# How to calculate Earth's velocity vector in ECEF?

I would like to write some AR mobile app that will show Earth velocity vector, but I have no any experience with astrodynamics. So how to calculate Earth's velocity vector in ECEF, having date and time as input data?

• Two questions: velocity relative to what? And how accurate do you need/want to be? Apr 27, 2016 at 8:55
• Velocity relative to Sun. Great precision is not required. And I don't need actual magnitude of velocity vector, only direction. Apr 27, 2016 at 9:40
• Do you mean how fast the Earth is revolving around the Sun?
– GdD
Apr 27, 2016 at 9:41
• Yes, ECEF mean Earth Centred, Earth Fixed. And, yes Earth is not moving relative to itself, but Earth velocity vector is moving in that reference frame, because of Earth's rotation and because of Earth is flying around Sun. And I'd like to know how to calculate that vector in ECEF. Apr 27, 2016 at 13:00
• Actually, because the reference frame is Earth-fixed the center of the reference frame is the Earth itself, meaning it's not moving or rotating in the ECEF reference frame. In ECEF the sun is moving relative to Earth and the center of Earth is always (0, 0, 0) Apr 27, 2016 at 15:00