The speed of the Earth in its orbit is v(orb) = (orbital velocity) = (2 pi * 1 AU) / (1 year) 11 9.3997 x 10 m v(orb) = -------------------- = 29,800 m/s 7 3.15576 x 10 s Now, this motion is in the plane of the Earth's ORBIT (not its equator). The maximum possible component along the line of sight to the star depends on the star's ECLIPTIC LATITUDE 'b'. max radial v = v(orb) * cos(b) = (29,800 m/s) * cos(b) At any moment, we can find the exact value of the radial component if we know the azimuthal angle between the target star and the vector running from the Sun to the Earth; that angle depends on the ECLIPTIC LONGITUDE 'l' of the star and of the Earth at this moment. azimuthal angle = (ecl long of star) - (ecl long of Earth) The ecliptic longitude of the Earth isn't usually listed, but we can use instead the fact that ecl long of Earth = (ecl long of Sun - 180 degrees) And so azimuthal angle = (ecl long of star) - (ecl long of Sun - 180 degrees) So, finally, we can determine the radial velocity component of the Earth's motion relative to the star: radial v = - (max radial v) * sin(azimuthal angle) = - (29,800 m/s) * cos(b) * sin(azimuthal angle)