HISTORY OF PHYSICAL ASTRONOMY.
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transit of the same planet that was actually observed, happened on Novem-ber 3, 1651. On'this occasion also the planet was passing through itsascending node * * * § . The phenomenon was observed at Surat in India , byShakerley, a young Englishman, who, having found by calculation that itwould be visible only in Asia, proceeded to India for the express purposeof witnessing its occurrence f.
The third transit of Mercury recorded by astronomers, happened onthe 3rd of May, 1661. Hevelius , who observed it at Dantzic, was asto-nished to find that the apparent diameter of the planet was so small.We have seen that Gassendi estimated it to be equal to 20"; Hevelius made its greatest value to amount only to 12", and the mean value to0" 3"'|. These results agree very nearly with the modern determinationsof the apparent magnitude of the planet. The same transit was observedat London by Huyghens, Street, and Mercator §.
The fourth transit of Mercury that is recorded to have been observed,was that of November 7, 1677. It was witnessed by Halley at St.Helena , where he was then residing, as well as by several persons inEurope . Halley was the first who observed both the ingress and egressof the planet.
The transits of Mercury that have been subsequently observed, arechiefly interesting on account of the indications they afford of the accuracyof the existing tables of the planet. With respect to the physical pheno-mena accompanying such occurrences, they are*not of such importance asmight be expected. It is only on a few rare occasions, indeed, that anypeculiarity worthy notice has been seen. In each of such instances,the phenomenon is manifestly referrible to some optical illusion. A briefdescription of the appearances observed during two or three transits of theplanet will serve to confirm the truth of this remark.
In an account of the transit of Mercury which happened on the 6tli ofMay, 1753, M. De Barros, a Portuguese , who witnessed the phenomenonat Paris, states that when the planet was about to leave the sun’s disk, heperceived the interior contact through a green glass placed before thesmoked glass of his telescope (which was an excellent Gregorian reflector, fourfeet long), and that immediately afterwards, viewing the planet through thesmoked glass alone, he found that a small thread of light was still visiblebetween the limbs of the two bodies. A second interior contact of theplanet was therefore observed at the commencement of the egress, whichdid not take place till four seconds after the first contact. A similar ap-pearance presented itself at the completion of the egress. The exteriorcontact was first observed by means of the two glasses; but upon removingthe green glass, the planet was again brought upon the sun’s disk, and didnot go off until six or seven seconds afterwards [|.
The phenomena above referred to are, in all probability, connected withthe irradiation of light. It has been already mentioned that the colour of
* The transits of Mercury happen necessarily, from the heliocentric position of itsnodes, always in May or November. When the transit takes place in May, the planet ispassing through the descending node of its orbit. The occurrence of the transit inNovember indicates the passage of the planet through the ascending node.
+ Wing, Astronomic/. Britannica, p. 312.
j “ Mercurius in Sole Visus.” Gedan, 1662, p. 92.
§ Wing, Astronomia Britannica, p. 312. This transit, happened on the day of thecoronation of Charles II. The observations of Huyghens and his companions are said byWing to have been made at Long Acre, with a telescope of excellent workmanship.
[1 Phil. Trans., 1753, p. 362.
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