Transit of Mercury

A transit of Mercury across the Sun takes place when the planet Mercury passes directly (transits) 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.

Transit of Mercury on November 11, 2019
Transit of Mercury on May 9, 2016
Transit of Mercury on November 8, 2006 with sunspots #921, 922, and 923
Mercury transiting the Sun as viewed by the rover Curiosity on Mars (June 3, 2014).[1]

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 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.

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]

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

Scientific observations

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.[2] Observed 2nd and 3rd contacts times for transits between 1677 and 1973 are given in Royal Greenwich Observatory Bulletin No.181, 359-420 (1975).[3]

Examples of the scientific investigations based on transits of Mercury are:

  • Measuring the scale of the solar system.[4]
  • Investigations of the variability of the Earth's rotation and of the tidal acceleration of the Moon.[5][6][7]
  • Measuring the mass of Venus from secular variations in Mercury's orbit.[2]
  • Looking for long term variations in the solar radius.[8][9]
  • Investigating the black drop effect, including calling into question the purported discovery of the atmosphere of Venus during the 1761 transit.[10][11][12]
  • Assessing the likely drop in light level in an exoplanet transit.[13]

Images of the November 15, 1999 transit from the Transition Region and Coronal explorer (TRACE) satellite were on Astronomy Picture of the Day (APOD) on November 19[14] Three APODs featured the May 9, 2016 transit.[15][16][17]

1832 Mercury transit

The Shuckburgh telescope of the Royal Observatory, Greenwich in London was used for the 1832 transit of Mercury.[18] It was equipped with a micrometer by Dollond and was used for a report of the events as seen through the small refractor.[18] By observing the transit in combination with timing it and taking measures, a diameter for the planet was taken.[18] They also reported the peculiar effects that they compared to pressing a coin into the Sun.[18] 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[18]

1907 transit

For the 1907 transit of Mercury, these were some the telescope used at the Paris Observatory include:[19]

  • Focault-Eichens reflector (40 centimetres (16 in) aperture)
  • Focault-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 observations.[19]

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 Mercury's nodal precession and Earth's axial precession.

Transits of Mercury occur on a regular basis. As explained in 1882 by Newcomb,[20] 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[21] 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
IntervalMay
transits		November
transits
After 6 years65′ 37″ S31′ 35″ N
After 7 years48′ 21″ N23′ 16″ S
Hence after 13 years (6 + 7)17′ 16″ S08′ 19″ N
... 20 years (6 + 2 × 7)31′ 05″ N14′ 57″ S
... 33 years (2 × 6 + 3 × 7)13′ 49″ N06′ 38″ S
... 46 years (3 × 13 + 7)03′ 27″ S  1′ 41″ N
... 217 years (14 × 13 + 5 × 7)00′ 17″ N00′ 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[22] 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,[22] SOLEX,[23] and Fourmilab.[24]

Partial transits of Mercury
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,[25][26] 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.[24]
  • 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. 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.

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. A transit of Venus was not observed until 1639 by Jeremiah Horrocks. The table below includes all historical transits of Mercury from 1605 on:

Past transits of Mercury
Date of
mid-transit		Time (UTC)Notes
StartMidEnd
1605 Nov 118:4720:0221:18[27]
1615 May 306:4410:0913:33[27]
1618 Nov 411:1013:4216:14[27]
1628 May 514:2317:3220:40[27]
1631 Nov 704:3907:2010:01[27] Observed by Pierre Gassendi.
1644 Nov 922:5500:5702:58[27]
1651 Nov 3–423:0900:5202:35Observed by Jeremy Shakerly in Surat, reported in letter to Henry Osbourne, January 1652. Shakerly is thought to have died in India around 1655.[28]
1661 May 313:0816:5420:40Occurred on the day of the coronation of King Charles II of England. Observed by Christiaan Huygens in London.
1664 Nov 415:5418:3221:10[27]
1674 May 722:0100:1602:31[27]
1677 Nov 709:3312:1114:48Observed by Edmund Halley in St Helena and Richard Towneley in Lancashire to determine solar parallax, also noted by Jean Charles Gallet in Avignon; as reported in a letter from John Flamsteed to Johannes Hevelius May 23, 1678.[29]
1690 Nov 1003:5905:4307:27[27]
1697 Nov 303:4005:4207:43[27]
1707 May 519:3723:3203:27Observed by Abraham Sharp.[30][27]
1710 Nov 620:4023:2202:03[27]
1723 Nov 914:2716:5919:30[27]
1736 Nov 1109:1110:3011:49[27]
1740 May 221:4223:0200:21[27]
1743 Nov 508:1510:3012:45[27] Coordinated scientific observations were organized by Joseph-Nicolas Delisle worldwide.
1753 May 602:1906:1310:06[27]
1756 Nov 701:2804:1006:54[27]
1769 Nov 9–1019:2321:4600:10[27] Observed by Charles Green and James Cook from Mercury Bay in New Zealand.[31] Noted that Mercury had little or no atmosphere.
1776 Nov 221:0321:3622:09[27]
1782 Nov 1214:4215:1615:50Observed from Cambridge U.K.[32][27]
1786 May 403:0105:4108:21[27]
1789 Nov 512:5315:1917:44[27]
1799 May 709:1012:5016:31Observed by Capel Lofft in England.[33]
1802 Nov 906:1608:5811:41Observed by William Herschel and Capel Lofft in England.[34][35]
1815 Nov 1200:2002:3304:46 [27]
1822 Nov 501:0402:2503:45 [27]
1832 May 509:0412:2515:47 [27]
1835 Nov 717:3520:0822:41 [27]
1845 May 816:2419:3722:49 Observed by William Lassell.[36]
1848 Nov 911:0713:4816:28 [27]
1861 Nov 1205:2107:1909:18 Partially observed from Malta by William Lassell.[37]
1868 Nov 505:2807:1409:00 [27]
1878 May 615:1619:0022:44 Observed from Greenwich Observatory.[38]
1881 Nov 6–722:1900:5703:36 Observed by John Tebbutt.[39]
1891 May 8–923:5702:2204:47 [27]
1894 Nov 1015:5818:3521:11Observed from Sidmouth, Devon by H.H. Turner and A. F. Lindemann[40]
1907 Nov 1410:2412:0713:50Observed from Johannesburg by R. T. A. Innes[41]
1914 Nov 709:5712:0314:09Seen by several observers across the U.K. including A. Grace Cook and T. E. R. Phillips.[42]
1924 May 8–921:4401:4105:38 Final stages observed by Basil Brown.[43]
1927 Nov 1003:0205:4608:29 Final stages observed from the U.K.[44]
1937 May 1108:5308:5909:06Only visible as partial transit in Southern Africa, Southern Arabia, South Asia, and Western Australia.
1940 Nov 11–1220:4923:2101:53 Observed from New South Wales.[45]
1953 Nov 1415:3716:5418:11 [27]
1957 May 5–623:5901:1402:30 [27]
1960 Nov 714:3416:5319:12[46]
1970 May 904:1908:1612:13[47]
1973 Nov 1007:4710:3213:17[48]
1986 Nov 1301:4304:0706:31[49]
1993 Nov 603:0603:5704:47[50]
1999 Nov 15
21:1521:4122:07[51] Partial transit in Australia, Antarctica, and New Zealand's South Island.
2003 May 705:1307:5210:32[52]
2006 Nov 8
18:1220:4123:10[53]
2016 May 9
11:1214:5718:42
Entire transit in S. America, eastern N. America, western Europe; part of transit everywhere else except Australia and far eastern Asia.[54]
2019 Nov 11
12:3515:2018:04 [27]
Future transits of Mercury
Date of
mid-transit		Time (UTC)Notes
StartMidEnd
2032 Nov 1306:4108:5411:07 [27]
2039 Nov 707:1708:4610:15 [27]
2049 May 711:0314:2417:44 [27]
2052 Nov 8–922:5301:2904:06 [27]
2062 May 10–1118:1621:3601:00 [27]
2065 Nov 11–1219:2422:0601:48 [27]
2078 Nov 1411:4213:4115:39 [27]
2085 Nov 711:4213:3415:26 [27]
2095 May 8–917:2021:0500:50 [27]
2098 Nov 1004:3507:1609:57 [27]
2108 May 1201:4004:1606:52 [27]
2111 Nov 14–1522:1500:5303:30 [27]
2124 Nov 1516:4918:2820:07 [27]

See also

References
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Sources
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