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
referred to the same meridian. Since, however, the meridian of
Greenwich is most convenient outside of our own territory, and
since but a small portion of the eclipses are visible within it,
it is much the best to have the eclipses referred entirely to the
meridian of Greenwich. I am the more ready to adopt this change
because when the eclipses are to be computed for our own country
the change of meridians will be very readily understood by those
who make the computation.

It may be interesting to say something of the tables and theories
from which the astronomical ephemerides are computed. To