In article .com,
"John Nagelson" wrote:

Hello, the US Nautical Almanac lists 173 "navigational stars", of
which a shorter list of 57 is sometimes extracted.
...

Actually, the table that you found in the back of the Nautical Almanac
is NOT a table of the 173 "navigational stars".

It indicates which stars in the table ARE navigational stars by giving
their number. All 57 of them are there with their average SHA and Dec
for each of the 12 months of the year.

These particular stars were chosen as "navigational stars" based on
their brightness and the fact that their distribution on the celestial
sphere made them useful for establishing one's position. That's why
you'll find them listed on every one of the daily pages in the almanac.
These are the ones that navigators will normally use to determine their
position at sea - or in the air.

So what's with all the other stars listed in a table in the back of the
almanac called "Stars" (note that the title of the table is NOT
"Navigational Stars")?

What is listed in the Stars table is all the stars down to a magnitude
of 3.0 (and a few that are even dimmer). If a navigator was to bring
down a star, in all likelihood it would be in this range and thus in
this table.

When might one be called on to use the other 116 stars?

Well, at sea the sky is not always cloudless. If, through a break in the
clouds you find a star that would be great for getting a fix, you bring
it down and record the time of observation and the star's sextant
altitude. At this point, you then have the task of figuring out which
star it was so that, with it's SHA and Dec, you can determine the
corresponding LOP.

This is how you do that:

You work out your corrected altitude for the star and then determine
your DR position at the time of the sight. With this information, you
can then use a sight reduction table (or your calculator/computer
program) to determine the "calculated/estimated" SHA and Dec of that
star. Entering the Stars table, you can, with the calculated SHA, narrow
down the list of possible candidates for the star sighted and, with the
calculated Dec, determine the actual star that was brought down. If it
was one of the navigational stars, you go back to your daily page and
perform the usual ritual. If not, you simply extract the SHA and Dec
listed in the table for the actual star that you brought down and go
from there.

Now you know what that table is all about.

Lots of luck with your project.

ron