Page:Popular Astronomy - Airy - 1881.djvu/187

Rh whole, and therefore, the result for the sun's distance will be more accurate. I also mentioned that, in the transit of 1769, Wardhoe and other places in Lapland answered very well to the former of these conditions, and Otaheite and other places in the Pacific answered well to the latter; and that these in fact were the places of observation upon which the measure of the sun's distance principally depends.

I may now mention that, although the principles of the method are stated with most perfect correctness in the explanation given above, and although the process must be thus contemplated by an astronomer, in order to enable him to select stations in the most advantageous positions, yet an astronomer's calculation is not made in that form. His calculation is made entirely by the method of parallax. The process, strictly speaking, is algebraical; but it may be correctly described in the following manner. He assumes a certain value in seconds for the sun's horizontal equatoreal parallax; then from the known proportion of the distances of Venus and the sun, he computes the horizontal equatoreal parallax of Venus (the parallax being greater as the distance is less). Thus, if the sun's horizontal equatoreal parallax be assumed at ten seconds, and if it be known that at that time the distances of Venus and the sun from the earth are in the proportion of 28 to 100, then he must take the horizontal equatoreal parallax of Venus at thirty-five seconds and five-sevenths. Then, from knowing the earth's form, he computes the horizontal parallax of each at the place of observation; and then from knowing the apparent elevation of the sun and Venus, he calculates the actual parallax of each at the time of observation, that is, how much each of them is apparently depressed by parallax. Venus being