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Transit of Earth from Mars


A transit of Earth across the Sun as seen from Mars takes place when the planet Earth passes directly between the Sun and Mars, obscuring a small part of the Sun's disc for an observer on Mars. During a transit, Earth would be visible from Mars as a small black disc moving across the face of the Sun. They occur every 26, 79 and 100 years, and every 1,000 years or so there is an extra 53rd-year transit.

Transits of Earth from Mars usually occur in pairs, with one following the other after 79 years; rarely, there are three in the series. The transits also follow a 284-year cycle, occurring at intervals of 100.5, 79, 25.5, and 79 years; a transit falling on a particular date is usually followed by another transit 284 years later. Transits occurring when Mars is at its ascending node are in May, those at descending node happen in November. This cycle corresponds fairly closely to 151 Mars orbits, 284 Earth orbits, and 133 synodic periods, and is analogous to the cycle of transits of Venus from Earth, which follow a cycle of 243 years (121.5, 8, 105.5, 8). There are currently four such active series, containing from 8 to 25 transits. A new one is set to begin in 2394. The last series ending was in 1211.

No one has ever seen a transit of Earth from Mars, but the next transit will take place on November 10, 2084. The last such transit took place on May 11, 1984.

During the event, the Moon could almost always also be seen in transit, although due to the distance between Earth and Moon, sometimes one body completes the transit before the other begins (this last occurred in the 1800 transit, and will happen again in 2394).

A transit of Earth from Mars corresponds to Mars being perfectly uniformly illuminated at opposition from Earth, its phase being 180.0° without any defect of illumination. During the 1879 event, this permitted Charles Augustus Young to attempt a careful measurement of the oblateness (polar compression) of Mars. He obtained the value 1/219, or 0.0046. This is close to the modern value of 1/154 (many sources will cite somewhat different values, such as 1/193, because even a difference of only a couple of kilometers in the values of Mars' polar and equatorial radii gives a considerably different result).


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