Thread: Royal Marines
View Single Post
Old 13-07-2014, 10:56 PM   #143
Senior Member
Join Date: Apr 2008
Location: Inactive
Posts: 36,483
Likes: 237 (190 Posts)
Lightbulb Celestial Navigation

In celestial navigation, lunar distance is the angle between the Moon and another celestial body. A navigator can use a lunar distance (also called a lunar) and a nautical almanac to calculate Greenwich time. The navigator can then determine longitude without a marine chronometer..The method relies on the relatively quick movement of the moon across the background sky, completing a circuit of 360 degrees in 27.3 days. In an hour then, it will move about half a degree, roughly its own diameter, with respect to the background stars and the Sun. Using a sextant, the navigator precisely measures the angle between the moon and another body.. That could be the Sun or one of a selected group of bright stars lying close to the Moon's path, near the ecliptic; Regulus was particularly commonly used. At that moment, anyone on the surface of the earth who can see the same 2 bodies will observe the same angle(after correcting for parallax error). The navigator then consults a prepared table of lunar distances and the times at which they will occur.. By comparing the corrected lunar distance with the tabulated values, the navigator finds the Greenwich time for that observation. Knowing Greenwich time and local time, the navigator can work out longitude.. Local time can be determined from a sextant observation of the altitude of the Sun or a star.. Then the longitude (relative to Greenwich) is readily calculated from the difference between local time and Greenwich Time, at 15 degrees per hour..Having measured the lunar distance and the heights of the 2 bodies, the navigator can find Greenwich time in 3 steps...A lunar distance changes with time at a rate of roughly half a degree, or 30 arc-minutes, in an hour.. Therefore, an error of half an arc-minute will give rise to an error of about 1 minute in Greenwich Time, which (owing to the Earth rotating at 15 degrees per hour) is the same as one quarter degree in longitude (about 15 nmi (28 km) at the equator)..Captain Joshua Slocum, in making the first solo circumnavigation in 1895-1898, somewhat anachronistically used the lunar method along with dead reckoning in his navigation..The work of the lunarian, though seldom practised in these days of chronometers, is beautifully edifying, and there is nothing in the realm of navigation that lifts one’s heart up more in adoration.. they were traveling somewhere?..A superior species, no's fairly evident they were in the process of leaving, before things went to pot..Leaving to go where?.Earth..Sometimes to create, one must first destroy.. If you're receiving this transmission, make no attempt to come to its point of origin...

Last edited by lightgiver; 13-07-2014 at 10:57 PM.
lightgiver is offline   Reply With Quote