Having two meridians to look after, the form of the American
Ephemeris, to be best adapted to the wants both of navigators and
astronomers was necessarily peculiar. Had our navigators referred
their longitudes to any meridian of our own country the
arrangement of the work need not have differed materially from
that of foreign ones. But being referred to a meridian far outside
our limits and at the same time designed for use within those
limits, it was necessary to make a division of the matter.
Accordingly, the American Ephemeris has always been divided into
two parts: the first for the use of navigators, referred to the
meridian of Greenwich, the second for that of astronomers,
referred to the meridian of Washington. The division of the matter
without serious duplication is more easy than might at first be
imagined. In explaining it, I will take the ephemeris as it now
is, with the small changes which have been made from time to time.
One of the purposes of any ephemeris, and especially of that of
the navigators, is to give the position of the heavenly bodies at
equidistant intervals of time, usually one day. Since it is noon
at some point of the earth all the time, it follows that such an
ephemeris will always be referred to noon at some meridian.
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