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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2023 Jun 30, 11:12 -0700
Feeling like an expert yet?
...Now for something completely different. A sextant altitude greater than 90 degrees. It's called a "back sight" because you're shooting with your back toward the target star. And it makes sense, too. More than an hour after that Capella sight, the northern horizon is still bright, even at local midnight this time of year, but the southern horizon is not distinct. And yet we have a nice opportunity: Vega is reaching the meridian. For almost five minutes from 23:29, the altitude of Vega off the north horizon is 106°12'. Back sight raw altitude:
Vega: 106°12' at 23:30 UT.
We only need a dip correction at that high altitude. From the corrected altitude, what would be its zenith distance on the southern side? We can use this ZD as normal for a standard meridian latitude. Finally, although the horizon is not great to the west, less that a minute after the Vega sight, you shoot Arcturus (visual azimuth is West by South --that's "West and a bit South"). Its raw altitude:
Arcturus: 29°07' at 23:30:48 UT.
What then is your 23:31 fix?
