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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: Unexpected USNO height correction precepts
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.
```
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