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that divides the hemispheres; and the places where they should join cannot be perceived so readily by those who are not well skilled in the nature of stereographical projections. The like may be said of many of the dotted curve-lines, on which are expressed the hours and minutes of the beginning or ending of the transit, which are the absolute times at these places through which the lines are drawn, computed to the meridian of London.
Containing an account of Mr. Horrox's observation of the transit of Venus over the Sun, in the
year 1639; as it is published in the Annual Register for the year 1761.
76. When Kepler first constructed his (the Ru. dolphine) tables upon the observations of Tycho, he soon became sensible that the planets Mercury and Venus would sometimes pass over the Sun's disc; and he predicted two transits of Venus, one for the year 1631, and the other for 1761, in a tract published at Leipsick in 1629, intitled, Ad. monitio ad Astronomos, &c. Kepler died some days before the transit in 1631, which he had predicted was to happen. Gassendi looked for it at Paris, but in vain (sie Mercurius in Sole visus, & Venus invisa). In fact, the imperfect state of the Rudolphine tables was the cause that the transit was expected in 1631, when none could be observed; and those very tables did not give reason to expect one in 1639, when one was really observed.
When our illustrious countryman Mr. Horrox first applied himself to astronomy, he computed ephemerides for several years, from Lansbergius's tables. After continuing his labours for some time, he was enabled to discover the imperfection of these tables; upon which he laid aside his work, intending to determine the positions of the stars from his own observations. But that the former part of his time spent in calculating from Lansbergius might not be thrown away, he made use of his ephemerides to point out to him the situations of the planets. Hence he foresaw when their conjunctions, their appulses to the fixed stars, and the most remarkable phenomena in the heavens would happen ; and prepared himself with the greatest care to observe them.
Hence he was encouraged to wait for the important observation of the transit of Venus in the year 1639; and no longer thought the former part of his time mispent, since his attention to Lansbergius's tables had enabled him to discover that the transit would certainly happen on the 24th of November. However, as these tables had so often deceived him, he was unwilling to rely on them entirely, but consulted other tables, and particularly those of Kepler : accordingly in a letter to his friend William Crabtree, of Manchester, dated Hool, October 26, 1639, he communicated his discovery to him, and earnestly desired him to make whatever observations he possibly could with his telescope, particularly to measure the diameter of the planet Venus; which, according to Kepler, would amount to 7 minutes of a degree, and according to Lansbergius to 11 minutes ; but which, according to his own proportion, he expected would hardly exceed one minute. He adds, that according to Kepler, the conjunction will be No. vember 24, 1639, at 8 hours 1 minute A. M. at Manchester, and that the planet's latitude would be 14' 10" south; but according to his own corrections he expected it to happen at 3 hours 57 min. P. M. at Manchester, with 10' south latitude. But be. cause a small alteration in Kepler's numbers would greatly alter the time of conjunction, and the quantity of the planet's latitude, he advises to watch the whole day, and even on the preceding afternoon, and the morning of the 25th, though he was entirely of opinion that the transit would happen on the 24th.
After having fully weighed and examined the se. veral methods of observing this uncommon phenomenon, he determined to transmit the Sun's image through a telescope into a dark chamber, rather than through a naked aperture, a method greatly commended by Kepler; for the Sun's image is not given sufficiently large and distinct by the latter, unless at a very great distance from the aperture, which the narrowness of his situation would not allow of; nor would Venus's diameter be well defined, unless the aperture were very small; whereas his telescope, which rendered the solar spots distinctly visible, would shew him Venus's diameter well defined, and enable him to divide the Sun's limb more accurately.
He described a circle on paper which nearly equalled six inches, the narrowness of the place not allowing a larger size; but even this size admitted divisions sufficiently accurate.
He divided the cir. cumference into 360 degrees, and the diameter into 30 equal parts, each of which was subdivided into 4, and the whole therefore into 120. The subdivision might have still been carried farther, but he trusted rather to the accuracy and niceness of his eye.
When the time of observation drew near, he adjusted the apparatus, and caused the Sun's distinct image exactly to fill the circle on the paper : and though he could not expect the planet to enter upon the Sun's disc before three o'clock in the afternoon of the 24th, from his own corrected numbers, upon which he chiefly relied; yet, because the calculations in general from other tables gave the time of conjunction much sooner, and some even on the 23d, he observed the Sun from the time of its rising till nine o'clock; and again, a little before ten, at noon, and at one in the afternoon; being called in the intervals to business of the highest moment, which he could not neglect. But in all these times he saw nothing on the Sun's face, except one small spot, which he had seen on the preceding day; and which also he afterward saw on some of the following days.
But at 3 hours 15 minutes in the afternoon, which was the first opportunity he had of repeating his observations, the clouds were entirely dispersed, and invited him to seize this favourable occasion, which seemed to be providentially thrown in his way; for he then beheld the most agreeable sight, a spot, which had been the object of his most sanguine wishes, of an unusual size, and of a perfectly circular shape, just wholly entered upon the Sun's disc on the left side: so that the limbs of the Sun and Venus perfectly coincided in every point of contact. He wasimmediately sensible that this spot was the planet Venus, and applied himself with the utinost care to prosecute bis observations.
And, First, with regard to the inclination, he found, by means of a diameter of the circle set perpendicular to the horizon, the plane of the circle being somewhat reclined on account of the Sun's altitude, that Venus had wholly entered upon the Sun's disc, at 3 hours 15 minutes, at about 62° 30' (certainly between 60° and 65°) from the vertex toward the right hand. (These were the appear. ances within the dark chamber, where the Sun's image and motion of the planet on it were both inverted and reversed.) And this inclination continued constant, at least to all sense, till he had finished the whole of his observation.
Secondly, The distances observed afterward be. tween the centres of the Sun and Venus were as follows: At 3 hours 15 minutes by the clock, the distance was 14' 24"'; at 3 hours 35 minutes, the distance was 13' 30"; at 3 hours 45 minutes, the distance was 18' 0". The apparent time of sun-setting was at 3 hours 50 minutes the true time 3 hours 15 minutes, refraction keeping the Sun above the horizon for the space of 5 minutes.
Thirdly, He found Venus's diameter, by repeated observations, to exceed a thirtieth part of the Sun's diameter, by a sixth, or at most a fifth subdivision. -The diameter therefore of the Sun to that of Ve. nus may be expressed, as 30 to 1.12. It certainly did not amount to 1.30, nor yet to 1.20. And this was found by observing Venus as well when near the Sun's limb, as when farther removed from it.
The place where this observation was made, was an obscure village called Hool, about 15 miles northward of Liverpool. The latitude of Liverpool had been often determined by Horrox to be 53° 20'; and therefore, that of Hool will be 53° 35'. The longitude of both seemed to him to be about 22° 30' from the Fortunate Islands : that is, 14° 15' to the west of Uraniburg:
These were all the observations which the shortness of the time allowed him to make upon this most remarkable and uncommon sight; all that could be done, however, in so small a space of time, he very
, happily executed; and scarce any thing farther remained for him to desire. In regard to the inclination alone, he could not obtain the utmost exactness; for it was extremely difficult, from the Sun's rapid motion, to observe it to any certainty within the de. gree. And he ingenuously confesses that he neither did, nor could possibly perform it. The rest are
. very much to be depended upon; and as exact as he could wish.
Mr. Crabtree, at Manchester, whom Mr. Hor. rox had desired to observe this transit, and who in mathematical knowledge was inferior to few, very readily complied with his friend's request; but the sky was very unfavourable to him, and he had only one sight of Venus on the Sun's disc, which was about 3 hours 35 minutes by the clock; the Sun then, for the first time, breaking out from the clouds: