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.