A Transit of Mercury across the Sun’s disk is a relatively rare occurrence that averages about thirteen each century. They are however more frequent than Venus transits, which occur in pairs roughly every 120 years. (See Transit of Venus Page) Mercury cannot transit once every 116 days (synodic period) because its orbit is highly elliptical and tilted some 7o to the ecliptic. Transits may only occur when the planetary orbit intersects or move across the ecliptic. This is specifically during the times of the ascending or descending nodes, when the Earth is exactly aligned with Mercury and the Sun. Therefore the transit can only happen when Mercury both passes through its inferior conjunction AND when the nodes are in alignment. Subsequently, these particular geometric positions do mean that all transits are six months apart - in either early-May or early to mid-November. Average transit dates of the event are either the 8th May or 10th November, and these can vary only by one or two days around these dates.
Three of the last transits happened on 15th November 1999, 07th May 2003 and 08th November 2006. From Australasia, the May 2003 was not a full transit across the solar disk, and only the initial ingress contact was seen in the mid-afternoon around 15h 14m A.E.S.T. (05 U.T.) Observers throughout most of Asia, Russia, Europe and Africa were able to see both ingress and egress events. On the 08th November 2006 the last Mercury transit was able to be seen from New Zealand and the eastern coast of Australia, yet was invisible in Europe and much of North America. The next observable transit will be on 10th May 2016, but will not be seen again from the Australian shores until 13th November 2032.
Mercury in 2003 glanced only near the edges of the Sun, whose contact enters at the mean position angles of 14.9oand exiting at 291.7o. The average of the entire transit event will take 05h 18m 50.2seconds. The least possible angular distance is 11′ 48″ - roughly one-third of the solar diameter. This mid-event was invisible from everywhere Australia eastward of an imaginary line roughly between Darwin and Perth. Ingress and egress are divided into two phases - being the exterior (I and II) and interior (III and IV) contacts of each side of the Mercurian disk. The time it takes to enter the solar disk is about 04 minutes 27 seconds, and at the mid-time, it will seem as if a smallish bite has been taken out of the Sun. The whole transit takes around 5 hours, which averages between 1600 AD and 2300 AD as 04 hours 53 minutes.
Slight differences in the placement of Mercury’s disk occur with different latitudes on Earth. In this case the difference will not be much amounting only to a few minutes of time. Interestingly during the last transit in 1999, there was a significant difference where you were placed on the Earth, as Mercury only just grazed the solar limb.
Its general appearance appears as a small blackened disk, similar to like a small black dot on a piece of paper, subtending some 12.0 arcsec across. If Mercury’s orbit passes quite near aphelion, this makes Mercury to be near its greatest maximum size. To the eye, at least via solar telescopic projection, it does not look like a sunspot because the planet is quite inky black and too round. Some general motion will be detectable in only a few minutes when compared to some solar surface feature like a sunspot, who movement can only be really detected daily. he relative size of Mercury compared to the Sun is about two-hundredths the diameter.
Most recommend that the best telescopic magnification is about 80× to 120×, with higher magnifications often limited because of the general poor seeing conditions. Of course, observers should always heed the important warning below;
The only real serious scientific endeavour is in timing the contact events with the solar limb. This is not as easy as it seems because of the effects of atmospheric seeing and when the events start are subject to considerable uncertainty. Some have recommended doing timings by Hydrogen-alpha (Hα) filters rather than ordinary projected white-light. This is because prominences can be placed to backdrop Mercury’s disk prior to the contact points, thus increasing the observational accuracy. The U.S. Naval Observatory would likely be interested in observation, requiring the exact geographical positions taken from a topographic map, and including some estimation of the error in the timing, method of timing and observation.
All November transits can recur over intervals of 7, 13, or 46 years, however, the May transits recur in either 13 or 46 year periods. These variations are caused by the differing relative motions of Mercury during the times of either aphelion or perihelion. Overall, the November perihelion transits are longer but do show Mercury with the smaller 10 arcsec disk. All the May aphelions are generally shorter in length but display a larger Mercurian disk up to 12 arcsec. Furthermore, the May transits of Mercury are 1.9 times less likely than November ones.
Between 1600AD and 2300AD, of the ninety-five transits predicted to occur, only twenty-four are May transits and the other seventy-one are November transits. Ie. 34%. Regarding the height differences between the different transits, similar transit circumstances recur once every 217 years.
********************************** Time Time Cord Date (AEST) (UT) Size (″) ********************************** 09 May 1970 22:16 08:16 114 10 Nov 1973 00:32 10:32 026 13 Nov 1986 18:07* 04:07 471 06 Nov 1993 19:57* 03:57 927 15 Nov 1999 11:41 21:41 963 07 May 2003 17:52* 07:52 708 09 Nov 2006 07:41 21:41 423 10 May 2016 00:57 16:57 319 12 Nov 2019 01:20 15:20 076 13 Nov 2032 18:54* 08:54 572 07 Nov 2039 20:46 10:46 822 08 May 2049 00:24 14:24 512 09 Nov 2052 14:30* 02:30 319 10 May 2062 11:37 21:37 521 11 Nov 2065 10:07 20:07 181 14 Nov 2078 03:42 13:42 674 07 Nov 2085 03:36 13:36 718 08 May 2095 11:08 21:08 310 10 Nov 2098 21:18* 07:18 215 ********************************** * Previous Day
According to the useful table produced by Fred Espenak, the longest Mercury transit last occurred on 15th May 1707 with the long duration of 07 hours 56 minutes - passing within 64.5 arcsec of the very centre of the Sun. The next transit lasting above 07 hours will happen on 09th May 2016, but is totally invisible from Australasia. The shortest of these previous transitd during in this period occurred in 11th May 1937 and transited in only thirteen (13) short minutes.
According to Jean Meeus, the shortest transit time in the future for Mercury in the same time-frame of 1600AD to 2300AD will occur on 18th November 2216AD. Then the transit is only partial and the transit’s short duration will only be about twenty (20) minutes. Only contacts I and IV will occur! The widest transit occurs in 12 November 2190 and will crosses only 9 arcsec from the Sun’s centre. This latter event shall also be one of the longest of any of the May transits in this 700-year period - taking fractionally over nine (9) hours to complete thr transit.
1. Espenak, F., “Transit of Mercury : 07th November 1999”., NASA Website (13 Aug 1998)
2. Meeus, J., “Transits of Mercury, 1920 to 2080.”, J.BAA;, 67, 30 (1956)
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