### Terrestrial Time

**(TT)**
(current measure of time for astronomical observations on Earth)

**Terrestrial Time** (**TT**) is a Time Standard designed for describing
astronomical observations from Earth, that allows simple arithmetic
to determine time intervals, precise to the millisecond level.
Certain observations require that time precision, e.g., observations
of moving planets and stars and observations of Pulsar pulses
and glitches.
The J2000.0, B1950.0, and B1900.0
are defined in terms of Terrestrial Time.

Such a time standard is non-trivial and *Terrestrial Time* is the
latest standard by the International Astronomical Union to provide one. Other time standards
are also used, e.g., to compensate for Earth's orbit: light from a nearby
star viewed from Earth will reach Earth earlier by minutes if
Earth is currently at the point of its orbit closer to the star.

Terrestrial Time is based on the SI second, and a point in
time was chosen when it matches other standards, but it "drifts"
from other standards.

(*coordinate,time*)
http://en.wikipedia.org/wiki/Terrestrial_Time

**Referenced by:**

Time Standard

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