Astronomy: Measuring Earth's Motion

Astronomers have long known pre cisely how fast the earth moves as it cir cles around its own star, the sun (66,500 m.p.h.), and how fast the solar system revolves around the core of its own galaxy, the Milky Way (481,000 m.p.h.).

They have even been able to compute the astounding speed with which the Milky Way moves around the center of a supercluster of 2,500 neighboring galaxies (1,350,000 m.p.h.). Yet the earth and the Milky Way are also in motion with respect to the vast reaches of the universe itself.*

While scientists have often speculated about this universal speed,...