South! Ernest Shackleton (affordable ebook reader .TXT) 📖

star for a latitude, and an east star and a west star for a longitude. The refraction error will then usually mean out. This error affects observations both with the theodolite and the sextant, but in the case of the sextant another cause of error occurs. In using the sextant, the angle between the heavenly body and the visible horizon is measured directly. Even in dense pack-ice, if the observations are taken from the deck of the ship or from a hummock or a low berg, the apparent horizon is usually sharp enough for the purpose. In very cold weather, however, and particularly if there are open leads and pools between the observer and the horizon, there is frequently a great deal of mirage, and the visible horizon may be miraged up several minutes. This will reduce the altitude observed, and corrections on this account are practically impossible to apply. This error may be counterbalanced to some extent by pairing observations as described above, but it by no means follows that the mirage effect will be the same in the two directions. Then again, during the summer months, no stars will be visible, and observations for latitude will have to depend on a single noon sight of the sun. If the sun is visible at midnight its altitude will be too low for accurate observations, and in any case atmospheric conditions will be quite different from those prevailing at noon. In the Antarctic, therefore, conditions are peculiarly difficult for getting really accurate observations, and it is necessary to reduce the probability of error in a single observation as much as possible. When possible, observations of the altitude of a star or of the sun should be taken with the theodolite, since the altitude is referred to the spirit-level of the instrument, and is independent of any apparent horizon. During the drift of the Endurance both means of observation were generally employed. A comparison of the results showed an agreement between sextant and theodolite, within the errors of the instrument if the temperature was above about 20° F. At lower temperatures there were frequently discrepancies which could generally be attributed to the mirage effects described above.

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, 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.

South Atlantic Whales and Whaling

By Robert S. Clark, M.A., B. Sc., Lieut. R.N.V.R.

Modern whaling methods were introduced into sub-Antarctic seas in 1904, and operations commenced in the following year at South Georgia. So successful was the initial venture that several