Heliocentric Julian Day
The Heliocentric Julian Date (HJD) is the
Magnitude and limitations
The correction is zero (HJD = JD) for objects at the poles of the ecliptic. Elsewhere, it is approximately an annual sine curve, and the highest amplitude occurs on the ecliptic. The maximum correction corresponds to the time in which light travels the distance from the Sun to the Earth, i.e. ±8.3 min (500 s, 0.0058 days).
JD and HJD are defined independent of the time standard. Rather, JD can be expressed as e.g. UTC, UT1, TT or TAI. The differences between these time standards are of the order of a minute, so that for minute accuracy of timings the standard used has to be stated. The HJD correction involves the heliocentric position of the Earth, which is expressed in TT. While the practical choice may be UTC, the natural choice is TT.
Since the Sun itself orbits around the
The common formulation of the HJD correction assumes that the object is at infinite distance, certainly beyond the Solar System. The resulting error for
Calculation
In terms of the vector from the heliocentre to the observer, the unit vector from the observer toward the object or event, and the speed of light :
When the
where is the distance between Sun and observer. The same equation can be used with any
See also
References
- Eastman, Jason; Siverd, Robert; Gaudi, B. Scott (2010). "Achieving Better Than 1 Minute Accuracy in the Heliocentric and Barycentric Julian Dates". Publications of the Astronomical Society of the Pacific. 122 (894): 935–946. S2CID 54726821.
- A. Hirshfeld, R.W. Sinnott (1997). Sky catalogue 2000.0, volume 2, double stars, variable stars and nonstellar objects, p. xvii. Sky Publishing Corporation (ISBN 0-521-27721-3).
External links
- http://astroutils.astronomy.ohio-state.edu/time/: Online converter from UTC to BJDTDB, BJDTDB to UTC, or HJD (UTC or TT) to BJDTDB.