# Geo Helio and RA

please assist me , i'm using Zet 9 and there is Geo longitude , Helio longitude and RA , what is the difference between them.

example :

                     Mars
Date        Time     HLng       Lng        RA
01.01.2000  0:00:00  359.12584  327.57547  330.14521


some books use geo long. and the RA interchangeably as if they are the same thing , but they are not.

please help , thanks in advance.

## 2 Answers

Those three different values are measured using different coordinates systems and from different locations:

• Helio longitude (HLng) is the ecliptic longitude as seen from the Sun. The calculation of the position of solar system bodies always begin with the heliocentric ecliptic coordinates (longitude and latitude) because these object orbit the Sun. The heliocentric coordinates then get converted to other coordinate systems.
• Geo longitude (Lng) is the ecliptic longitude as seen from the Earth (Geo), but otherwise, the coordinate system is the same. Based on the heliocentric coordinates of Mars and Earth, the geocentric longitude can be calculated. Naturally, the parallax of observing from the Earth causes the values to be different.
• RA is the right ascension as seen from the Earth. The ecliptic longitude and latitude are measured along the ecliptic; the right ascension and declination are measured along the celestial equator (which is a projection of the Earth's equator). Based on the 23.5 degree (approximate) inclination of the ecliptic, the geocentric ecliptic coordinates can be converted to right ascension and declination.

Ecliptic longitude is measured in the plane of the Earth's orbit, while right ascension is measured in the plane of the Earth's equator. Their values are different in general but coincide in a few special cases. Both are 0° at the vernal equinox, where the two planes intersect.

Geocentric longitude λ and heliocentric longitude l originate from different points of view but are relative to the same reference direction. In this diagram, longitude 0° is up and 90° is to the left.

Solar System Simulator image courtesy NASA/JPL-Caltech, annotated by MG