# NavList:

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

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Re: Horizontal Parallax
From: Frank Reed
Date: 2014 Nov 23, 10:37 -0800

Evangelos Kontos, you wrote:
"Thank you Mr Creed, I got it!"

You're welcome. Incidentally, it's "Reed" not "Creed" (no problem), but please feel free to address me as "Frank".

"If for any other reason I wanted moons HP at my Local Time, would there be a way to find it without knowing GMT? My professors at the university of Athens, Greece don't seem to know. Thanks a lot!"

Of course measuring the lunar distance is the most efficient way if that's what we're doing anyway, but yes, there is another way. The HP is simply a measure of the Moon's distance from the Earth. If you divide the Earth's radius in km by the distance to the Moon in km (center-to-center), then multiply that ratio by 3438, you get the Moon's HP in minutes. You can easily get the distance to the Moon at any instant of time by setting up a high-power laser in your backyard and timing the roundtrip travel time for a pulse fired at the Moon. The retro-reflectors left by the Apollo moon landing missions and the Soviet Lunokhod missions are still working just fine. You time the round-trip in seconds, divide by two, and then multiply by the speed of light 299792.458 to get the distance from your observing location in km. So you're all set. You do have a high-power laser, right?

That was a joke. :) It would work, but the necessary laser is an expensive toy!

Instead, use your sextant to measure the angular diameter of the Moon in minutes of arc, D, and measure the approximate altitude, H, of the Moon (within 5° is good enough for this task). Then
HP = 1.836·D - 1.1·sin(Hmoon).
This works quite well. You're always better off getting the HP from almanac data (and why wouldn't you?), but measuring it can be satisfying, too. Note that the observation of the Moon's angular diameter, D, is best done by measuring the Moon's pole to pole diameter off-arc and then on-arc. The average of the two values will usually be accurate to 0.1 minutes if you make the observation with a 7x or better scope. Index correction can be ignored if you do this off-arc/on-arc averaging process. One nice thing about this observation is that it comes equipped with an easily understood answer to the inevitable question from passers-by: "Hey, what are you doin' with that thing?" In this case, you can turn to them and say plainly, "I am measuring the distance to the Moon!"

-FER

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