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Frank Reed wrote:

> I wonder why they felt it was necessary to fix the list at 57?

I don't have the faintest idea, but the question reminds me of a debate
over the reason why Ptolemy chose an 18-year interval for mean motion
tables in the Almagest (basically, multiplication tables). Toomer put an
end to it. Ptolemy says that 18 years will "produce symmetry in the
layout of the tables". Toomer points out that 45 lines was the standard
height of tables in the Almagest, presumably chosen to conform to a
standard height of papyrus roll. Loose a couple of lines for headers.
Ptolemy combines two columns on the same sheet: In the first, a table
for 12 months and one for 30 days (= 42 entries); in the other column, a
table for 24 hours and one for n individual years. But 42 - 24 equals
18. Get it?

> They added six stars and deleted six others including Polaris, which
> always seems surprising.

Indeed, surprising. I never understood the obsession of _modern_
navigators with the special treatment of Polaris. We don't need to point
out to this forum the historical significance of Polaris. But nowadays
it is hardly more than a boy-scouts thing, just like finding north in
the woods with your watch: same importance and same accuracy.

It's a myth that one can find latitude from Polaris without knowing
time: one needs _local sidereal_ time. In modern times, this is
equivalent to an estimate of longitude and Greenwich time. (When did you
last use a nocturnal?) If one has such an estimate (and who doesn't?),
but does not trust it, it is much easier to find latitude with a regular
two star sight, reduced a la St. Hilaire, and to discard the longitude.
If the almanac gave GHA and Dec of Polaris, one could use it in the
regular way, if one really must, and forget the special procedures and
special tables consuming three pages in the almanac. But why would one
want to use a star of magnitude 2.1 in the first place?

It is true, since Polaris is always nearly on the meridian, a shot of
Polaris can give a very good latitude, if (!) one has local time; but
this feature is inherent in the geometry of the observation, not in the
method of reduction. Therefore, there is no need to make head stands for
a special method of reduction. Besides, since Sumner's great discovery,
we have come to understand that latitude is just one specially named
line of position.

Herbert Prinz

   

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