Historical Transits of Mercury

One of the first valuable avenues of astronomical research with the inner planets was to use transits to determine the value astronomical unit (AU), as to, in turn, finding the true scale of the Solar System. During the expedition of Lt. James Cook, he famously observed the Transit of Venus on 3rd June 1768, but unbeknown to most, Cook also observed a transit of Mercury on 10th November 1769 from New Zealand for the use of the same purpose. (Also see article The Dawn of Australian Astronomy for further discussion.)

Later, serious discussion of these transits was made by French astronomer Urbain Jean Joseph Le Verrier (1811-1877) during mid-1800s, who used the perturbations of Mercury as a means of determining the mass of Venus I.e. Williams (1938) He used observations from twelve Mercury transits between 1697 and 1832, and organised new timings for the transits of 1845 and 1848 appearing in Annales de l’Observatoire de Paris, 5 (1859).

Using these results, were also paramount in better deriving an improved set of orbital elements for much needed corrections for its planetary motion. In the 15th through to the early 18th Century, predicting the position of Mercury in the sky was always a significant problem. The main issue was trying to obtain accurate astrometric positions when the planet was near the horizon, whose placement was hindered by the solar proximity, but also issues like atmospheric refraction and the murky twilight an uneven horizon. Infrequent observations along with measures made only during transits or maximum elongations meant the orbit elements for Mercury had gross errors, and hence, poor predictions with ephemerides that could be out by an intolerable ½°. This also had consequences predicting future times of transits.

Le Verrier later published small book entitled “Théorie du Mouvement de Mercure” (1845), which discussed this problem directly and in much better detail, leading to a transit prediction to about one second. From this work, it was later found an unknown ambiguity with the precession of the perihelion of Mercury existed, that was eventually explained by Einstein’s Theory of Relativity. Le Verrier found that motion precesses perihelia by 38.3 arcsec; per century, but he was unable to explain the cause. By the 1870s, Simon Newcomb reinvestigated the problem and agreed with this disparity, finding in 1898 that perihelion did increased by 43.37 arcsec per century. Debate continued on the veracity of the accuracy of these earlier transits, and this appeared in a paper by R.T.A. Innes.

Recent Transits of Mercury

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 from around 15h 14m A.E.S.T. (05 U.T.) Observers throughout much of Asia, Russia, Europe and Africa were able to see both ingress and egress events. On 08th November 2006 the last Mercury transit could be seen from New Zealand and the eastern coast of Australia, yet was invisible in Europe and much of North America. The next locally observable transit will be on 10th May 2016, but will not be seen again from Australian shores until 13th November 2032.

Mercury in 2003 glanced only near the edges of the Sun, whose contact entered the mean position angles (P.A.) of 14.9° and exiting at 291.7°. Average duration of the entire transit event will take 05h 18m 50.2 second with the least possible angular distance being 11′ 48″ — roughly one-third 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 — 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, such differences only amount to several minutes. Interestingly, during the last 1999 transit there was significant differences where you were placed on Earth. Then Mercury only just grazed the solar limb.

Appearance and Observing Mercury Transits

Transits general appear like a small-blackened disk similar to one small black dot on a piece of paper. When the orbit of Mercury passes near aphelion, this makes the planet near its observed greatest maximum apparent size, which subtending about 12.0 arcsec across. By eye, at least by the safest observational method of solar telescopic projection, the planet does not look like any sunspot because it is quite inky black and obviously too symmetrically round and without the usual sunspot area of an outer penumbra.

Although motion of this event is fairly sluggish, some general movement will be detectable over some minutes when compared to other solar feature like nearby sunspots, whose own progress across the solar disk can only be really detected daily. Its relative small size of Mercury compared to the Sun is about two-hundredths its diameter.

Most recommend that the best telescopic magnification should be about 80× to 120×. Higher magnifications often are limited because of the general poor seeing conditions. Of course, observers should always heed the important warning given below;

!! WARNING !!

NEVER POINT ANY TELESCOPES TOWARDS the SUN UNLESS
SUITABLY FILTER PROTECTED FROM THE SOLAR RAYS.

It is strongly recommended that if is observed in daylight hours
MERCURY TRANSITS should ONLY be observed by
projecting the image of the SUN.

One of the only real serious scientific endeavours is knowing the contact events timings at the solar limb edges. This may not be easy as it seems because of the effects of atmospheric seeing and when the events start are subject to considerable uncertainty. Worse is, unlike Venus, the size of the apparent disk is small, requiring a decent aperture above 20cm. or so. Some, because of this, have recommended making timings in Hydrogen-alpha (Hα) filters rather than ordinary projected white-light. This is because prominences can be focussed on or placed to backdrop Mercury’s disk before to the contact points, thus increasing the observational accuracy. Perhaps the U.S. Naval Observatory would likely be interested in such observations. If they did, they would require the exact known geographical position that have been taken from some accurate topographic or even by way of Google maps. Also need would be the times, timing method, estimation of observational errors, and the observation conditions.

OCCURRENCES and FUTURE TRANSITS

All November transits can recur over intervals of seven, thirteen, or forty-six years, but May transits recur in either thirteen, or forty-six years. These differing periods are caused by the relative motions of Mercury during either aphelion or perihelion. Overall, November perihelion transits are longer, though Mercury has a smaller 10 arcsec disk. All May aphelions are generally shorter but have slightly larger Mercurian disk of up to 12 arcsec. Furthermore, the May transits of Mercury are 1.9 times less likely than November ones.

Between 1600 AD and 2300 AD, of the predicted ninety-five transits, only twenty-four are May transits and the other seventy-one are November transits. I.e. 34%. Regarding the height differences between the different transits, similar transit circumstances recur once every 217 years.

TRANSITS of MERCURY : 1970 and 2100 A.D.

NOTES

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 observed centre of the Sun. One of the next lengthy transit lasting above 07 hours will happen on 09th May 2016, but is totally invisible from Australasia. Shortest of these previous transits during in this period occurred in 11th May 1937 and transited in only thirteen short minutes.

Jean Meeus has calculated the shortest Mercury transit time in the future within the same time frame of 1600 AD to 2300 AD will occur on 18th November 2216AD. This transit is only partial and of short duration of only be about twenty minutes. Only contacts I and IV will occur! Widest of all the transits in this periods occurs in 12 November 2190 AD and will crosses only 9 arcsec from the centre of the Sun. 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 hours to complete the entire transit.

References

1. Espenak, F., “Transit of Mercury : 07th November 1999&”
2. Meeus, J., “Transits of Mercury, 1920 to 2080.”, J.BAA., 67, 30 (1956)
3. Orchiston, W., “Transits of Venus: New Views of the Solar System and Galaxy Proceedings”, IAU Coll., 196, 52 (2004)
4. Williams, K.P., “The Transits of Venus”1, 4, Pub. Kirkwood Observatory, University of Indiana (1939)

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Last Update : 22nd September 2019

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