It was supposed that we were already pretty accurately acquainted with the distance of the moon from the earth, it being so much nearer to us, by observing its parallax, or the difference of its place in the heavens as seen from the surface of the earth, from that in which it would appear if seen from its centre(51). But the parallax of the sun is so exceedingly small, as scarcely to afford the basis of a mathematical calculation(52). The parallax of Venus is however almost four times as great as that of the sun; and there must therefore be a very sensible difference between the times in which Venus may be seen passing over the sun from different parts of the earth. It was on this account apprehended, that the parallax of the sun, by means of observations taken from different places at the time of the transit of Venus in 1761 and 1769, might be ascertained with a great degree of precision(53). (51) Bonnycastle, Astronomy, 7th edition, p. 262, et seq. (52) Ibid, p. 268. (53) Phil. Transactions, Vol. XXIX, p. 457. But the imperfectness of our instruments and means of observation have no small tendency to baffle the ambition of man in these curious investigations. "The true quantity of the moon's parallax," says Bonnycastle, "cannot be accurately determined by the methods ordinarily resorted to, on account of the varying declination of the moon, and the inconstancy of the horizontal refractions, which are perpetually changing according to the state the atmosphere is in at the time. For the moon continues but for a short time in the equinoctial, and the refraction at a mean rate elevates her apparent place near the horizon, half as much as her parallax depresses it(54)." (54) Astronomy, p. 265. "It is well known that the parallax of the sun can never exceed nine seconds, or the four-hundredth part of a degree(55)." "Observations," says Halley, "made upon the vibrations of a pendulum, to determine these exceedingly small angles, are not sufficiently accurate to be depended upon; for by this method of ascertaining the parallax, it will sometimes come out to be nothing, or even negative; that is, the distance will either be infinite, or greater than infinite, which is absurd. And, to confess the truth, it is hardly possible for a person to distinguish seconds with certainty by any instruments, however skilfully they may be made; and therefore it is not to be wondered at, that the excessive nicety of this matter should have eluded the many ingenious endeavours of the most able opetators."(56). (55) Ibid, p. 268. (56) Phil. Transactions, Vol. XXIX, p. 456.