Page:Popular Astronomy - Airy - 1881.djvu/154

140 degrees, and therefore we have the means of computing the length of GM or CM expressed in reference to our yard measure; that is, of ascertaining how far off the moon is from the earth as expressed by a yard measure.

In the middle of the last century, the celebrated French Astronomer, the Abbé de la Caille, was sent to the Cape of Good Hope to make the requisite observations; observations were also made at the same time at the Observatories at Paris and at Greenwich, to determine the angle spoken of. A few years ago, Mr. Henderson and Mr. Maclear, who were successively sent to the Cape of Good Hope, were partly employed in making observations for the same purpose; and from the observations also made at the same time at Greenwich and at Cambridge, the distance of the moon from the earth has been determined.

I will now mention the only failure likely to take place in consequence of pursuing the method which I have described. It is this: I pointed out to you in the first lecture that a correction for refraction is necessary for every observation made with the mural circle. In consequence of that refraction all objects, in every part of the heavens, appear higher than they really are; a correction is applied for that circumstance, but that correction may be liable to a small uncertainty. The angular distance of the line GM from the line GP, directed to the North Pole of the heavens, may therefore be slightly in error: we can come very near the truth indeed, and perhaps we should not be wrong a single second, or half a second—but still there is always some uncertainty; and I may say, that refraction is the very abomination of astronomers. In like manner there may be a small error in the angle MCP', and it may happen that the two errors