The Astronomical Methods

Otago Witness, Issue 1917, 17 August 1888, Page 32

The Astronomical Methods

for determining a ship's position depend on simple considerations, though their details are often exceedingly complicated. In travelling over the terrestrial globe, the voyager changes his distance from the equator (that is, the circle midway between the poles) and from the first meridian (that is, a certain circle through the poles, from which distances east and west are measured) ; or technically he changes his latitude and his longitude. Changes of the first kind are readily measured, because the elevation of the pole of the heavens increases while the elevation of the celestial equator due south diminishes as the equator is left and vice versa. The stars of course partake in these changes. But the sun is the best object to observe /or its determination. His distance from the equator on any day throughout the year, and at any hour, is known. If then he be observed at solar noon, when he is due south, and moving parallel to the horizon, his observed height at once gives the means of deducing the height of the celestial equator — whence the latitude- is determined. (For whatever the height of the equator at any place the latitude of the place is 90deg diminished by that angular height.) Changes of longitude are by no taeans determined so readily as changes of latitude. To an observer in the same latitude in North America as that of- Greenwich in England, the star skies present absolutely the same appearance. The sun, moon, and planets are in slightly different positions when southing or in any other definite directions with respect to the cardinal points, simply because they reach these apparent directions some five or six hours later, by which time they are in slightly different directions with regard to the earth. But the differences thus existing are far too small to be available for determining longitude at sea, except in the case of the moon, whose movements are rapid enough to be measurable (not merely recognisable) in shorb periods of time. Even in observing the moon in this way, which is

one of the methods actually employed for determining the longitude, it is found that the results deduced are less satisfactory than can be obtained by the simpler method of keeping Greenwich (or other known meridianal) tisae on board ship and ascertaining local time from observations from the sun. Suppose, for instance, a captain could ascertain that the sun was due south when his chronometer, showing Greenwich time, pointed to shr 27min (p.m.). This would show that solar time at the ship's place was 5hr t 27min later than mean solar time at Greenwich. Correcting from true solar into mean solar time from the column " clock before sun " or " clock after sun " in the almanac our captain could, let us say, correct the difference of shr 27min into shr 20min. This would show him that the ship's place was 5J twenty-fourths of SGOdeg, or 80deg west of Greenwich. As a matter of fact, though, it is not at noon that a navigator tries to determine solar time, though many ocean travellers imagine that noon is the time, noting that at that time ship-noon is determined. Shipnoon is only noon for fixing the " watches," not for purposes of navigation. It will easily by seen how unsuitable is noon for timing the sun at sea, if it be remembered that the only available way of determining when it is solar noon is by noting when the sun is at his highest, and as at noon he is moving parallel to the horizon on the change of height it about this time takes place at its slowest. (On land solar noon can be readily and neatly determined by observing when he sun souths — as by a sun dial ; but at sea there is no means of determining the south except from the rough indication of the magnetic compass.) The proper hour for