As the ‘Endurance’ was carried by the ice-drift well to the west of the Weddell Sea, towards the position of the supposed Morrell Land, the accurate determination of longitude became a matter of moment in view of the controversy as to the existence of this land.  During a long voyage latitude can always be determined with about the same accuracy, the accuracy merely depending on the closeness with which altitudes can be measured.  In the case of longitude matters are rather different.  The usual method employed consists in the determination of the local time by astronomical observations, and the 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,