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In agreement with a nomenclature used in other astronomicalcalculations, one-half of this angle, or one such as the Earth ’sradius of 4000 miles would subtend at the Sun , is called theSolar Parallax. It will, therefore, he understood, that,whether we speak of the determination of the Sun ’s distance,or of the Solar Parallax, the same astronomical problem isinvolved.
In place of the surveying method, which is for the abovereason inapplicable in the case of the Sun , notwithstandingthat it is most useful and efficient when applied to terrestrialmeasurements, or even to determine the distance of the Moon from the Earth , other methods, less direct, but moreinteresting, are therefore employed. Of these, that whichdepends upon observations of transits of the planet Venus across the Sun ’s disc is the most generally known and themost full of popular and historical interest, although the estima-tion of its scientific value has, as we shall presently see, of latedecidedly diminished.
To two young Englishmen, Jeremiah Horrox , Curate ofHoole, Lancashire , and William Crabtree , a clothier , or draper,of Broughton, near Manchester, and especially to the former,whose early death cut short a career of the highest promise,belongs the credit of observing the first recorded transit of thisplanet , on November 24th, 1639 (O.S.) Horrox calculated thedate before its occurrence, and communicated it to his friend Crab-tree. These two enthusiastic students had for some time been inthe habit of corresponding with each other, and with SamuelFoster, who was afterwards Professor of Astronomy in Gresham College , and they arranged to observe the transit by a methodwhich may still be found useful by those who are not possessedof elaborate mechanical appliances, viz., by allowing the lightof the Sun to pass through a very small hole in a shutter,so as to form a distinct image upon a screen in a darkened room.Horrox tells us that at a quarter past 3 p.m., an opening inthe clouds rendered his observations successful. “ Oh, mostgratifying spectacle! ” he says, “ the object of so many earnestwishes. I perceived a new spot of unusual magnitude, and ofaperfectly round form, that had just entered upon the left limb