The Astronomical Ephemeris Volume 1842 (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. 1838 Excerpt: ...represent its motion, is denominated mean time. We cannot immediately obtain mean time from observation; but, from an observation of the true Sun, with the aid of the equation of time, which is the angular distance in time between the mean and the true Sun, we may readily deduce it. Suppose the true Sun to be observed on the meridian of Greenwich, Jan. 1, 1842; it would then be apparent noon at that meridian; the equation of time at this instant apparent time"; hence it appears that the corresponding mean time is 0h 3m 5 l, -02, or that the mean Sun had passed the meridian-previously to the true Sun, and that at the instant of observation the mean time clock or chronometer ought to indicate this time. A mere inspection of the columns of the Ephemeris is, of itself, sufficient to show that the quantities are continually varying, and that some reduction is necessary where data are to be obtained for any time differing from that for which the quantities are registered. Take, for instance, the Suns Right Ascension on Page II. of the month of January; on January I, it is 18h 46m 45'-08; on January 2, it is 18b 51m 9-85; in the course of 24 mean hours it has therefore increased by 4m 24, -77-Ifj then, the Right Ascension were required for any time between the Mean Noons of January 1 and 2, as at 6h from Mean Noon of January 1, it would be necessary to increase the Right Ascension on January 1, by the proportional part of the daily increase due for the 6h, viz. by one-fourth part, or lm 6 19. This would in all cases be required, even under the meridian of Greenwich, for which the quantities have been specially computed. Let a person be now supposed to be under a meridian 15 West of Greenwich. The positions of the heavenly bodies, as referred to the centre of the...