A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: GMT from Moon and Body Observed Altitudes...??
From: Fred Hebard
Date: 2019 Feb 21, 15:13 -0500
Thank you very much for clarifying the history of longitude by lunar distance and altitude and why distance is the preferred method at sea, in most cases.
A mirror used as an artificial horizon definitely needs to be carefully leveled, and as flat as possible. However, liquid horizons are leveled and flattened automatically, other than vagaries introduced by deviation from the vertical or menisciip. If mercury is undesirable, black molasses is a good substitute, with the added advantage of being more viscous and thus less subject to the wind. The little glass covers for artificial horizons also keep the wind out. With a cover, even plain water is a good medium for artificial horizons when examining brighter objects.
These uncertainties in the horizon disappear if you're doing this on land with an artificial horizon, assuming it's been carefully leveled.
Those of you that have backyard artificial horizons, carefully leveled, should give this a try.
The advantage gained by shooting lunar altitudes instead of lunar distances is modest (more familiar process of observation and calculation) while the disadvantage is substantial (considerably reduced accuracy unless an excellent artificial horizon can be used and greatly reduced opportunities for the observation especially in higher latitudes).
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