Modern Astronomy; Being Some Account of the Revolution of the Last Quarter of a Century (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1901. Excerpt: ... massive piers--the gun-carriage is to be fixed as rigidly as possible. Attempts to remove this limitation have generally failed; and a conspicuous instance of this is to be found in the fact that Airy, who designed a perfectly satisfactory transit-circle, was unsuccessful in his altazimuth. The need for this latter instrument arose in this way: --there is a special importance attaching to observations of the Moon's place, as will presently be explained. Now the transit-circle, with its limited opportunities, did not get enough observations: it was for instance an aggravating, but in our erratic climate too frequent experience for the Moon to shine brightly until the time came for making the observation with the transit-circle, and then for clouds to come up and obscure her, only to roll away again when the opportunity was gone for that day. More than this, in the first and last quarters the Moon is so near the Sun that the time for the meridian (or transit-circle) observation is too near noon, and the Moon cannot be seen owing to the brightness of the surrounding sky. When the Sun has set and the Moon is shining against a dark sky, she is away in the west quite out of range of the transit-circle. Hence Airy made the attempt to set up an altazimuth, which may be compared to a gun capable of being elevated at any angle as before, with a carriage which could be swung round to any point of the compass, the exact compass-point or "azimuth" being read off on a carefully divided horizontal circle. With this instrument the Moon could be observed at any time, if she would only show for a brief interval; but, oh the cost to the observer on a fitful night With the transit-circle there is disappointment enough for him when he comes to the instrument at the proper ti...