Barycentric Julian Date

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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.