A Treatise on Surveying Volume 2 (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. 1911 Excerpt: ...Z P S depends to a great extent upon the side Z S, for, of the other sides, P Z is constant, and P S varies only slightly with the change of declination, during the time taken in making an observation. Now Z S is the complement of the altitude, and thus in a position of the heavenly body, such as is shown in the figure, the hour angle changes in accordance with the changes of the altitude. Obviously the most favourable period to observe altitudes for time will be when they are changing as rapidly as possible, i.e., when for a given change ot altitude, the corresponding alteration in time, is as small as possible. Now the altitude is changing fastest (and at the same rate for all stars observed from the same spot), when the star is on the prime vertical, hence, this is the best position in which to observe it. It is sometimes convenient to be able to compute the altitude of a star when it crosses the prime vertical. This can be done from the following formula, s'n 5 ( Vide page 128.) Altitude of Stars on the Prime Vertical. Limit of Accuracy in Time Deter-minations. Time should be found by either of the methods hereafter described to an accuracy of less than one second of time, and may be applied for correction of the chronometer or whatever timekeeper the observer is using. Instead of altering the chronometer it is generally sufficient to record its error (or correction), which for any instant is the difference between the time it shows and the time it should show, this error being styled fast when it is ahead of the proper time, and slow when it is behind it. The error of a chronometer is generally recorded for some convenient epoch--such as mean noon--on a given day, i.e., at the commencement of a given astronomical day. The 'correction' is that which sho...