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Motion of the Sun

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The Sun is in motion, just like any other star.

Motion of the Sun Relative to Local Stars

First, the Sun and the other stars in its vicinity partake of the general rotation of the galaxy (the Milky Way Galaxy rotates once about every 225 million years). This corresponds to an average velocity of about 220 km/s. The space velocities that we measure for other stars then correspond to deviations from this average motion for the stars around the Sun. This happens because the Sun and the stars near it are on somewhat different orbits around the center of the galaxy, so at any one time the Sun is overtaking some stars and being passed by others.

The Solar Apex and Antapex

This motion of the Sun with respect to the local field of stars is in the direction of an imaginary point in the constellation Hercules, near the bright star Vega. This point is called the solar apex, and the Sun is moving toward it (relative to the nearby stars) at a net speed of about 19.7 km/s. The point on the opposite side of the sky from which the Sun appears to be moving away is called the {\em solar antapex}.

Thus, every second we move about 20 km closer to the star Vega. However, there is plenty of time before we get there: Vega is 26.5 light years away! As an exercise, calculate how long it will take the Sun (and therefore the Earth) to travel 26.5 LY at a speed of 20 km/s.

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