Determining Solar Time at Sea

Otago Witness, Issue 1917, 17 August 1888, Page 33

Determining Solar Time at Sea

in order to compare it with first-meridian time, is somewhere about 9 in the morning or 3 in the afternoon, when the sun's elevation changes more quickly than at other times, so that the observer can readily determine the moment when the sun is at a determined height, or (which is equally effective) can readily determined the height of the sun at a particular moment. It takes two persons to make the observation properly, one noting the time while the other with a sextant measures the sun's elevation above the horizon. (The sextant is so planned that an observer looking through a small telescope at the horizon below the sun can, by means of a reflector, bring the sun's image into the telescope's Held of view so as just to touch the horizon. A graduated arc enables him then to ascertain through what angle he had brought the sun down, and so to ascertain what is at the moment the sun's elevation above the horizon. Other objects can in like manner be brought to the horizon, or two objects, as the moon and a star, brought together, and the corresponding angular distance between the horizon and any object, or between the moon ani a star, very accurately ascertained. Thus may a ship's latitude and longitude 1)0 ascertained from da}- to day in clear weather, and nearly always a,t sufficient intervals throughout a long journey to correct sufficiently the results obtained from dead reckoning. But of course as the seaman draws near land he becomes increasingly watchful. Often he deems it necessary to supplement by the lead line, which* may be regarded as feeling the ground, the indications alike of astronomical observation and of the dead reckoning. I have no space to consider here, as fully as I could wish, the determination of the shortest and best course from port to port, or from a ship's actual position at any moment to her haven. But I may remark that Mercator's charts are intended for what i 3 called