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. During a transit, Mercury appears as a tiny black dot moving across the Sun as the planet obscures a small portion of the solar disk. Because of orbital alignments, transits viewed from Earth occur in May or November. The last four such transits occurred on May 7, 2003; November 8, 2006; May 9, 2016; and November 11, 2019. The next will occur on November 13, 2032. A typical transit lasts several hours. Mercury transits are much more frequent than transits of Venus, with about 13 or 14 per century, primarily because Mercury is closer to the Sun and orbits it more rapidly.

The transit of Mercury on May 9, 2016. Mercury is visible to the lower left of center. A sun spot is visible above center.

Mercury transiting the Sun as viewed by the rover Curiosity on Mars (June 3, 2014).[1]

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.[1]

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Scientific investigation
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The orbit of the planet Mercury lies interior to that of the Earth, and thus it can come into an inferior conjunction with the Sun. When Mercury is near the node of its orbit, it passes through the orbital plane of the Earth. If an inferior conjunction occurs as Mercury is passing through its orbital node, the planet can be seen to pass across the disk of the Sun in an event called a transit. Depending on the chord of the transit and the position of the planet Mercury in its orbit, the maximum length of this event is 7h 50m.[2]

Transit events are useful for studying the planet and its orbit. Examples of the scientific investigations based on transits of Mercury are:

Measuring the scale of the solar system.[3]
Investigations of the variability of the Earth's rotation and of the tidal acceleration of the Moon.[4][5][6][7]
Measuring the mass of Venus from secular variations in Mercury's orbit.[8]: 367 
Looking for long term variations in the solar radius.[9][10]
Investigating the black drop effect, including calling into question the purported discovery of the atmosphere of Venus during the 1761 transit.[11][12][13]
Assessing the likely drop in light level in an exoplanet transit.[14]
Occurrence
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Transits of Mercury can only occur when the Earth is aligned with a node of Mercury's orbit. Currently that alignment occurs within a few days of May 8 (descending node) and November 10 (ascending node), with the angular diameter of Mercury being about 12″ for May transits, and 10″ for November transits. The average date for a transit increases over centuries as a result of Mercury's nodal precession and Earth's axial precession.

Transits of Mercury occur on a regular basis. As explained in 1882 by Newcomb,[8]: 477–487  the interval between passages of Mercury through the ascending node of its orbit is 87.969 days, and the interval between the Earth's passage through that same longitude is 365.254 days. Using continued fraction approximations of the ratio of these values, it can be shown that Mercury will make an almost integral number of revolutions about the Sun over intervals of 6, 7, 13, 33, 46, and 217 years.