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11.2.2 Astronomical Time Scales

Various time scales used in astronomy are related as follows:


physically realized

offset; observed, nominally +32.184 seconds

terrestrial time

rate adjustment (L_G)

part of the definition of TT


time scale for the Geocentric Coordinate Reference System (GCRS)

semi-periodic terms

These terms depend on the masses and positions of the bodies in the Solar System. There is an annual term of approximately 1.66 ms, plus planetary terms of up to about 20 μs, plus lunar and diurnal terms of up to about 2 μs. Included in dtdb.

rate adjustment (L_C)

function of solar-system ephemeris


time scale for BCRS

rate adjustment (-L_B)

part of the definition of TDB


TCB scaled to track TT. Depends on solar-system ephemeris. The relationship is currently fixed, but might need to be adjusted in the future to the order of 1 ns per century.

semi-periodic terms

Included in dtdb.


terrestrial time