Handbook of Astronomy (Paperback)

Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: with great precision the proportion in which the wire divides the distance between the points s and s', and can therefore determine the fraction of a second after being at s, at which it was upon the wire m rri'. Although the art of constructing chronometers has attained a surprising degree of perfection, it is not perfect, and the Bate of even the best of such instruments is not absolutely uniform. It is therefore necessary from time to time to check the indications of the clock by observing its rate. If the clock were absolutely perfect, the pendulum would perform exactly 86,400 vibrations in the interval between two successive returns of the same star to the meridian. Now a good astronomical clock will seldom make so many as 86,401 nor so few as 86,399 vibrations in the interval. In the one case its rate would he too fast, and in the other too slow by I in 86,400. Even with such an erroneous rate the error thrown upon an observation of one hour would not exceed the 24th part of a second. If, however, the rate be observed, even this error may be allowed for, and no other will remain save the remote possibility of a change of rate since the rate was last ascertained. 23. The transit instrument.?All the most important astronomical observations are made at the moment when the objects observed are upon the celestial meridian, and in a very extensive class of such observations the sole purpose of the observer is to determine with precision the time when the object is brought to the meridian by the apparent diurnal motion of the firmament. This phenomenon of passing the meridian is called the Transit; and an instrument mounted in such a manner as to enable an observer, supplied with a clock, to ascertain the exact time of the Transit is called a Transit Instrument. Such a...