Transcript: The reflex motion of stars caused by planets that orbit them has the effect of creating a slight wobble, but it also has a second important consequence, a Doppler effect. Jupiter, for example, causes the Sun to wobble as Jupiter moves in its orbit in a twelve year period. The distance that Jupiter moves the Sun in twelve years can be converted into a speed or Doppler shift of the Sun as it wobbles; it’s 13 meters per second, about the speed of a car. For a smaller planet further away the leverage is less, and the speed is slower. Uranus on its own would create a Doppler shift of 0.3 meters per second in the Sun. The Earth is closer to the Sun but a much smaller mass, so its leverage is even less. The Doppler effect caused by the Earth on the Sun is only 0.09 meters per second, about the walking pace of an ant. The Doppler effect cannot always be detected because geometry comes into play, as with eclipses or transits. In general, the Doppler effect will not show unless the motion of the planet is in the plane of the observation. For example, if we were staring down on the orbits of the solar system all of the motions would be perpendicular to our line of sight, and we’d see no Doppler effect. On average, astronomers will see some fraction of the full size of the Doppler shift.