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
theoretical astronomer these are the most convenient epochs.

Another part of the ephemeris gives the position of the heavenly
bodies, not at equidistant intervals, but at transit over some
meridian. For this purpose the meridian of Washington is chosen for
obvious reasons. The astronomical part of our ephemeris, therefore,
gives the positions of the principal fixed stars, the sun, moon, and
all the larger planets at the moment of transit over our own meridian.

The third class of data in the ephemeris comprises phenomena to be
predicted and observed. Such are eclipses of the sun and moon,
occultations of fixed stars by the moon, and eclipses of Jupiter's
satellites. These phenomena are all given in Washington mean time as
being most convenient for observers in our own country. There is a
partial exception, however, in the case of eclipses of the sun and
moon. The former are rather for the world in general than for our own
country, and it was found difficult to arrange them to be referred to
the meridian of Washington without having the maps referre