Why do constellations look the same after several years even though all the stars are moving? (Intermediate)

It is said that all stars are moving at different speeds and directions. Believable enough, but why, after being around for 73 years, do the stars of the Big Dipper appear to me to be in the same relative position to one another?

The motion of stars is quite small at a few or a few tens of km/s. However, they are situated several light years away from us. let us take an example. Let a star be situated about 10 light years away from us (note that this is a nearby star) and move at 10 km/s. Then, in 100 years, the movement is approximately 30 billion km. The distance of the star from us in comparison is 90,000 billion kilometers. So its motion in 100 years is so small compared to its distance that we see the star in the same spot in the sky. However, if one waits for a few hundred thousand years, then one can definitely see the constellations change.

This page updated on June 27, 2015

About the Author

Jagadheep D. Pandian

Jagadheep built a new receiver for the Arecibo radio telescope that works between 6 and 8 GHz. He studies 6.7 GHz methanol masers in our Galaxy. These masers occur at sites where massive stars are being born. He got his Ph.D from Cornell in January 2007 and was a postdoctoral fellow at the Max Planck Insitute for Radio Astronomy in Germany. After that, he worked at the Institute for Astronomy at the University of Hawaii as the Submillimeter Postdoctoral Fellow. Jagadheep is currently at the Indian Institute of Space Scence and Technology.