It often happens that an astronomer needs to measure the precise position of an object in the sky. Accurate positions are required for many astronomical investigations. To determine proper motions, for example, positions measured over several months or years are needed. It is frequently necessary to compare the position of optically-visible objects with that of nearby radio sources.
All ground-based observations are made from the surface of the rotating Earth. Because of the rotation, the stars appear to move across the sky. It is clear therefore that the measurement of the position of an object requires an accurate record of the precise time at which the position is measured. Man has measured time since antiquity, and has frequently exploited the regular motion of celestial objects (especially the Sun and Moon) in this endeavour. Today several systems of measuring time are used in astronomy.
Although we cannot see the stars close to the Sun because of the bright sky, the apparent position of the Sun slowly moves so that after a year it returns to the same point in the sky relative to the background of stars. The TROPICAL YEAR is defined as the time taken by the Sun to return to its starting point with respect to the distant stars. This starting point is arbitrarily taken to be the vernal equinox (defined below). The tropical year gives rise to a system called EPHEMERIS TIME. One second of Ephemeris time is j defined as 1/31556925.97474 of the tropical year 1900. The tropical j year is 365.24220 mean solar days.
The need for Ephemeris time arose in the early part of the twentieth century, when it was realized by E.W. Brown in the United States and H. Spencer Jones in England that irregularities in the apparent motions of the Sun and Moon were due to the erratic rotation of the Earth. To free astronomical calculations from the vagaries of an irregular clock (Earth), Ephemeris time was introduced, with the second defined relative to the length of the year 1900 Ephemeris time and Greenwich Standard Time (universal time) differ by an amount (currently less than 60 seconds) that is derived by comparing the observed and computed positions of the moon
UNIVERSAL TIME is derived from atomic clocks, the fundamental physical standards. It is broadcast throughout the world by station MSF in England, and by station WWV in the USA.
The Sun does not move through the sky uniformly; it moves more rapidly through the background of stars when Earth is at perihelion that when it is at aphelion. For this reason the Sun is a poor indicator of time, although sundials served man well for millennia. The modern world needs a uniformly-moving fictitious sun, known as the MEAN SUN to regulate daily life; this imaginary sun moves along the equator at a constant rate, and it defines MEAN SOLAR TIME. The difference between mean solar time and APPARENT SOLAR TIME (sundial time) varies throughout the year and is given by the EQUATION OF TIME.
CIVIL TIME is based on the length of mean solar day. It is the same as universal time at Greenwich, England (0° longitude).
The fact that Earth orbits the Sun has an important consequence for the measurement of time. Because of the orbital motion it actually needs to turn slightly more than one complete revolution with respect to the distant stars in order to present the same face to the Sun. This means that the aspect of the stars will slowly change if they are viewed on successive days at a fixed time as registered by a normal (civil time) dock. As the astronomer is more concerned with the time it takes for the Earth to rotate once relative to the background of distant stars and not relative to the Sun, he finds it convenient to work in terms of a SIDEREAL DAY (23hr 56min 4.091 sec) rather than a solar day. As a result of Earth’s Orbital motion, the sidereal day is about 4 minutes shorter than a solar day; the stars seem to rise about 4 minutes earlier each day as reckoned by civil time. In practice, the sidereal day is defined. relative to the vernal equinox; since this point moves due to precession, a sidereal day is about 8.4 ms less than if it were defined with respect to a fixed point in space.
When long time intervals are involved, it is often more convenient to reckon time entirely in days rather than in terms of days, months and years. The number of days since noon (1200 hours Ephemeris time) of 1 January 4713 BC is called the Julian date. Note that there is no year 0; 1 BC is immediately followed by 1 AD. The Julian date of 1 January 1975 at midnight (the start of the new year) is 2442413.5.