# NavList:

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

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Re: Lunar trouble, need help
From: Paul Hirose
Date: 2008 Jul 07, 15:45 -0700

```KENT AE NORDSTRÖM wrote:
> I all cases there is an angle between the distance line and the vertical
> used for measuring the altitude. Refraction compresses the body - more on
> the LL than on the UL - which means that there is a need to correct also
> the distance for refraction. The body is not circular but compressed in
> directions apart from th the compression in the vertical. The arguments for
> this correction is altitude and angle between the distance and the vertical.

The last angle is the position angle. It's the "azimuth" from point A to
point B on the celestial sphere. If the coordinates of those points are
expressed in a horizontal reference system (any system could be used,
e.g., RA and dec.), the zenith is the "north pole". In that case, if B
is directly above A, the position angle from A to B is zero. The angle
increases counterclockwise. This is consistent with the way we measure
azimuth: 0 at "north", increasing "east".

Here are the International Astronomical Union routines for position angle:
http://www.iau-sofa.rl.ac.uk/2008_0301/SeparationPA.html

> This correction should be done for the moon and the sun. As an example for
> altitude 8d 30s and angle of 45d the correction is about 5 arcsec's, while
> when the angle is 0d, as in altitude reduction, the correction is 10
> arcsec's. If the other body is the sun then a similar correction should be
> used. So it is not negligible to avoid this
> correction when reducing the distance.

I agree with your numbers, but are those corrections significant for a
sextant shot at 8.5° altitude from the deck of a ship?

My lunar distance program corrects the Moon and Sun semidiameters for
refraction, and even adjusts the Moon position for the difference
between its center of mass and center of figure. However, in the real
world I'm sure human error, sextant error, and non-standard refraction
have much greater effect than the little corrections I'm applying.

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