Transit of Mercury

A transit of Mercury across the Sun takes place when the planet Mercury passes directly between the Sun and a superior planet, becoming visible against (and hence obscuring a small portion of) the solar disk. During a transit, Mercury appears as a tiny black dot moving across the disk of the Sun.

Transits of Mercury with respect to Earth are much more frequent than transits of Venus, with about 13 or 14 per century, in part because Mercury is closer to the Sun and orbits it more rapidly.

Transits of Mercury occur in May or November. The last four transits occurred on November 15, 1999; May 7, 2003; November 8, 2006; and May 9, 2016. The next will occur on November 11, 2019, and then on November 13, 2032. A typical transit lasts several hours.

On June 3, 2014, the Mars rover Curiosity observed the planet Mercury transiting the Sun, marking the first time a planetary transit has been observed from a celestial body besides Earth.

More generally, transits can also occur for Venus and were investigated in the context of searching for the hypothetical inner planet Vulcan.

The most common observation to be made at a transit is to record the times when the disk of Mercury appears to be in contact with the limb of the Sun. Those contacts are traditionally referred to as the 1st, 2nd, 3rd and 4th contacts – with the 2nd and 3rd contacts occurring when the disk of Mercury is fully on the disk of the sun. As a general rule, 1st and 4th contacts cannot be accurately detected, while 2nd and 3rd contacts are readily visible within the constraints of the Black Drop effect, irradiation, atmospheric conditions, and the quality of the optics being used.

Observed contact times for transits between 1677 and 1881 are given in S Newcomb's analysis of transits of Mercury. Observed 2nd and 3rd contacts times for transits between 1677 and 1973 are given in Royal Greenwich Observatory Bulletin No.181, 359-420 (1975).

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