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.
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