As a Delta T ( ) in astronomy is the difference of the Terrestrial Dynamic Time ( TDT, also TT ) and the Universal Time (UT ) indicates, that the difference in time scale that is based on the movement of the bodies of the solar system and that is determined by the actual rotation of the earth.
The current value of can be determined from the data provided by the International Earth Rotation and Reference Systems Service ( IERS ). At the beginning of the 21st century is about 64 seconds at the end of this century, the time difference will be increased to about 204 seconds. History for can be determined approximately by traditional observations with present calculation results are compared. Furthermore, there are several derived from these data polynomials for the approximate calculation. Such polynomials are also to predict future values .
Due to the irregularity of the earth's rotation, the Universal Time (UT ) is not strictly uniform time scale and therefore is unsuitable for the ephemeris, so is not suitable for example for the longer-term prediction of planetary constellations. Even the deduced Universal Time Coordinated Universal Time Coordinated ( UTC) is not suitable, because in this leap seconds are inserted to align it with the universal time at irregular intervals. Therefore, the Ephemeris Time (ET ) was introduced in 1960, in 1984 by the Terrestrial Dynamic Time ( TDT, also TT ) has been replaced. In contrast to UT and UTC are ET and TDT strictly uniform time scale, the basic unit of TDT is the SI second and a day is always exactly 86,400 seconds long.
The entry time for astronomical events is therefore calculated as a rule, in TDT. To be able to specify the local conditions for the observation of the Earth's surface, but the precise current rotation angle of the Earth's rotation is taken into account. This is, for example, during solar eclipses need to be able to indicate which places on earth are swept by shadows. To this end, the present in TDT calculation result in UT or UTC must be converted, for which the forecast made at that time for use is.
Current value and forecast future values
The current value for can be determined using published by the International Earth Rotation and Reference Systems Service ( IERS ) Earth rotation parameters:
The 32.184 seconds, the constant difference between TDT and the International Atomic Time ( TAI ). The difference between TAI and Coordinated Universal Time ( UTC) is the number of the inserted date in UTC leap seconds ( since 1 July 2012 and until the next leap second: 35). The contribution of the last term part is less than one second, it is the difference between the Polschwankungen considered variant of Universal Time (UT1 ) and UTC, which is also called DUT1. The current and forecasted values for this published the IERS
The following approximate formula is used for the calculation of 2050:
This is the value for the date as an annual value and is determined as follows:
Historical values for can be determined from the comparison of traditional with modern observations recalculations. Useful observations go back until about the year -700. With the invention of the telescope in the early 17th century, the observation accuracy increased sharply, so that it is possible from this point determine more accurate.