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. What
meridian this shall be is purely a practical question, to be
determined by convenience and custom. Greenwich noon, being that
necessarily used by the navigator, is adopted as the standard, but
we must not conclude that the ephemeris for Greenwich noon is
referred to the meridian of Greenwich in the sense that we refer a
longitude to that meridian. Greenwich noon is 18h 51m 48s,
Washington mean time; so the ephemeris which gives data for every
Greenwich noon may be considered as referred to the meridian of
Washington giving the data for 17h 51m 48s, Washington time, every
day. The rule adopted, therefore, is to have all the ephemerides
which refer to absolute time, without any reference to a meridian,
given for Greenwich noon, unless there may be some special reason
to the contrary. For the needs of the navigator and the