Heliocentric Julian Day (HJD) is a system of time measurement used in astronomy. It is based on the number of days that have passed since January 1, 4713 BC at 12:00pm (noon) in the Julian calendar.
HJD is calculated by adding the Julian Day (JD) value to the difference between the local sidereal time and the right ascension of the Sun. This calculation takes into account the Earth's rotation and orbit around the Sun.
The main difference between HJD and JD is that HJD takes into account the Earth's orbit around the Sun, while JD only measures time based on the Earth's rotation. In other words, HJD is a heliocentric system, while JD is a geocentric system.
HJD is important in astronomy because it allows for more precise and accurate calculations of planetary positions and orbits. It also allows for the comparison of data from different observatories and telescopes around the world.
HJD is often used in conjunction with other time systems, such as Universal Time (UT) and Terrestrial Time (TT). This allows for the conversion of HJD values to different time systems and ensures consistency in astronomical calculations and observations.