Hi Stuart,

I agree that the text 'Dist from home planet' is a little ambiguous;
it ought to read something more like 'Dist from current location'.

For dates near the present, a geocentric distance to the moon should
be good to within about a meter (the interpolation error for the JPL DE
ephemeris). Good enough for all ordinary purposes not involving lunar
retro-reflector ranging.

A topocentric distance would only be good to within about 25 meters.
That's because some oddities in the earth's orientation (which way its
axes are pointing at a given time) are handled with low precision. The
orientation has a "predictable" part due to precession and nutation,
plus some "unpredictable" parts you can only figure out by observations:
changes in Delta-T, polar motion, and nutation offsets. Guide doesn't
include those "unpredictable" parts, except for a decent approximation
for Delta-T.

Guide gives a geocentric distance, at 11:23 UTC on 14 November, of
356510 km (and also gets that time for perigee).

Can't say I'm all that "fanatical" about supermoons myself, but I
agree that it's a good excuse to get people out and looking a bit.

Interestingly enough, generating ephems using JPL's _Horizons_ site :

http://ssd.jpl.nasa.gov/horizons.cgi#results

gives a perigee time of 11:28, at 356510.358 km. Note that this
is at perigee, i.e., the time when the distance to the moon is barely
changing at all.

However... it occurs to me that one can look at line 325 in the file
'strings.dat', which looks like this :

Dist from home planet: {%.8f} ^AU (

Just change that ".8" to ".11", and Guide will produce distances in
AU to eleven decimal places rather than eight. (There's no similar
trickery to get the kilometer distances to greater precision. For
the following, I just took the AU distances and multiplied by
1 au= 149597870.700 km.

Using this, and checking times at one-minute intervals, I got
a minimum distance of 356610.367 km at 11:28. The time agrees with JPL,
but the distance disagrees by nine meters. Which, considering it ought
to agree to within about a meter, is a little distressing. I will
investigate.

-- Bill

On 2016-11-11 12:41, s.kranz1@... [guide-user] wrote:
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> So how accurate is Guide regarding data on the Moon such as distance?
>
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> With the upcoming Full Moon and all the publicity of the Supermoon, I was checking out some lunar data on Guide.
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> According to most of the sites and stories I've read, the closest distance to the Moon is supposed to be at 11:23 UT on Nov 14 with a distance of 356,509 km.
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> BUT at that time, Guide shows the Moon being at 354,837 km away.
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> According to Guide, the closest point will be at 05:53:30 UT on the 14th with a distance of 350,854 km.
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> Now, I am in no way a fanatic about this whole supermoon business, but if it gets people out looking up, then I'm all for it.
>
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> But the differences between Guide and the reported time and distance has me worried.
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> Thoughts?
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> -Scott Kranz
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