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.
- Scientific observations
- Occurrence of transits
- Partial transits of Mercury
- Past and future transits
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).
Examples of the scientific investigations based on transits of Mercury are:
Images of the 1999 Nov 15 transit from the Transition Region and Coronal explorer (TRACE) satellite were on Astronomy Picture of the Day (APOD) on 1999 Nov 19 Three APOD's featured the 2016 May 9 transit.
Occurrence of transits
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 the longitude of the nodes of Mercury's orbit increasing by about 1.1 deg per century.
Transits of Mercury occur on a regular basis. As explained in 1882 by Newcomb, 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. From the ratio of these values, it can be easily shown that Mercury will make an almost integral number of revolutions about the Sun over intervals of 4, 6, 7, 13, 33, 46, 171 and 217 years.
In 1894 Crommelin noted that at these intervals, the successive paths of Mercury relative to the Sun are consistently displaced northwards or southwards. He noted the displacements as:
Displacements at subsequent transits
Comparing these displacements with the solar diameter (about 31.7’ for May, and 32.4’ for November, transits) the following may be deduced about the interval between transits:
Transits that occur 46 years apart can be grouped into a series. For November transits each series includes about 20 transits over 874 years, with the path of Mercury across the Sun passing further north than for the previous transit. For May transits each series includes about 10 transits over 414 years, with the path of Mercury across the Sun passing further south than for the previous transit. Some authors have allocated a series number to transits on the basis of this 46-year grouping.
Similarly transits that occur 217 years apart can be grouped into a series. For November transits each series would include about 135 transits over 30,000 years. For May transits each series would include about 110 transits over 24,000 years. For both the May and November series, the path of Mercury across the Sun passes further north than for the previous transit. Series numbers have not been traditionally allocated on the basis of the 217 year grouping.
Predictions of transits of Mercury covering many years are available at
Partial transits of Mercury
Sometimes Mercury appears to only graze the Sun during a transit. There are two possible scenarios
The possibility where, at mid-transit, Mercury is seen to be fully on the solar disk from some parts of the world, and to completely miss the Sun as seen from other parts of the world, cannot occur.
Past and future transits
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.