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## 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 13, 17:43 -0800

Jim, you wrote:
"I have just done the altitude corrections per the outline handed out in class and am about to clear the lunar using the series method.  Where those corner formulas refer to Alt m or s, should I be using Alts as adjusted for the refraction and parallax figured out in the Altitude Corrections, or the original from Stellarium.  My assumption would be use the altitudes from Stellarium as adjusted for parallax and refraction."

No, those altitudes, Alts and Altm, at this point in the work are just the "pre-cleared" altitudes of the Sun and Moon (so in your case, the altitudes from Stellarium directly). You wouldn't want to apply refraction and parallax corrections because that's exactly what the primary clearing process does. It figures out the portion (percentage) of the corrections that act along the lunar arc. The "corner cosine" at the Moon corner is a number between 0 and 100% (actually between -100% and +100%) that tells us what fraction of the Moon's correction in altitude (which is always vertical) is aligned along the arc of the lunar distance that you measured (which is typically inclined at some angle relative to vertical). To get a solid grip on the significance of those "corner cosines", try working out a case where the two bodies are aligned one above the other. For example, if Altm is 14° and Alts is 74° and the pre-cleared LD is 60°, what do you get for the corner cosines? And does that make sense? Another to consider: if Altm is 45° and Alts is 50° and the pre-cleared LD is 85°, what do you get for the corner cosines?

"Also,  does a dh for the moon of -48.8' and for the sun of 2.0' seem in the ball park for a raw moon alt of 14.2° and for the sun of 22.9°?"

Yes, those certainly sould like they're in the right ballpark. :)

Frank Reed

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