Observation Time Systems

Sidereal Time, Solar Time

There are two ways of mesuring Earth's rotation. The one is to measure the time elapsing between the two passages of a same star at the meridian of the observer. This is the sidereal time. The other is to measure the time elapsing between two passages of the Sun -from noon to noon- at the meridian. This is the solar time
Technically the sidereal time is referring to the vernal equinox as, due to the motion of the Earth on its orbit, a same star may not be observed at night all year long. The vernal equinox is one of the point where the ecliptic -the plane in which most planets are orbiting, projected unto night sky- is crossing the celestial equator -the terrestrial equator projected onto the celestial sphere. The vernal equinox is where the Sun is in spring. In the celestial sphere equatorial coordinate system, the vernal equinox is one of the point from where one of the coordinates of stars is measured. Hence sidereal time is called too the "hour angle of the vernal equinox", that is that sidereal time is inferred from the passage of any star, as any star position may be referred to the vernal equinox. A sidereal day lasts 23 hours 56 minutes and 04.0905 seconds

Solar time yields a solar day, which varies by ± 15 minutes relative to the sidereal day. This due to the fact that, in one day, the Earth moves on its orbit. Hence each day the Sun is seen slightly more East. Hence it needs an additional rotation of 4 minutes (1°) to see the Sun at the observer's meridian. On the other hand, this difference of 4 minutes is not steady along the year. The Earth rotates more rapidly when it's nearer the Sun (perihelion) than when it's further (aphelion) or at the equinoxes the ecliptic (the apparent path of the Sun on the celestial sphere) is at an angle with the celestial equator, making that the motion of the Sun appears swiftier than at the solstices. It's the solar dials which are giving this solar time -which is called the "true solar time"- and that's why they are featuring a looping line to take in account the variations of Sun's motion along the year. To get a more even solar time, one has deviced a "mean solar time" which is based on a constant velocity of the Sun along the celestial sphere. The difference between this "mean solar time" and the "true solar time" is called the "equation of time". This conception of the solar time brings too assymetrical shifts of sunrise and sunset at the time of the winter solstice as of morning and afternoon length at the equinoxes. The earliest sunset occurs about 11 days before winter solstice as the latest sunrise about 10 days after. see more at the tutorial Night and Day's Length at Solstices and Equinoxes. To be accurately measured solar time is too measured in terms of hour angle to the vernal equinox. The length of an Earth day also, generally, over the course of a year, varies by about 1 millisecond, getting longer in the winter and shorter in the summer as such seasonal changes are driven by exchanges of energy between the solid Earth and fluid motions of Earth's atmosphere (blowing winds and changes in atmospheric pressure) and its oceans. The length of an Earth day also fluctuates over much longer timescales. A dominant longer timescale mode that ranges from 65 to 80 years was observed to change the length of day by approximately 4 milliseconds. Those changes are due to the flow of liquid iron within Earth's outer core, where Earth's magnetic field originates. This fluid interacts with Earth's mantle to affect Earth's rotation. This flow of liquid iron in Earth's outer core oscillates, in waves of motion that last for decades, such timescales corresponding closely to long-duration variations in Earth's length of day

Current Time Systems

Several time systems may be found today about astronomy

-> -Time UT: Timezones and Basic Astronomical Time
-Coordinated Universal Time (UTC): Civil Time
-Dynamical Time (TDT, TDB): Ephemerides
-Sidereal Time: Ephemerides
-GPS: A Built-In Time
-Atomic Time (TAI): Science

Time UT: Timezones and Basic Astronomical Time

The "Universal Time" (UT) is an astronomical time. That is that it is defined by reference to celestial events. On one hand, the time UT replaced about 1926 the 1884 GMT system which was defining an international time system based on timezones and the Greenwich meridian (see more details at the tutorial "Time-Keeping: Time"). Since then the time UT is used for the timezones system. On the other hand, UT is the basic astronomical time. It's time in which most amateur astronomy events are given as astronomers are using UT because it's synchronized with Earth's rotation. UT is used when an accuracy of about 1 second is enough. Astronomically, time UT is a sidereal time. A mathematical formula just translates it into a solar time. Hence time UT is apparently the mean solar time for the Greenwich meridian. Technically the passage from the sidereal time to UT yields a first time, called "UT0". This time is corrected for Earth's axis wobble and yields in turn a time called "UT1". It's UT1 which serves for the international timezones system and for the astronomical purposes. A further correction, taking in account the Earth's rotation irregularities, yields "UT2". UT2 has few practical uses in astronomy
As far as terminology is concerned, Universal Time may be labeled "Universal Time", "UT", or "Z". The latter mostly used by the military and weather services. The two first by the astronomical community. Universal Time is a 24-hour clock format, that is the a.m./p.m. format is not used. 2:42 p.m. UT reads 14:42. The written format for UT is of the type: 02:17 UT, 02:17 Universal Time, 14:42 UT, 14:42 Universal Time. The military, weather -and some advanced astronomical- format is: 0217Z, 0217UT, 1442Z, 1442UT

Coordinated Universal Time (UTC): Civil Time

The advance of clock-making led after WW2 to the atomic clocks by which time is kept with an extreme accuracy. Atomic clocks are working based on the frequency of electromagnetic waves emitted or absorbed by an atom of Cesium-133. Such devices led to defining the second of the International System of Units (SI), the SI-second. It's based on such an atomic time that the Coordinated Universal Time (UTC) was devised. The time UTC is the worldwide system of civil time. It's the time at your watch, or distributed by time services/or radio stations. The time UTC has no astronomical reference. As the Earth's rotation has not this precision of atomic clocks, the time UTC has to be maintained synchronized with UT1. UTC must not differ from UT1 by more than 0.9 second. The synchronization is made through a positive, or negative, "leap second" which is added to, or substracted from, UTC. From 1972, when leap seconds were first implemented, through 1999, leap seconds were added at a rate averaging close to one per year. Since then, leap seconds have become less frequent. This June 2015’s leap second will be only the fourth to be added since 2000. Before 1972, adjustments were made in a different way. Scientists don’t know exactly why fewer leap seconds have been needed lately. Sometimes, sudden geological events, such as earthquakes and volcanic eruptions, can affect Earth’s rotation in the short-term, but the big picture is more complex

Dynamical Time (TDT, TDB): Ephemerides

Dynamical Time was created in 1977 by the International Astronomical Union in the purpose of providing a more accurate frame system to the computation of the astronomical ephemerides and the definition of high accuracy predictions (like for the pulsars). Until then the ephemerides were already using a specific time: the "Ephemerides Time" (ET) to allow for more accurate data than by using the time GMT or UT. ET, too already was a dynamical time in that sense that it worked using dynamical law of motions (like Newton's law of force) to predict celestial objects position. A further verification of the actual position of these allowed to further constraint ET. ET was using a definition of the second based on the year 1900. The new dynamical times, TDT and TDB, are now using the atomic-kept SI-second and they are taking in account the dynamics induced by Einstein's Relativity
Terrestrial Time (TT) or Terrestrial Dynamical Time (TDT) is the base for geocentric ephemeris predictions as Barycentric Dynamical Time (TDB) is used for predictions referring to the solar system barycenter (the solar system center of gravity). It's computed from TDT. Further technical such dynamical times were defined in 1991 and are related to Relativity-related applications and computations (Terrestrial Time (TT) -equivalent to TDT, Geocentric Coordinate Time (TCG), and Barycentric Coordinate Time (TCB))

Due to tidal frictions or other causes, Earth rotation rate is decreasing. This amounts to 0.002 seconds per century only. It's not back to the dinosaurs era that the day was actually 2 hours shorter than today. Compared to an ideal rotation of the Earth, this amounts to a difference however. When ephemerides are given in TDT, like for solar eclipses, this has to be taken in account to convert data into UT time. The difference between time TDT and time UT is called "delta-T" (TDT+UT). Such a value is usually built in astronomy software. Due to historical records, delta-T is known for ancient times. Such data may be found at Fred Espenak's http://www.eclipsewise.com/help/deltat.html as this site is providing the value for last years too (future values may be extrapolated only)

Sidereal Time: Ephemerides

The sidereal time is still used as it is reliable. Any star will be accurately measured each night at or relative to the observer's meridian. Sidereal is used for some ephemerides as the U.S. Naval Observatory computes the mean solar time from the mean sidereal time

GPS: A Built-In Time

As far as the GPS (Global Positioning System) time aspect is concerned, the clocks both on Earth devices and aboard satellites (these are atomic clocks) started with an offset of 19 seconds compared to UTC. This was due to the GPS beginning to work on January 6th, 1980 when such an offset existing between time TAI and UTC. GPS receivers simply translate this offset, plus the current offset, when they receive the satellite data, and they display the time UTC

Atomic Time (TAI): Science

The atomic SI-second provided by atomic clocks is defining the time TAI (from the French "Temps Atomique International"). Time TAI is only used for high accuracy scientific work