Now largely replaced by more accurate atomic clocks, does
the monitoring of ephemeris time has become a largely esoteric astronomical
exercise?
By: Ringo Bones
In recent years, modern atomic clocks and Global Positioning
System based timekeeping have largely relegated the computation of ephemeris
time – also known as terrestrial time – into something of an arcane esoteric
astronomical exercise. Despite its lesser relevance to contemporary
ultra-accurate timekeeping, it is worth noting that the “timekeepers” at the US
Naval Observatory still do this with refreshing regularity.
Ephemeris time may be defined by the motion of any planet of
the Solar System or by any of their satellites. Astronomers have formerly
defined ephemeris time by the orbital motion of the Earth around the Sun. It
may be obtained by observing the position of the Sun with respect to the stars;
however, in practice this is difficult to do and it is obtained from the
orbital motion of the Moon above the Earth.
This concept of astronomical time is based on the monthly
motion of the Moon among the stars may be considered to form a clock wherein the
stars represent the hour marks and the Moon represents the pointer. To utilize
the Moon for this purpose, its ephemeris must be calculated with great
exactitude and its position must be precisely determined by observation. The
ephemeris of the Moon is based upon the mathematical researches of Ernest
William Brown (1866 – 1938) at Yale University.
The position of the Moon has usually been determined from
meridian transits and from occultations of stars. A more recent method is to
photograph the Moon among the stars with a dual-rate camera developed by
William Markowitz at the US Naval Observatory in 1951. By means of a central,
dark, plane-parallel glass filter, which is tilted during an exposure of about
20 seconds, the image of the Moon is given an artificial motion which cancels
the normal motion of the Moon with respect to the stars. Thus, the Moon is held
fixed relative to the stars and sharp images of both the Moon and the stars are
obtained. Since the positions of the stars are known, measurement of the plate
gives the position of the Moon with respect to the stars. This is entered in
the lunar ephemeris and the corresponding ephemeris time is obtained.
