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this month, in the morning, the planet Mercury will cross the disc of the sun, and appear on it as a circular black spot for nearly seven hours; this celestial phenomenon will be visible from its commencement to its termination to the whole of Europe, and a great part of Africa; the ingress will be visible to Asia, and the egress to America.
There is no doubt but that every lover of the science of astronomy, within the limits of the visibility of the transit, will endeavour to witness the spectacle,--to see this bright and beautiful gem, that shines with such a rosy brilliancy as the morning or evening star, now melting away in the full effulgence of the rising day, and then heralding the bright hosts of stars to glitter on the midnight sky,- to see this lovely jewel of the ruddy dawn, or evening shades, enter on the sun's glowing orb, with not merely dimmed splendor, but shrouded in intensest blackness, pursuing its course over a field of glory, yet clad in gloom; such a phenomenon will not fail to interest, and the observer, as he marks the blackness of the planet, in contrast with the solar effulgence, may apply the celebrated line to the messenger of the gods:
“ Dark with excessive light his robes appear.” The following are the particulars of this interesting phenomenon :
Apparent time. Mean solar time. External ingress, or beginning of the Transit
9 2 57 8 59 28 Internal ingress
9 6 17 9 2 48 Time of Mercury's nearest approach. 12 28 50 12 25 20 Internal egress.
3 47 41 External egress, or end of the Transit, 354 31 3 51 Duration of the Transit., 6h. 51m. 34s.
The following diagram will illustrate the points of ingress, nearest approach, and egress of the planet :
Transit of Mercury over the Sun, 5th May, 1832.
In observing the transit, the same course may be pursued with Mercury that has been recommended for the planet Venus,-ti at previous to the time of the beginning of the transit, the observer should have bis telescope properly fixed, and prepared with dark glasses to defend the eye, which he should keep fixed upon that point of the sun's limb where the planet is expected to
enter; at the instant he suspects the contact to take place, he must note the time, and proceed to observe, in order to be certain that he was not mistaken. If he find that he was mistaken, he must continue to wait for it, always noting the time when he suspects it, in order that he may not miss it when it really happens; having entered on the sun's disc, wait for the internal contact and note the time; the same method is to be pursued for the internal and external contact when Mercury passes off the disc.
At the transit of Venus, in 1761, some singular phenomena were observed. At Madras, a kind of penumbra was noticed, preceding the first external contact two or three seconds of time ; this dusky shade was so remarkable, that the observer was assured that the contact was near, which happened accordingly. In the transit of 1769, Dr. Maskelyne was very attentive to observe if this circumstance took place, but he could perceive no such effect. When Venus was a little more than half immersed into the sun's disc, he saw its whole circumference completed, by means of a vivid, but narrow, ill-defined border of light, which illuminated that part of its circumference that was off the sun ; but this disappeared about two or three minutes before the internal contact. In the same transit, an observer had warning of the approach of Venus to the contact, by the sudden appearance of a violent corruscation, ebullition, or agitation, of the upper edge of the sun, five or six minutes before the limb of Venus broke in upon the sun; this, it was supposed, might have been owing to the atmosphere of Venus. Some perceived at the first external contact a kind of watery pointed shadow, appearing to give a tremulous motion to that part of the sun's limb. Some astronomers observed a luminous crescent at the times of the ingress and egress which enlightened that part of Venus's circumference, which was off the sun, so that the whole circumference was visible. At the internal contact the limb of Venus seemed, to most observers, to be united to the sun's limb by a black protuberance, or ligament, which was not broken by the thread of light till some seconds after the regular circumference of Venus seemed to have coincided with the sun's. Others observed that the thread of light between the limbs did not break instantaneously,—the points of the threads darting into each other, and parting again, in a quivering manner, several times before they finally adhered.
The last transit of Mercury occurred 4th November, 1822, but invisible in this country: it was witnessed in India, and an account transmitted to Dr. Olinthus Gregory, by Major J. A. Hodgson, Revenue SurveyorGeneral of India ; the communication was inserted in the Memoirs of the Royal Astronomical Society of London, and is as follows:
Russell Street, Chouringhy, Calcutta,
4th November, 1822. The sun was frequently obscured by flying clouds at the commencement of the transit, that I thought it safest to use a large field of view until I saw the planet enter the sun. A cloud prevented my seeing the first external contact; I observed the first internal contact, but I cannot be certain to less than four or five seconds, as it was rather cloudy, and a power of 45 in the 42-inch telescope is not great.
At the end of the transit I used the 60-inch telescope, but reduced the aperture of 3.7 to 2.5, as the sun's limb is better
defined with a small than a large aperture. The magnifying power applied was 60. It would have been more desirable to use a greater magnifying power; but as clouds were flying about occasionally, and partially obscuring the sun, I was afraid of the inconveniences attending a small field.
The observations of the preceding and following limbs of the planet, when leaving the sun, were made under favorable circumstances, and I am well satisfied with them, particularly with that of the following limb. No colored glass was used, the light of the sun was so faint that the eye could bear it; its limb appeared white and sharp, and the planet was dark, round, and well defined. I could not see it with the telescopes of the sextant, or reflecting circles, nor with a small Brewster's double object glass micrometrical telescope, though it showed the limbs of the sun perfectly well defined. The most important observations are these :
h. Internal contact.-Ingress or following limb 18 56 16.38 Ditto.
21 38 34.11 External ditto.
21 40 56.11 Mean time:-Estimated longitude
5 53 10 Captain Herbert did not see the beginning of the transit. He was at the same place and observed the
h. Internal contact at egress.
21 38 42.11 External ditto.
21 40 55.11 using the 42 inch telescope with a power of 80, and no colored shade to the eye piece. He also took several micrometrical measurements with a Dolland's parallel wire micrometer applied to the same telescope. The astronomical clock was by Earnshaw, No. 14.
The transit was observed at the cantonment of Kurnaul, latitude north 29° 41' 25", estimated longitude 5h. 8m. Is. by Walter Ewer, Esq. ; telescope by Dolland, 5 feet focal length, power about 100 ; dark glasses used in the eye piece.
h. Mean time of 2nd internal contact
20 53 46.5 Ditto. external ditto.
20 56 16.5 J. A. HODGSON.