses do not only occur on Earth, but can be seen from all planets in the Solar System that have satellites, all with the exception of Mercury and Venus.

Mars for example has two moons, Phobos and Deimos, which periodically pass in front of the solar disk, as shown in the second image taken from the Martian surface by the Curiosity rover. However, both Phobos and Deimos are too small to completely obscure the solar disk, resulting only ever in partial eclipses. Deimos in particular is so small and distant from Mars that its passages on the Sun are classified as transits and not as eclipses.

Moving towards the outer Solar System we see that the four gas giants, Jupiter, Saturn, Uranus and Neptune, are all surrounded by numerous moons. Solar eclipses from these planets are therefore quite common phenomena, with the type of eclipse (total, partial, annular) depending on the size and distance of the satellite in question from its planet. For example, on Uranus there are twelve satellites close enough to the planet and large enough to produce an eclipse.

Solar eclipses can then also be observed from Pluto. From this position the Sun appears as a very bright star, approximately the same angular size as Jupiter seen from Earth. Charon seen from Pluto has instead an angular diameter four times than that of the full Moon seen from Earth. Because of this large difference in size between the Sun's and Charon's discs, a solar eclipse seen from Pluto is more like an occultation of a star by the lunar disk than a real eclipse.

Credit: Joshua Cripps, NASA, JPL, MSSS.

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