Our entire galaxy rotates about its center. The spiral arms revolve in a clockwise direction as seen from the north galactic pole and the velocity of revolution of stars about the center of the galaxy will decrease with increasing distance from the center. This is also true for the planetary orbits in the solar system; the far-out planets take longer to circle the Sun than do the inner planets. This is called differential rotation.

Figure B. will help to illustrate differential rotation. Stars or gas clouds that were lined up at one time (points 1) are spread out by the time they have gone 1/4 of the way (points 2) or about 3/4 of the way (points 3) around the galaxy. The small diagrams show the net velocity that stars or gas clouds at different distances from the GC would have with respect to our Sun. Objects within the Sun's orbit around the GC are orbiting faster than does the Sun and objects farther out than the Sun's orbit are orbiting more slowly than does the Sun. Our Sun complete one revolution about the GC in some 200 million years. The Sun was last on this side of the milky way center (with respect to the universe of external galaxies) at about the time that small dinosaurs were beginning to develop on the Earth's surface. It has moved through an angle of about 120° of its orbit since the last great dinosaurs vanished. Altogether, our Earth and Sun have completed only about 20 to 25 revolutions around the GC since their formation some 5 billion years ago.

Copyright (c) 1997-99 Michael Erlewine