Heliocentric Julian Date

Calendar sets UT or JD


h m s

Star for which HJD is required

(from Simbad)

OR enter coordinates directly
(any reasonable format):

A table will build here, selectable with mouse for copying to a spreadsheet

For variable stars of short period there is a non-neglible time-scale variability of period 1 year due to the Earth's orbit about the Sun. Although the radius of the Earth's orbit is only 8.3 light-minutes, this nevertheless advances or retards the timing of observations by up to that amount. The effect ranges from a maximum for stars on the ecliptic to zero for any star at either ecliptic pole.

Ephemerides of variable stars are therefore specified for UT as if the observer were at the Sun, and are said to be given in "heliocentric time". A correction must be applied to observation times to allow for the Earth-Sun light-time component in the direction of the star. The resulting corrected time is often referred to as Heliocentric Julian Date (HJD) but this is misleading because it implies a universal time scale instead of being specific to the star being observed.

The present page contains a calculator for both HJD and the difference (dt) between HJD and JD. It is necessary to specify three inputs to the calculation: the required JD (or, for convenience, date/time in UT on a calendar) and the particular variable star's celestial position, its Right Ascension and Declination. Strictly speaking the position is for epoch J2000.0 but the difference for the current year is negligible.

There is a "morsel" about Julian Dates and how they are computed.

Script written by Graham Relf (www.grelf.net)