# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: The Moon and Parallax
From: Frank Reed
Date: 2019 Oct 14, 10:32 -0700

"This is the bit which always baffles me. Why do they use the name Horizontal Parallax when it leads to a vertical correction?"

Odd, isn't it? I try to talk about this whenever I teach about the HP. It's one of my favorite bits of navigation linguistic trivia. It's because the word has shifted somewhat in meaning since the late 18th century. Back then "horizontal" was understood to mean "horizon-based". The HP is the value of the Moon's parallax at the horizon. Of course, your line of sight looking at the Moon is "horizontal" when the Moon is at the horizon, so you can manage to avoid thoughts of changing word meaning if you want. The Moon's parallax in altitude reduces with a factor of cos(Alt) to zero at the zenith. If you want a more sensible turn of phrase that stays very close to the original, then just say "Horizon Parallax".

And don't forget the useful trick that relates the HP to the angular size of the Earth. If you're standing on the Moon, the Moon's HP is just the SD of the Earth! When the Moon is closer to the Earth, the Earth gets bigger in the lunar sky. So when you see an HP value over 60, you know the Moon is near perigee. When the HP is, let's say, 54 or below, then the Moon is near apogee. This also makes it easy to see why the Moon's SD is directly proportional to HP and makes it clear that the proportionality factor is simply the ratio of the Moon's actual diameter to the Earth's diameter. The size of the Moon is 27.24% of the Earth, so SDmoon = 0.2724 · HP.

Frank Reed

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