HJD

Heliocentric Julian Date (HJD) is a time standard used in astronomy that represents the Julian date corrected to the time a photon would have been observed at the center of the Sun. The correction accounts for the finite speed of light and the motion of the Earth around the Sun, removing timing variations due to Earth's changing position and perspective.

Calculation: The adjustment is computed from the observer’s position relative to the Sun and the direction to the observed object. A common form is t_HJD = t_JD + (r · ŝ)/c, where r is the vector from the Sun to the observer, ŝ is the unit vector toward the source, and c is the speed of light. The resulting HJD is the time at the Sun’s center corresponding to the original observation time. In practice, many datasets provide HJD values computed from standard solar-system ephemerides.

Relation to BJD: The Barycentric Julian Date (BJD) is a closely related quantity that uses the Solar System's barycenter as the reference point rather than the Sun’s center. For most routine work the difference between HJD and BJD is small, but it can be important for high-precision timing.

Applications and notes: HJD is widely used in time-domain astronomy for folding light curves of variable stars, exoplanet transits, and pulsars, where removing Earth’s orbital timing variations improves phase coherence. Users should be aware of the convention used (heliocentric vs barycentric, JD vs MJD) and the availability of relativistic and clock corrections in modern ephemerides.