Four time conventions have been used for solar observations since observations began in the Bureau in the 1960s.
Coordinated Universal Time (UTC) is the primary world standard for time, with time zones around the world being expressed as positive or negative offsets from it. It is essentially the mean solar time (see below) for any location on the Greenwich meridian. The Greenwich meridian is also called the prime meridian.
Other times conventions are dependent on longitude and Greenwich Hour Angle. Longitude is the angular distance on the earth's surface, measured east or west from the prime meridian, to the meridian passing through a location. It is expressed in degrees (or hours), minutes, and seconds, with longitudes between 0° and 180° west of the prime meridian often expressed as a negative value.
Greenwich Hour Angle (GHA) is the angular distance west of the prime meridian, and is measured from 0° to 360°. For example, a location which has a longitude of 45° W (or -45°) has a GHA of 45°, while the GHA of a location at a longitude of 45° E is 315°. While longitude is the most commonly used approach to specify positions east or west of the prime meridian on the earth's surface, GHA is similarly used in celestial navigation for objects in the sky.
Local standard time is the clock time for the time zone in which the observing site is situated, but which does not include any shift in time due to the implementation of daylight saving time. There is a fixed relationship between local standard time and Coordinated Universal Time based on the longitude that represents the local time zone.
True solar time is determined by the difference between the Greenwich Hour Angle of a location (which is fixed) and that of the sun, which changes by approximately15° an hour as it traverses the sky. The reference time for true solar time is solar noon (12:00:00 TST), which is defined as the time at which the sun's GHA is equal to the location's GHA. Solar noon is commonly referred to as being when the sun's direction is true north and its position is highest in the sky.
The Coordinated Universal Time for the same sun GHA changes from one day to the next because the Earth's orbit is elliptical and its axis is tilted to the plane of revolution around the sun. Thus, while solar noon for a location is always constant in true solar time, in local standard time and Coordinated Universal Time it occurs at different times from one day to the next. For this reason, the World Meteorological Organization World Radiation Data Center requires solar data to be referenced to true solar time or mean solar time (see below).
Mean solar time is the solar time which would apply if there was no day to day variation in the Coordinated Universal Time of solar noon at a location. It is determined by the local standard time and the difference between the GHA of the location and the GHA equivalent to the local time zone. The difference between true solar time and mean solar time can range between + 16 and – 16 minutes throughout a year (see below for an explanation).
Bureau stations and years of observation (csv text) where solar data were measured according to mean solar time.
The relationship between true and mean solar times is described by the equation of time (shown graphically in Figure EOT) and is the difference between true solar time and mean solar time as a function of the Coordinated Universal Time of the year. Because of the complexity of the defining equation the relationship is often approximated.
This relationship is true for every location on the Earth, but the form of the relationship varies very slowly over millennia as the Earth's orbit changes its shape (eccentricity), and the Earth's axis wobbles (called nutation) about is mean orientation (called precession).