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exactly south you can turn it on the axis until the line of sight passes through the zenith, and still farther until it passes through the pole on the north horizon; but you cannot turn it east or west. This might seem to restrict its usefulness, but it is on this restriction of its motion that its usefulness depends. The great value of this instrument is that it enables us to determine the right ascension of a star without taking any measurement but one of time. In a former chapter we described sidereal time, the units of which are slightly shorter than those of our ordinary time, so that a sidereal clock gains about two hours every month on an ordinary clock. The sidereal time at which a star crosses the meridian is the same as its right ascension; the problem of determining the latter, therefore, is the simplest in the world. We start our sidereal clock, set it on the exact sidereal time, point the telescope of the meridian circle to various stars as they are about to cross the meridian, and note the exact moment at which each star passes. In the instrument the meridian is shown by a very fine fibre or spider’s web fixed in the focus of the telescope. The moment when the image of the star as seen in the telescope crosses this spider line is that of passing the meridian. The time by the sidereal clock then shows the star’s right ascension. If the clock could be set with perfect exactness and the instrument revolved exactly in the plane of the meridian, right ascensions would be determined in the very simple way we have described.

It unfortunately happens, however, that no clock can be set with such exactness as to satisfy the requirements