Why is light from stars not blurred across the sky?

Say light from the stars you see at night has traveled billions of years to reach you. When you look at the stars, all are seen as a point of light.

It seems to me that given the constant movement of the stars and our planet that each point of light we call a star might be light from the previous position of a star seen elsewhere in the sky.

Just like if you swirled a flashlight around quickly, the flashlight makes what looks like a circle of light.

The difference is the stars and our planet are constantly moving but the light from the moving stars stays pretty much constant to our perception. Not swirling.

3 Answers

  • Anonymous
    7 years ago
    Favourite answer

    It's because you look away before the swirl can become visible. If you point a camera north and leave the shutter open for a while you will see circles. Like this: http://apod.nasa.gov/apod/ap130303.html

  • 7 years ago

    Stars are blurred across the sky. The blur is just so small because our eyes have such a short exposure time and low spatial resolution that our naked eyes cannot pick it up very well. But high power telescopes that have long exposure times and high spatial resolution pick up the blur just fine. In fact, telescopes have to deal with this motion blur if they want a crisp picture. The dominant motion of the stars is in circles around the north and south celestial poles once a day due to the rotation of the earth, so the stars blur in circular arcs along this path:


    Telescopes compensate for this motion blur by tracking along with the stars. If stars traveled faster, then the blur would be more apparent to the naked eye. Since the dominant motion is caused by earth's rotation, the earth would have to spin faster to make them blur more. The earth spins once a day. Your flashlight spins once every tenth of a second or so. If the earth spun once every tenth of a second, then the stars would look as blurred as a spinning flashlight and we would all vomit from motion sickness.

  • 7 years ago

    It's because they aren't moving that quickly relative to how far away from us they are. They are zooming through the galaxy at amazing speeds, but they are so far away that we don't notice just by looking at them for a few minutes.

    It is the same principle as when you are driving through a tunnel in a car. The tunnel walls are really close and seem to go by really fast. But if you don't change your speed when you exit the tunnel, you'll notice that trees in the distance don't "move" quite as fast as the tunnel walls did.

    I hope that makes sense!

Still have questions? Get answers by asking now.