The speed of the observatory will be v = (equatorial velocity) * cos(latitude) If we define the earth's angular velocity relative to the stars as 2 pi radians 2 pi radians -5 rad omega = --------------- = --------------- = 7.29212 x 10 --- 23h 56m 4s 86164 sec sec then we have v = (radius of earth) * omega * cos(latitude) = 465 m/s Now, this motion is in the plane of the Earth's equator. The maximum possible component along the line of sight to the star depends on the star's Declination -- which is its angle away from this plane. max radial v = v * cos(Dec) = (465 m/s) * cos(Dec) 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 Earth's center to our observatory: that angle is called the HOUR ANGLE, or HA for short. radial v = (max radial v) * sin(HA) = (465 m/s) * cos(Dec) * sin(HA)