# NavList:

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

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Re: Lunars with Young's formula: always off by several minutes
From: Paul Hirose
Date: 2018 Nov 4, 21:12 -0800

```On 2018-11-02 16:15, William Porter wrote:
>
> *Moon-Aries 19Oct18 18:46:47U Obs distance (near): 41deg 16.8’*
> Lat: 51.432 deg Long -0.219 deg
> *Ephemera:*
> Moon (from USNO polynomials)
> RA: 332.155 deg Dec: -13.891 deg HP:3274” AD:1784”

I don't agree. First, the time. I assume "19Oct18 18:46:47U" is UTC.
Currently UTC is 37 seconds behind TAI, and TAI is a fixed 32.184 s
behind Terrestrial Time. Therefore, TT at observation time = 18:46:47 +
37 s + 32.184 s = 18:47:56.2 = .783289 day.

The applicable polynomial coefficients are:

R.A.   322.7938744    12.0578390    -0.1199279     0.0092096
0.0018500    -0.0000684

Dec.   -16.3099733     2.8134374     0.3553752    -0.0179380
-0.0005172    -0.0000195

H.P.     0.9060852     0.0032932     0.0013016    -0.0000520    -0.0000059

(http://asa.usno.navy.mil/static/files/2018/lunarpoly_2018.txt)

With a calculator and the polynomials I get geocentric apparent coords:

332.1701° -13.8970°  RA, dec (true equator and equinox of date)
.9094° (3274″) HP

An independent computation with my astronomical software confirms those
values. The position is .95′ different from what William got. I do agree
with his parallax. Don't know what his AD:1784” means.

> Aries
> RA J2000: 19:50:47=297.696deg
> *PLUS TO A FIRST ORDER APPROX, 18.8years’ precession at 360/25771 deg year=0.263deg=297.959deg*
> *Thank you Navlist for helping me find this error*
> Dec, ignoring precession for now: 8.868 deg
> GST-UT: 1:52:47 (from astropixels.com)

The Aries computation I will skip since I don't know what's going on.

> 1) Preparation
> GST (GHA Aries) = 20:39:34 = 309.890 deg
> GHA moon = 337.735 deg
> GHA Altair = 12.194 deg
> “d”, observed distance = 41.528 deg after adding semi-diameter

My geocentric values:
309.8858° Greenwich apparent sidereal time
337.7156° GHA Moon
11.9647° GHA Altair

(UT1-UTC is negligible since it's only 32 milliseconds.)
```
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