HISTORY OF PHYSICAL ASTRONOMY.
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to induce astronomers generally to observe the phenomenon. He did notfail, however, to acquaint his friend Crabtree, who lived in the neighbour-hood of Manchester , with the approaching transit, and accordingly thesetwo individuals made suitable preparations, each at his own residence, toobserve its occurrence.
Horrocks employed a mode of observation similar to that practised byGassendi on the occasion of the transit of Mercury in 1631, as alreadyexplained. He divided the diameter of the white circle upon which horeceived the image of the sun into 30 parts, and each of these into 4smaller parts, so that by supposing the apparent diameter of the sun tobe 30', each of the more minute subdivisions would represent an apparentmagnitude of 15". After rectifying the motion of the planet, he foundthat its conjunction would not take place before three o'clock in the after-noon of the 24tli of November. However, as all the tables of the planetindicated the conjunction to be earlier—some of them even made it to takeplace on the 23rd—he did not consider it prudent to trust too implicitlyto his own calculations. He therefore did not omit to examine attentivelythe image of the sun from time to time on the 23rd.
On the 24th he continued to observe the solar image from sunrise tillthe hour appointed forgoing to church*. During all this time, he sawnothing upon the sun except an ordinary spot of small dimensions whichhe had noticed on the preceding days, and which could not, therefore, beVenus. At a quarter past three o’clock in the afternoon, as soon as hewas again at leisure, he proceeded to resume his observations. “ At thistime,” says he, “ an opening in the clouds, which rendered the sun dis-tictly visible, seemed as if divine Providence encouraged my aspirations,when, Oh most gratifying spectacle! the object of so many earnest wishes j,I perceived a new spot of unusual magnitude, and of a perfectly round form,that had just wholly entered upon the left limb of the sun, so that the mar-gins of the sun and the spot coincided with each other, forming the angle ofcontact .” The planet, in fact, had then entered upon the eastern limb ofthe sun, at the distance of 62° 30' from the lower extremity of the verticaldiameter of his disk. By a careful comparison of the relative magnitudesof the image and the round spot visible upon it, he concluded theirdiameters to be in the proportion of 30 to l-J- or 1-j- at the utmost. Theapparent diameter of the planet did not therefore exceed 1' 12".
Owing to the near approach of sunset, Horrocks was unable to observethe planet longer than half an hour. During this brief interval, he mea-sured its distance from the sun three different times. His observationsof the phenomenon were made at Hoole, a small village in Lancashire ,about fifteen miles to the north of Liverpool.
Crabtree, who resided at Broughton near Manchester , had made ar-rangements for observing the transit similar to those employed by Hor-rocks. The sky, however, unfortunately continued overcast during thewhole day, and he had abandoned all hopes of witnessing the phenomenon,when, just a little before disappearing below the horizon, the sun burstthrough the clouds. Repairing immediately to the chamber in which hehad made preparations to receive the image of the sun, he beheld, to his
* The words of Horrocks on this occasion, “ ad majora avocatus qvee utique ob litreparerga negligi non decuit,” manifestly have reference to religious duties, and this in-ference is further confirmed by the fact that the 24th of November, 1639, fell uponSunday, as will be apparent to any person by a slight computation.
f “ Ecce gratissimum spectaculum et tot votorum materiem!”