Heliocentric Julian Day

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

It is important to note that the HJD itself does not depend only on the local time (such as JD) but the celestial position of the observed object or event. 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 apparent celestial coordinates are $(lambda\_0,varphi\_0)$ and the coordinates of the observed object or event are $(lambda,varphi)$, the distance between the Sun and Earth is $d$, then $\{\; m\; HJD\}=\{\; m\; JD\}-(d/c)sin(varphi)sin(varphi\_0)+cos(varphi)cos(varphi\_0)cos(lambda-lambda\_0)$, where $c$ is the speed of light. Since the orbit of Earth is nearly circular, the ratio $d/c$ is roughly $0.005775$ days, but depending on the season, can be any between $0.005678$ and $0.005872$ day.