A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2021 Feb 19, 22:56 -0800
Geoffrey, you wrote:
"That is more easily said than done."
I think it's easy. But I'm probably not describing it well enough. I'll see what I can come up with. :)
"But when I am doing a lunar I usually take the telescope out of the sextant to get a good field of view. Then, sighting the Moon through the horizon mirror, I rack the index arm until the Sun appears. The Sun will only appear, however, if I set the sextant perpendicular to the Moon's Horns. I am, effectively, putting the sextant on the plane of the great circle between the Moon and the Sun."
Yes. Exactly. You orient the instrument to place the sextant frame perpendicular to the line through the Moon's horns. Works great! The part about "racking" the index arm is, I think, un-necessary. You can preset to the correct angle within a degree or so, and this isn't cheating. There's no circumstance where we don't know (or didn't know at any point in the past 300 years) the GMT to the nearest hour or two, and that means we know the lunar distance to the nearest degree. You preset to that angle first. Next you sight the Moon directly in the horizon glass. Then you rotate about that axis to the Moon until the line through the Moon's horns is more or less perpendicular to the frame of the instrument, just as you said. And there it is. The Sun pops right into view. It's almost foolproof once we get past basic visualization issues.
There are a couple of other tricks. When shooting a Moon-Sun lunar, you can sight at the Moon through the horizon glass, and then move the sextant away from your eye, maybe by a foot or so. Now look for the shadow of the sextant on the deck. Maintaining the alignment on the Moon through the horizon glass (even approximately is fine here), you rotate the sextant until its shadow on the deck (or the ground in your backyard, or your driveway, or a convenient wall) is minimized. This is the same trick I recommend for getting the azimuth right on a standard Sun altitude when the Sun is high. Minimize the shadow and you are aligned with the Sun. In the case of a Moon-Sun lunar, if the horizon glass view is aimed at the Sun, and the shadow of the sextant is minimized which places the frame of the instrument in the same plane as the Sun and Moon, then you have the right orientation to see the Sun and Moon. You then bring the sextant back to your eye, and it's only a small motion to get both bodies in view. This is a really quick way to get the orientation right. Note that you don't necessarily have to look at the instrument's shadow per se. You can also rotate the instrument until sunlight just barely grazes across the side of the instrument that was previously shadowed. There are usually a few small bits of dust on the frame, and as the instrument turns to the Sun, they "light up". The correct orientation will be very close to that "light up" angle.
If you're not shooting a Moon-Sun lunar, the shadow trick doesn't work. In that case I recommend holding the instrument out in front of you a bit further and trying to sight both Sun and star along the frame of the instrument. The plane of the sextant frame has to match the plane containing the Moon, star, and the observer. So you hold the sextant up in front of you in an orientation where you can sight right along the frame (not looking through the scope, just looking across the frame). Once you have that orientation, it's much easier to bring it back to your eye and maintain that orientation. As before, since you have preset to the proper angle, the star will pop into view almost immediately.