This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1922 edition. Excerpt: ...with the time of reception of signals emitted at a definite Greenwich time enables the error of the chronometer to be determined and the constancy or otherwise of its rate verified. (iii) Eclipses of the Satellites of Jupiter may be used to determine longitude, since they occur at the same instant for all observers and therefore provide a common reference signal. They also occur with sufficient frequency to be of use. Unfortunately, the disappearance of a satellite when eclipsed is gradual and not instantaneous as is the case when a star is occulted by the Moon; the accuracy obtainable by this means is therefore not very high. (iv) Observations of the Moon.--One of the oldest methods of determining longitudes is based upon the use of the Moon as a clock, and although the telegraphic method is now used almost exclusively, this method is not without interest. The Moon changes its place amongst the stars, and therefore also its right ascension and declination, much more rapidly than any other celestial object. Its position in the sky is given in the Nautical Almanac for every hour of Greenwich time throughout the year. If then the position of the Moon amongst the stars is observed and corrected for parallax, so as to reduce the observation to one made by an observer at the centre of the Earth, the Greenwich time of the observation can be estimated by interpolation from the Nautical Almanac tables, and a means provided for the determination of longitude. The disadvantage of the method is that the motion of the Moon amongst the stars is relatively slow, so that errors of observation enter into the deduced longitude magnified about thirty times. The observation of the Moon may consist either (i) in the determination of its right ascension at...