May 9, 2016 - Mercury Transit Black Dot

May 9, 2016 - Mercury Transit Black DotA transit of Mercury across the Sun takes place when the planet Mercury comes between the Sun and the Earth, and Mercury is seen as a small black dot moving across the face 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 in 1999, 2003, 2006, and May 9, 2016. The next will occur on November 11, 2019 and then on November 13, 2032. 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. Transits of Mercury can happen in May or November with May transits being about half as frequent as November transits. They currently occur within a few days either side of May 8 and November 10. The interval between one November transit and the next November transit may be 7, 13, or 33 years; the interval between one May transit and the next May transit may be 13 or 33 years.

May transits are less frequent than November transits because during a May transit, Mercury is near aphelion whereas during a November transit, it is near perihelion. Perihelion transits occur more frequently due to two effects: firstly, Mercury moves faster in its orbit at perihelion and can reach the transit node more quickly, and secondly at perihelion Mercury is closer to the Sun and so has less parallax. During May transits Mercury has an angular diameter of 12" and these transits take place at the descending node of Mercury's orbit. During November transits Mercury has an angular diameter of 10" and these transits occur at the ascending node. Transits of Mercury are gradually drifting later in the year; before 1585 they occurred in April and October.

May 9, 2016 - Mercury Transit Black DotSometimes Mercury appears to only graze the Sun during a transit. In this case it is possible that in some areas of the Earth a full transit can be seen while in other regions there is only a partial transit (no second or third contact). The transit of November 15, 1999 was such a transit, and the previous one before that was on October 28, 743. The next such transit will occur on May 11, 2391. It is also possible that a transit of Mercury can be seen in some parts of the world as a partial transit, while in others Mercury misses the Sun. Such a transit last occurred on May 11, 1937, and the previous one was on October 21, 1342. The next such transit will occur on May 13, 2608.

The first observation of a transit of Mercury was on November 7, 1631 by Pierre Gassendi. Johannes Kepler had however predicted the occurrence of transits of Mercury and Venus some time before that. Gassendi unsuccessfully attempted to observe the transit of Venus just one month later, but due to inaccurate astronomical tables he did not realize that it was not visible from most of Europe, including Paris. A transit of Venus was not observed until 1639, by Jeremiah Horrocks. The Table below does not include all historical transits of Mercury.
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