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Dr. Christopher S. Baird

Why are stars not blurred even though the earth is moving?

Category: Space      Published: May 17, 2013

star trails
Stars do experience motion blur due to earth's rotation. Our eyes are fast enough that the blur is too small to notice. Public Domain Image, source: U.S. National Park Service.

Stars are blurred when viewed from earth. We just don't notice it much with our naked eyes because of the high temporal and low spatial resolution of the human visual system. Take a glow stick, sparkler, or flashlight and spin it very quickly in the dark. You don't see an individual object anymore, but see instead a circular blur of light. The reason for this is that the spin rate is faster than the rate at which our eyes can discern distinct images. The distinct images of the object get smeared together in a process known as motion blur. So why don't the stars look like spinning glow sticks? The rotation of the earth does cause the stars to spin in the sky, but the spinning is much slower. Whereas it takes the stars one day to trace out a circular path in the sky, it takes tenths of a second for the glow stick to spin in a circle. Our eyes can mostly keep up with the motion of the stars because they move slowly, but they cannot keep up with the motion of the spinning glow stick. There is still motion blur on the stars, it is just much smaller than that of the glow stick because they are moving much slower. The motion blur of the stars seen by the naked eyes is small enough that the relativity low spatial resolution of our eyes cannot detect it. But using cameras or telescopes, the motion blur of the stars can be detected.

There are two ways to make the motion blur more apparent: 1) reduce the temporal resolution of your imaging system (increase exposure time), and 2) increase the spatial resolution of your imaging system (use a higher powered telescope). For astronomers and telescope hobbyists, motion blur is usually undesirable, as the blur reduces the clarity of the image. High power telescopes therefore use either short exposure times or tracking systems to minimize the motion blur of stars. Tracking system keep track of the location of the star of interest, and keep moving the telescope to ensure the star stays at the same spot in the field of view.  Because the earth spins around its North Pole-South Pole axis, the stars in the sky appear to follow circular paths around these poles. There are other relative motions of the stars due to earth's orbit around the sun, the solar system's orbit around the galaxy, etc., but these effects are much less significant than the motion due to earth's rotation.

Topics: astronomy, blurred stars, motion blur, resolution, star, stars, telescope, tracking