# NavList:

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

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Re: Modern Lunars
From: Paul Hirose
Date: 2016 Sep 23, 16:47 -0700

```On 2016-09-20 7:43, Antoine Couëtte wrote:
> One thing puzzles me ... the difference between our Azimuts:
> For the Moon, you get  85.4227° , while I get: *Moon Z = 85.42280° including
a Parallax in Azimut equal to dZ = -14.59022” , which - without the parralax
in Azumuth - would put the Moon at 85.4268 °*
> For the Sun, you get 85.4223°, while I get: *Sun Z = 85.42227°, including a
parallax in Azimut equal to dZ = - 0.02538”*

This table shows the residuals (me - Antoine) in azimuth, altitude, and
lunar distance.

Moon az     alt    Sun az    alt    distance

-0.0001° -0.0001° +0.0000° +0.0012° -0.0009° #2 sea level
-0.0001° -0.0001° +0.0000° +0.0012° -0.0009° #2 400 meters

-0.0003° -0.0001° -0.0001° +0.0002° -0.0002° #3 sea level
-0.0003° -0.0001° -0.0001° +0.0001° -0.0002° #3 400 meters

+1.0001° -0.0002° -0.0004° +0.0001° -0.0002° #4 sea level
+1.0001° -0.0002° -0.0004° +0.0001° -0.0002° #4 400 meters

I think these are mainly due to roundoff error and small differences in
the refraction models, especially in #2 where the Sun is very low.

The exceptions are the Moon azimuths in #3 and Sun azimuths in #4. I
think they're too large to be to be easily explained.

JPL HORIZONS agrees with my azimuths and unrefracted altitudes in #3 and
#4 within 0.0001°. But note that its delta T is 7.477317, not the 7.68
in Antoine's document. (The Astronomical Almanac says 7.6 at 1855.0 and
7.7 at 1856.0.)

HORIZONS delta T is .203 s less than Antoine, so it thinks Earth has
rotated further east. To get compatible azimuth and altitude you must
move the observer west to compensate. Convert .203 s to arc and get a
longitude correction of 0.00085° west.

I think the +1.0001° Moon azimuth residual in #4 is due to a typo in
Antoine's document.

```
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