Transit of Mercury

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.

In 1894 Crommelin[15] 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

Interval	May transits	November transits
After 6 years	65′ 37″ S	31′ 35″ N
After 7 years	48′ 21″ N	23′ 16″ S
Hence after 13 years (6 + 7)	17′ 16″ S	08′ 19″ N
... 20 years (6 + 2 × 7)	31′ 05″ N	14′ 57″ S
... 33 years (2 × 6 + 3 × 7)	13′ 49″ N	06′ 38″ S
... 46 years (3 × 13 + 7)	03′ 27″ S	1′ 41″ N
... 217 years (14 × 13 + 5 × 7)	00′ 17″ N	00′ 14″ N

Comparing these displacements with the solar diameter (about 31.7′ in May, and 32.4′ in November) the following may be deduced about the interval between transits:

• For May transits, intervals of 6 and 7 years are not possible. For November transits, an interval of 6 years is possible but rare (the last such pair was 1993 and 1999, with both transits being very close to the solar limb), while an interval of 7 years is to be expected.
• An interval of 13 years is to be expected for both May and November transits.
• An interval of 20 years is possible but rare for a May transit, but is to be expected for November transits.
• An interval of 33 years is to be expected for both May and November transits.
• A transit having a similar path across the sun will occur 46 (and 171) years later – for both November and May transits.
• A transit having an almost identical path across the Sun will occur 217 years later – for both November and May 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[16] 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 NASA,[16] SOLEX,[17] and Fourmilab.[18]

At inferior conjunction, the planet Mercury subtends an angle of 12″, which, during a transit, is too small to be seen without a telescope.[19] A common observation made at a transit[20] is recording the times when the disk of Mercury appears to be in contact with the limb of the Sun.[21] 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.[22] As a general rule, 1st and 4th contacts cannot be accurately detected,[7] while 2nd and 3rd contacts are readily visible within the constraints of the Black Drop effect,[23] irradiation, atmospheric conditions, and the quality of the optics being used.[24]

Observed contact times for transits between 1677 and 1881 are given in S Newcomb's analysis of transits of Mercury.[8]^: 367 Observed 2nd and 3rd contacts times for transits between 1677 and 1973 are given in Royal Greenwich Observatory Bulletin No.181, 359-420 (1975).[25]

November 15, 1999 simulated transit of Mercury across the Sun.

Sometimes Mercury appears to only graze the Sun during a transit. There are two possible scenarios:

• Firstly, it is possible for a transit to occur such that, at mid-transit, the disk of Mercury has fully entered the disk of the Sun as seen from some parts of the world, while as seen from other parts of the world the disk of Mercury has only partially entered the disk of the Sun. The transit of November 15, 1999 was such a transit,[26][27][28] with the transit being a full transit for most of the world, but only a partial transit for Australia, New Zealand, and Antarctica. The previous such transit was on October 28, 743 and the next will be on May 11, 2391. While these events are very rare, two such transits will occur within 2+1⁄2 years in December 6149 and June 6152.[18]
• Secondly, it is possible for a transit to occur in which, at mid-transit, the disk of Mercury has partially entered the disk of the Sun as seen from some parts of the world, while as seen from other parts of the world Mercury completely misses the Sun. Such a transit last occurred on May 11, 1937, when a partial transit occurred in southern Africa and southern Asia and no transit was visible from Europe and northern Asia.[29] The previous such transit was on October 21, 1342 and the next will be on May 13, 2608.

The possibility that, at mid-transit, Mercury is seen to be fully on the solar disk from some parts of the world, and completely miss the Sun as seen from other parts of the world cannot occur.

The Shuckburgh telescope of the Royal Observatory, Greenwich in London was used for the 1832 Mercury transit.[35] It was equipped with a micrometer by Dollond and was used for a report of the events as seen through the small refractor.[35] By observing the transit in combination with timing it and taking measures, a diameter for the planet was taken.[35] They also reported the peculiar effects that they compared to pressing a coin into the Sun.[35] The observer remarked:

I afterwards observed, that immediately around the planet there was a dusky tinge, making it appear as if, in a small degree sunk below the sun's surface.

— Royal Astronomical Society, Vol II, No. 13[35]

For the 1907 Mercury transit, telescopes used at the Paris Observatory included:[36]

• Foucault-Eichens reflector (40 centimetres (16 in) aperture)
• Foucault-Eichens reflector (20 centimetres (7.9 in) aperture)
• Martin-Eichens reflector (40 centimetres (16 in) aperture)
• Several small refractors

The telescopes were mobile and were placed on the terrace for the several observations.[36]