# Orientation of Earth at J2000

• I
• TheGalaxyOfGold
In summary, the individual is seeking calculations for the orientation of Earth and the position of the moon at J2000.0 for their earth, moon, sun system model. They can acquire this information from the JPL Horizons website and do not need to worry about the small difference between Universal Time and Terrestrial Time.

#### TheGalaxyOfGold

Hi there,

I am making a very basic model of an earth, moon, sun system in ECEF in C# using the Helix Toolkit and in order to begin transformations according to Earth's rotation, tilt, and orbit, I need a reference orientation/position/time of Earth with respect to the sun and moon.

So my question is, where can I acquire calculations for the orientation of Earth at J2000.0 as well as the position of the moon with respect to Earth at this same epoch. For instance, which of Earth's longitudinal lines is most-directly facing toward the sun at J2000.0 and given that orientation, on which of these longitudinal lines belonging to Earth is the moon most closely sitting directly over?

How accurate do you want to be? Since J2000.0 is defined at noon Universal Time, longitude 0 degrees is facing the sun. However, this is not quite true because of the difference between Universal Time and Terrestrial Time. How deeply do you want to get into these issues?

As for the position of the moon, the JPL Horizons web site will give you the position of any solar system object at any time.

jim mcnamara
I could just convert from UT to TT with a simple conversion equation, can't I?

I don't think accuracy is imperative, as long as its accurate too within a degree I should be fine.

TheGalaxyOfGold said:
I could just convert from UT to TT with a simple conversion equation, can't I?
I don't think accuracy is imperative, as long as its accurate too within a degree I should be fine.

I think so. This Wikipedia site says the difference is only 64 seconds ( about .25 degrees), so if one degree is your level of accuracy I just wouldn't worry about it.

thank you very much sir!

berkeman