Heliocentric Julian Date (HJD) is the same as the Julian Date (JD), but adjusted to the frame of reference of the Sun, and thus can differ from the Julian Date by as much as 8.4 minutes (507 seconds or 0.00587 days), that being the time it takes the Sun's light to reach Earth. Instants of astronomical events occurring outside our Solar System are usually specified in HJD instead of JD since otherwise the measured time instance depends on the observer's position in the Solar System due to the finite speed of light.

The HJD depends not only on the local time (such as JD) but the celestial position of the observed object or event and of the Sun. Near the poles of the ecliptic (those are RA = 18:00, Dec = +67°34' and RA = 06:00, Dec = −67°34'), the HJD is always very close to JD. If the Sun's celestial coordinates are (λ₀,φ₀) and the coordinates of the observed object or event are (λ,φ), and if the distance between the Sun and Earth is d, then

HJD = JD − (d/c) [sin(φ) sin(φ₀) + cos(φ) cos(φ₀) cos(λ−λ₀)]

where c is the speed of light. Since the Earth's orbit is nearly circular, the ratio d/c is roughly constant at 0.005775 days, but depending on the season, it can be anything between 0.005678 and 0.005872 days.

[edit] References

• Hirshfeld, A., Sinnott, R.W. (1997). Sky catalogue 2000.0, volume 2, double stars, variable stars and nonstellar objects, p. xvii. Sky Publishing Corporation (ISBN 0-933346-38-7) and Cambridge University Press (ISBN 0-521-27721-3).