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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: 18 june 2013 lunar distance
From: Paul Hirose
Date: 2013 Jul 02, 16:46 -0700

```Dave Walden wrote:
> The additional places are essentially 'free' in modern computers, and are correct, so why not?
>
> They amuse some of us!

When people explore the limits of their software and compare results,
there can be benefits beyond amusement. In December 2010 we had a long
discussion in the HASTRO-L group about whether or not a lunar
occultation of Spica was visible from Alexandria in November 283 BC. One
person using the Swiss Ephemeris software had a .0005° discrepancy in
Spica's position. It was due to a previously undetected bug in the
processing of star catalog data. Right ascension proper motion in the
Hipparcos catalog is expressed in great circle units, but the Swiss
Ephemeris was coded as if they were coordinate units. That was quickly
fixed.

> Using USNO NOVA's, DE405, SIMBAD data for Altair, and Paul's "Delta T
= 32.184 + 35 - 0.0661627", I get:
>
> true xld= in deg
> + 106 deg 34' 28.466124"   OR  106 deg 34.474435'   OR 106.574574deg

Note that each angle above has 60 times less precision that the one to
its left. A good rule of thumb is to use the same total number of
digits. For instance, DDDMMSS.s and DDD.ddddd have the same number of
digits and about the same precision.

In this case, Lunar3 says 106°34′28.453″. That's 13 mas different from
Dave. (2013-06-18 0330 UT1, 67.1178 delta T, DE406 ephemeris, Hipparcos
second reduction)

I think 1 mas is close enough, but 13 is too much. My use of DE406
should not matter. It's the same integration as DE405 but stored in a
more compact format, "only" accurate to 1 meter for the Moon. (That's
how well DE406 duplicates the original integration, not its accuracy
with respect to the true position of the body.)

What about SIMBAD vs. Lunar3's internal star catalog? With the SIMBAD data

I get 106°34′28.454″, only 1 mas more.

Is the discrepancy in the Moon position, or Altair? I would like to
compare the geocentric apparent places of both bodies. And to eliminate
any question about the precession / nutation model, I wonder if Dave can
put the coordinates in the GCRS. From Lunar3 I get Moon coordinates

-2.44969548e-003 -5.24187596e-004 -3.58294080e-004 au, or
192° 04′ 40.973″  -08° 08′ 21.886″

and Altair coordinates
4.59325899e-001 -8.74784757e-001 1.54179599e-001 au, or
297° 42′ 09.913″  +08° 52′ 09.188″

--
I filter out messages with attachments or HTML.
```
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