NavList:

I wrote earlier:
"It's easier since it's a very short angle. You should be able to measure the Moon's actual diameter to an accuracy of about 0.1' of arc by averaging a handful of observations. As always, the IC is everything here."

As soon as I stepped outside tonight and looked at the Moon, I realized that I ignored an obvious advantage when measuring the Moon's diameter. If you measure it on-arc and then off-arc and average, the IC cancels out. This is obvious. I simply wrote it up wrong earlier today.

I measured the Moon's diameter with two sextants tonight. An "International Nautical" or Tamay-alike from c.1976 and an Astra IIIB c.2000 both equipped with 7x scopes. The on/off average for the I.N. was 32.8'. The same for the Astra was 32.7'. What should it have been? Given the Moon's HP of 59.7', the expected geocentric SD was 15.26' and the expected diameter was 32.5'. But that's before the augmentation. Since the Moon was not on the horizon, it was larger than the geocentric value. The correction to the SD is about 0.3'*sin(h_moon) and h_moon was about 32° so the correction to the diameter was 0.3' yielding an expected apparent diameter, including augmentation, of 32.8', consistent with the results from both sextants.

-FER

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