- since 1880, the legal definition of time in Britain has been set according to the time in Greenwich, England where the Royal Observatory stood (it has now been moved 20km away), this is called Greenwich Mean Time (GMT) or Universal Time (UT)
- Universal time (UT) is simply the number of hours, minutes, and seconds which have elapsed since midnight (when the Sun is at a longitude of 180°) in the Greenwich time zone.
- Since the Earth's rotation is irregular at the 0.1 second level, a local approximation to universal time not corrected for polar motion is often used. This is called UT0, and also referred to as Greenwich mean time, abbreviated GMT. In UT0, 24 universal hours are defined to be a mean solar day.
- The actual universal time (denoted UT or UT1) is tied to the
rotation of the Earth. Because the Earth's rotation rate is rather
irregular and unpredictable at the 0.1 s level, Universal Time can
only be deduced from observations of star transits. Once known, UT can
be compared with known ephemeris time,
and the difference
can be derived (the value for this in 2003 is 68secs and increments by
~1sec per year). UT is always kept within 0.9 seconds of coordinated
universal time by the insertion or deletion of leap
seconds, usually at 23:59:59 UTC on either June 30 or December 31
- differs from international atomic time by an integral number of seconds and is the basis of most radio time systems and legal time systems. The step adjustments (leap seconds) are usually inserted after the 60th second of the last minute of December 31 or June 30. UTC is the time standard provided by WWV and other time broadcast services.
- other places in the world have their own time zones which are related to GMT according to their longitude, with each degree of longitude equivalent to 111.32km and every 15 degrees creating a new time zone 1 hour different to the previous (15deg x 24hours = 360 degrees = 1 day rotation of the earth) eg. AEST is UT + 10hrs
- these time zones are not adhered to strictly due to geographical & political boundaries, and also Summer Daylight times in various places
- at 180deg to Greenwich is the International Date Line where crossing this meridian from east to west results in traveller jumping to the next day. This line was designated such in 1894 and is regarded as the beginning of each day.
- NB. at the poles where all the meridians & thus time zones meet, the time is taken as being GMT.
- NB. astronauts in orbit are technically subject to the International Date Line system, but in practice keep to the same time as their ground control station.
- Time measured relative to the "fixed stars" instead of the sun.
- Sidereal time is useful because the offset of sidereal time from the
right ascension of a celestial object,
known as the hour angle, gives the time before or after transit of the
star across the meridian line of the sky.
- hour angle:
- The angle between an observer's meridian and the hour circle on which some celestial body lies.
- This angle is conventionally expressed in units of time (hours, minutes, and seconds), which gives the time elapsed since the celestial body's last transit at the observer's meridian (for a positive hour angle), or the time unit the next transit (for a negative hour angle).
- meridian:
- an imaginary circle in the local sky passing between the celestial poles and the zenith
- zenith:
- the highest point in the local sky
- celestial poles:
- the north and south points in the sky around which the celestial bodies appear to rotate
- hour circle:
- an imaginary circle passing through the celestial poles in a similar way to the longitude line on the map of earth.
- each hour circle is 15degrees of arc away from the next one (ie. there are 24 hour circles in the celestial globe)
- right ascension:
- similar to the hour circle, but having a zero point of right ascension is the first point in Aries, just as the zero point for longitude on the Earth is the prime meridian at Greenwich.
- thus any given star except the sun has an almost constant right ascension value but a constantly changing hour angle depending on the time of day and time of year and local position of the observer (ie. the local sidereal time) as the earth rotates on its axis and also revolves around the sun.
- Right ascension is usually measured in units of time (hours, minutes, and seconds), with one hour of time approximately equal to 15° of arc (360°/24 hours=15°/hour).
- declination:
- the "latitude" of a celestial object from the celestial equator & is independent of the observer's location on earth
- culmination:
- the time at which a celestial body transits the meridian.
- hour angle:
- The most useful form of sidereal time, it gives the right ascension of a transiting celestial object at a given location.
- = sidereal time + (1.0027379093 x UT) - (longitude degrees West / 15)
- A uniform time measure now kept by atomic clocks. Ephemeris Time (ET) was used in the Astronomical Almanac from 1960-1983, but was replaced by barycentric dynamical time when the IAU 1976 System of Astronomical Constants was implemented in the Astronomical Almanac in 1984.
- Dynamical time for barycentric phenomena (TDB) which replaced ephemeris time when the IAU 1976 System of Astronomical Constants was implemented in the Astronomical Almanac in 1984. The difference between terrestrial dynamical time and TDB is due to variations in the gravitational potential around the Earth's orbit combined with velocity terms, and is always less than 2 milliseconds. TDB is used as a time scale of ephemerides referred to the barycenter of the solar system.
- is dynamical time for geocentric phenomena which replaced Ephemeris Time when the IAU 1976 System of Astronomical Constants was implemented in the Astronomical Almanac in 1984. TDT is independent of the variable rotation of the Earth, and the lengths of the tropical year and synodic month are generally defined in days of 86,400 seconds of international atomic time. TDT is used as the time scale of ephemerides for observations from earth's surface, and differs from international atomic time (TAI) by an offset TDT = TAI + 32.184 s, which varies slowly with time