All > John Carmichael wrote: > > > > > > 2. To determine longitude at sea, Harrison's clock was carried on board a > > ship traveling west. The clock was set before the journey to show apparent > > solar time in Greenwich. The difference between local solar time on the > > ship and the time on the clock was used to determine longitude. > > > > To determine local solar time on the ship, Harrison used a sextant to look > > at the sun. How can a sextant, by itself, indicate the time? To determine > > high noon, wouldn't he only need a compass to know when the sun "souths" at > > high noon?
The following is from the manual that came with my sextant. It reminds me very much of a dialier's method of finding North. To find local noon, follow the sun up with a series of sights, starting about half an hour before estimated local noon. Note the time and the sextant reading carefully. Take a sight about every three minutes untill the sun's altitude is no longer increasing. During the meridian passage, the sun will seem to "hang" in the sky for a short period at its highest point, going neither up nor down. Carefully note the sextant reading. This is the sun's altitude at meridian passage. To determine the exact time of local noon, set your sextant at the altitude as your first sight. Wait for the sun to drop to this altitude, and note the time again. The time of local noon is exactly half way between the times of the two [first and last] sights. I think the time readings were taken by what was called in the movie, "a deck watch". ++ron
