NavList:

Yesterday afternoon the gibbous Moon was climbing in the east. It was a nice day, and I was inclined to shoot a lunar. It's easy to line things up if you have even a rough estimate of the correct distance. I find that I'm not very good at guessing the lunar distance angle when the Moon is past half-full. Maybe it's 100°... maybe 115°... It would have taken me only a moment to get a lunar distance estimate from an app in my pocket, but I wanted an alternative.

Looking at the Moon in the sky, I could see of course that by drawing a line across the sky perpendicular to the Moon's horns that line would lead me straight to the Sun (this can be counter-intuitive... when in doubt, grab a broomstick and hold it out "sideways" in front of you to make your line across the sky). Then I realized that I could extend that line in the opposite direction, away from the Sun, and it ends up on my shadow, specifically on the shadow of my own head, which would be 180° away from the Sun in the sky. Aha! So I grabbed the nearest sextant, a little Davis Mk.3 plastic sextant, and I pointed the horizon glass view at the Moon (the sextant was handle-side up, by the way). I rotated the sextant until the line through the Moon's horns was perpendicular to the frame of the sextant and then slid the index arm until the shadow of my head and sextant was visible in the reflected view next to the Moon. I wiggled it a bit until the portion of the shadow near my eye was close to the far limb of the Moon. And that angle was just about 65.5° give or take about half a degree. With this number I could now estimate the direct lunar distance from Sun to Moon! It would have to be 180°-65.5° or 114.5°. So I turned to try my direct lunar with the sextant now preset to... 

Well, no. The Davis Mk.3 stops at about 100° so I was out of range. I could have grabbed a better sextant, but that would have taken too much time from my "remote work" project in another hemisphere. Instead I decided to treat that crude Moon-to-shadow angle as an actual lunar. I loaded up my lunars clearing web app. For 114.5° near limb I found that my "anti-lunar" was only in error by some 9 minutes of arc. While that's basically useful for longitude, it's nice to see how close the estimate was.

Frank Reed

PS: photo composite is from today, but very similar to yesterday's conditions.

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