Page:South - the story of Shackleton's last expedition, 1914-1917.djvu/483

Rh comparison of this time with Greenwich time, as shown by the ship's chronometer, an accurate knowledge of the errors and rate of the chronometer being required. During the voyage of the 'Endurance' about fifteen months elapsed during which no check on the chronometers could be obtained by the observation of known land, and had no other check been applied there would have been the probability of large errors in the longitudes. For the purpose of checking the chronometers a number of observations of occultations were observed during the winter of 1915. An occultation is really the eclipse of a star by the moon. A number of such eclipses occur monthly, and are tabulated in the Nautical Almanac. From the data given there it is possible to compute the Greenwich time at which the phenomenon ought to occur for an observer situated at any place on the earth, provided his position is known within a few miles, which will always be the case. The time of disappearance of the star by the chronometer to be corrected is noted. The actual Greenwich time of the occurrence is calculated, and the error of the chronometer is thus determined. With ordinary care the chronometer error can be determined in this way to within a few seconds, which is accurate enough for purposes of navigation. The principal difficulties of this method lie in the fact that comparatively few occultations occur, and those which do occur are usually of stars of the fifth magnitude or lower. In the Antarctic, conditions for observing occultation are rather favourable during the winter, since, fifth-magnitude stars can be seen with a small telescope at any time during the twenty-four hours if the sky is clear, and the moon is also often above the horizon for a large fraction of the time. In the summer, however, the method is quite impossible, since, for some months, stars are not to be seen. No chronometer check could be applied until June 1915. On June 24 a series of four occultations were observed; and the results of the observations showed an error in longitude of a whole degree. In July, August, and September further occultations were observed, and a fairly reliable rate was worked out for the chronometers and watches. After the crushing of the ship on October 27, 1915, no further occultations were observed, but the calculated rates for the watches were employed, and the longitude deduced, using these rates on March 23, 1916, was only about 10´ of arc in error, judging by the observations of Joinville Land made on that day. It is thus fairly certain that no large error can have been made in the determination of the position of the Endurance at any time during the drift, and her course can be taken as known with greater certainty than is usually the case in a voyage of such length.