What Time Is It? There are man methods used to keep time, each having its own special use and advantage. Until recently, when atomic clocks became available, time was reckoned by the Earth's motions: one rotation on its axis was a "day" and one revolution about the Sun was a "year". An hour was one twenty-fourth of a day, and so on. It was convenient to use the position of the Sun in the sky to measure the various intervals. Apparent Time This is the time kept by a sundial. It is a direct measure of the Sun's position in the sky relative to the position of the observer. Since it is depedent on the observer's location, it is also a local time. Being measured according to the true solar position, it is subject to all the irregularities of the Earth's motion. The reference time is 12:00 noon when the true Sun is on the observer's meridian. Mean Time Many of the irregularities in the Earth's motion are due to its eccentric orbit and tidal effects. In order to add some consistency to the measure of time, we use the concept of mean time. Mean time uses the position of a fictious "mean Sun" which moves smoothly and uniformily across the sky and is insensitive to the Earth's irregularies. A mean solar day is 24 hours long. The "Equation of Time", is tabulated in almanacs and represented on maps by the analemma, provides the correction between mean and apparent time to allow for the eccentricity of the Earth's orbit. Local Mean Time (LMT) Local mean time is determined by the mean Sun's position relative to the local meridian of the observer. As with any "local" time, it depends on the observer's geographic location. The reference time is 12:00 noon when the mean Sun is on the local meridian. Mean Civil Time Also called clock time or zonal time, this is the standard time by which most of our non-astronomical activities are measured. The Earth's surface is divided into 24 time zones, each spanning 15 degree of longitude with some variance to accommodate political boundaries. The centrl meridian of each zone is precisely defined, however, to be an integral multiple of 15 degrees longitude. The reference time for the entire zone is 12:00 noon when the mean Sun is on the central meridian of the time zone. Universal Time (UT) This is the basis for all civil timekeeping and is very close to the LMT at 0 degrees longitide at Greenwich Observsatory. hence, it is sometimes called Greenwich Mean Time or GMT. The military often uses the term "zulu" to refer to universal time. Standard time broadcast by radio stations such as WWV or CHU is Coordinated Universal Time (UTC or UT1). This time is based on an atomic clock and is "corrected" by adding occasional "leap seconds" to keep it in reasonable agreement with universal time. International Atomic Time (IAT) International atomic time is the time kept by atomic clocks. The Systemme International (SI) second is defined so that the frequency of a certain resonance of the cesium atom is 9,192,631,770 herz. Sideral Time Sideral Time is measure relative to the stars and is based on the true rotation period of the Earth. Since the Sun appears to move relative to the stars, a sideral day is 3 minutes and 56 seconds shorter than a solar day. Sideral time is measure by the position of the vernal equinox relative to the meridian. Depending on the exact reference used, sideral time may be local (LST) or mean (MST). We use sideral time to adjust our setting circles. Ephemeris Time (ET) As the name implies, this is the time upon which the ephermeris is based. It is reckoned by the orbital periods of the moon and the planets and, therefore, is not subject to the irregularities of the Earth's rotation. It is a uniform measure which forms the basis of the theories of celestial dynamics. It was chosen to be close to UT during the 19th century. By the end of the 20th century, ephemeris time will differ from UT by some 50 seconds. While some almanacs list an estimated correction factor for the current year, the true correction is always determined after the fact by comparing measured planetary positions to the predicted positions. Hour Angle An object's hour angle is the difference between the local sideral time and the object's right ascension. The difference is taken in such a manner that an object west of the meridan has a positive hour angle, while an object east of the meridian has a negative hour angle. The hour angle diagram shows the relation between hour angle and siderial time.