# Heliocentric Julian Day

• Sastronaut

#### Sastronaut

Hi pf! I am working to better understand eclipsing binaries but to do so I need to understand a heliocentric Julian day; what it is and how it is advantageous for analyzing eclipsing binaries...could someone please explain to me how the Helio JD works and why its beneficial to use!? thanks!

I have. I am still a little bit puzzled by it...

In what way?

okay so the Heliocentric Julian Date (HJD) is the Julian Date (JD) corrected for differences in the Earth's position with respect to the Sun...but how is this beneficial when studying close orbiting eclipsing binary stars?

Say you take a few observations. Then you take a few more a month later. When you try to combine the data, you will find that the timing is off because of the extra distance the light has had to travel now that we are a month further along in our orbit. Using a heliocentric frame for timing, or better yet a baryocentric, would eliminate this timing error.

## What is Heliocentric Julian Day?

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.

## How is Heliocentric Julian Day calculated?

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.

## What is the difference between Heliocentric Julian Day and Julian Day?

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.

## Why is Heliocentric Julian Day important in astronomy?

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.

## How is Heliocentric Julian Day used in conjunction with other time systems?

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.