Do the stars move? Seems like a foolish question. Of course they move. Look at them now, then look again a couple of hours later. They are not where they were before. Look at them now, and look six months later at the same time. The stars and constellations you saw before aren't even there. They have been replaced by a different set. Furthermore, if we look at ancient star charts, one to two thousand years old, and compare the same day of year and time of day, the locations of the constellations in the sky have dramatically shifted. Yes, the stars move. But do they really?In fact, most of the apparent movements of stars by the hour, by the season, by the millennium, are not the movements of stars at all. They are the movements of the Earth!
- The apparent nightly dance of the stars across the sky is caused by the spinning of the Earth. The Earth spins around once every 24 hours, causing the stars and Sun to appear to move across the sky.
- The apparent seasonal march of the constellations across the sky is caused by the Earth's revolving around the sun once a year. The direction we face at night slowly changes during the year as the Earth travels in its long orbit around the Sun.
- The changes in position of constellations over thousands of years is caused by the Earth's wobble. Like a spinning top, the Earth slowly wobbles as it spins, taking 26,000 years to wobble around once. That wobble, called precession, slowly changes the direction we face at night, and therefore, over thousands of years, changes the stars we see on a given day of the year, at a given time.
Does that mean the stars don't really move? Not at all.
The stars do move. They move as individuals relative to each other, they move as members of clusters relative to other clusters, and they move as members of galaxies relative to other galaxies. But stars are so far away that we don't see these movements during our lifetimes, or even during the passage of several thousand years. What we see is a mere snapshot in the lifetimes of stars and galaxies.
To see the real movement of stars we would need to watch over periods of tens of thousands and hundreds of thousands of years, of millions of years. Stars move, in their own direction at their own speed. Litters of stars that are "born" or created from the same cloud of interstellar gas tend to travel through space together, as clusters, until each begins separating from the others, going off in its own direction. Many spend their "lives" as pairs or triplets revolving around each other as they move together through space. Stars contract, expand, and at times end their existence in cataclysmic explosions. Even the constellations, or familiar patterns we see in the sky, will look totally different hundreds of thousands of years from now because of the movements of the stars that make up these patterns. The galaxies spin slowly, like giant pinwheels, taking hundreds of millions of years to make a complete revolution. Galaxies even at times "collide", or actually pass through each other, profoundly changing each other's nature and structure. Clusters of galaxies continue to move apart from each other as the universe keeps expanding.
Do the stars move? YES! They surely do. Their movements are the grandest dance of all.
Back to Star Facts Home Page
Send suggestions, astronomy tips, corrections or comments to Pete Harris
via e-mail at odyssey@ccnet.com. Pete is also the creator of the website
The Child
Abuse Prevention Council of Contra Costa County