# NavList:

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

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Re: Stellarium, refraction, HP
From: Frank Reed
Date: 2020 Jan 12, 20:23 -0800

Hi Jim.

Let's set aside that essay that I wrote in 2004 for now. It was written referencing the Nautical Almanac altitude correction tables as a way of transitioning navigation enthusiasts from 20th century tools to modern tools, weaning folks off the familiar tables, you might say. At that time it was only five years since the 20th century but now it's just over twenty years since 1999, and I'm less worried about sticking to the familiar tools and the Nautical Almanac.

The tables that I gave you in the Lunars class that you attended are all you need, and you can work directly from the first page. Since you have the altitudes from Stellarium, you are partway through the Pre-Clearing step. You have the "pre-cleared" altitudes of the bodies centers directly. Next get the exact Sun SD and the exact Moon SD. It's all explained right there. Once you have the distance properly pre-cleared, you can move on to the altitude corrections. Post your result when you get this done!

For the altitude corrections, you need the refraction. You can calculate that yourself (see the instructions from class) and then compare against the refraction tables in the Nautical Almanac. Note that the correction for "stars" in the N.A. is the pure refraction value. We woon't even consider the Sun correction tables in the N.A. since they're awful. For the Moon altitude correction, you can either learn to master the "opaque" and complicated table in the N.A., or you can just continue with the calculation as described in the tables from class: dh = ref - P0 · cos(Alt). Note that P0 here is the exact HP. When you have the altitude corrections, post them, too, so we can make sure everything is going well.

After you have the altitude corrections, it's time to clear the lunar. You can either do the direct triangle approach (that's next in the class notes), or you can do the series approach (which is identical to the method outlined in "Easy Lunars"), or you can try the Fast Table method (which is a refined variation on one of the series methods). Or try all three! I suggest you try the series method with its "corner cosines" since you've already been working toward that in the "Easy Lunars" essay.

Having cleared the lunar, you can now get UT/GMT. Again, all of this is laid out in detail in the tables that I gave you in class. Please let me know if you get stuck again at any point. No point banging your head against a wall over some minor detail! :)

Frank Reed

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