Barycentric Julian Date

The Barycentric Julian Date (BJD) is the Julian Date (JD) corrected for differences in the Earth's position with respect to the barycentre of the Solar System. Due to the finite speed of light, the time an astronomical event is observed depends on the changing position of the observer in the Solar System. Before multiple observations can be combined, they must be reduced to a common, fixed, reference location. This correction also depends on the direction to the object or event being timed.

In 1991, the BJD replaced the Heliocentric Julian Date (HJD), which reduced times to the centre of the Sun, which itself orbits the barycentre. The difference between HJD and BJD is up to ±4 s.

The correction is small for objects at the poles of the ecliptic. Elsewhere, it is approximately an annual sine curve, and the highest amplitude occurs on the ecliptic. The maximum correction corresponds to the time in which light travels the distance from the barycentre to the Earth, i.e. ±8.3 min (500 s, 0.0058 days).

JD and BJD are defined independent of the time standard. JD can be expressed as e.g. UTC, TT, TAI, TDB, etc. The differences between these time standards are of the order of a minute, so for better than one-minute accuracy, the time standard must be stated. While many quote the BJD in UTC, UTC is discontinuous and drifts with the addition of each leap second, and should therefore only be used for relative timing over a short time span (~1 year). For high-precision, absolute timing, TDB should be used. However, applications for which ±1.7 ms precision is sufficient may use TT to approximate TDB, which is much easier to calculate.

Neglecting effects of special and general relativity, the correction of Terrestrial Time (TT) is

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