Hal Murray wrote: >> One can, of course, observe meridian passage for a variety of stars at >> night, and from that determine the time (given a calendar and the >> appropriate almanac data), so you could directly observe midnight. >> > > What does "directly observe" mean? > > Stars aren't conveniently located where I want them. I think I'd have to do > something like time when a star that's close to midnight crosses a > north-south line in the sky that's above my location. So in addition to the > almanac, I also need a (good?) compass. > > A compass isn't actually required for determining the meridian. If you have located the Meridian and have identified the star, then its not necessary to use one that has a meridan transit at local midnight. In any event if you dont know exactly where you are identifying local midnight may be problematic. A PZT or equivalent may be easier to setup.
> Maybe I don't understand how navigation by sextant works. They get latitude > from the elevation of the sun above the horizon. Do they also get time > (local noon) by noting when the sun is highest? I think that would be hard > to do since you are looking for the peak of a sine curve. That has a broad > flat top. There isn't any conspicuous peak. Do the numbers work out? Can I > get the peak close enough to be useful for navigation? > > > An accurate watch or equivalent source of UTC is required. Various techniques for determining the time from Astronomical observations are somewhat impractical at sea. >> Don't forget too, that solar noon varies quite a bit (minutes) from >> "mean solar time" over the year. If you're navigating your ship with >> noon sun sights, this is pretty important. >> > > What do I google for if I want that correction? > > Googling isnt a reliable technique for marine navigation, at least during the journey. > There are quite a few time scales. Which one do navigators use? > UTC is usually required in the calculations, as it is (currently) closely tied to the Earth's orientation angle. Bruce _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
