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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Unexpected USNO height correction precepts
From: Paul Hirose
Date: 2018 Dec 24, 02:02 -0800
From: Paul Hirose
Date: 2018 Dec 24, 02:02 -0800
On 2018-12-22 7:01, Mehmet Guzey wrote: > 1- phase angle = 139.313052 degrees I get 139.30. No great accuracy is necessary since the center of light correction is small. The exact phase angle depends on geometric place vs. astrometric place vs. apparent place. The JPL Horizons online calculator uses the apparent place of the Sun as seen from the body, i.e., it applies the aberration for an observer at the center of Venus. I think that's correct but I also think it's not necessary to be so precise. I use the heliocentric geometric place of Venus. For the other vector in the phase angle computation — the vector from Venus to the observer or its inverse — I don't know what's strictly correct so I use the geometric place. Anyway, your phase angle is accurate enough. > 2- position angle = 50.820597 degrees Position angle is zero in the direction of the "north pole" and increases east. So it depends on the coordinate system: ICRS, true equator and equinox, or horizontal. (In the horizontal system, "north" is the zenith.) I computed the position angle (Venus to Sun) all three ways: 233.9 ICRS 233.8 true equator 000.9 horizontal These are very different from your value. > 3- sd = 0.42 minutes Correct. > 4- offset = 0.312019 minutes Correct. Everything is correct except position angle.
