What is the speed of the Solar System?

(by Amara Graps)

Or, how fast is the Sun (Solar System) hurling towards the constellation Hercules?

From the book: _Guide to the Galaxy_, 1994; Henbest and Couper; Cambridge University Press.

The Sun is moving towards Lambda Herculis at 20 kilometers per second or 12 miles per second. Or in units "per hour": 72,000 kilometers per hour or 45,000 miles per hour. This speed is in a frame of rest if the other stars were all standing still.

The three-dimensional picture of the Sun's movement through the Galaxy is a little more complicated.

The Sun is moving upwards, out of the plane of the Milky Way, at a speed of 7 kilometers per second. Currently the Sun lies 50 light-years above the mid-plane of the galaxy, and its motion is steadily carrying it further away.

But the gravitational pull of the stars in the Galactic (Milky Way) plane is slowing down the Sun's escape. The astronomer Frank Bash estimates that in 14 million years the sun will reach its maximum height above the Galactic disk. From that 250 light-year position, it will be pulled back towards the plane of the Galaxy. Passing through, it will travel to a point 250 light-years below the disk, then oscillate upwards again to reach its present position 66 million years from now. We crossed the plane 2 million years ago. We are currently in the thick of the galactic disk and our view of distant regions is largely blocked by dust but 10-20 million years from now, our motion will allow a full view of our starry galaxy.

The Sun-Galactic center distance is 25,000 light-years -- plus or minus 2,000 light-years. The galaxy is thought to be 100,000 light-years in diameter and we are thought to be about halfway out from the center (used to be thought that we were two-thirds out).

If you have a good idea of the Sun's distance from the Galactic center, then the solar system's speed can be approximated. Using speed measurements of the gas at different distances from the Galactic center, the Sun appears to be cruising along at 200 kilometers per second and it takes 240 million years to complete the grand circuit around the Galaxy. This speed is an absolute speed.


1 light-year = 9.46*10^{15} meters
1 parsec = 3.08*10^{16} meters

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Special Thanks to A. Graps