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Every star in the sky has "proper motion", meaning its own motion in some direction, besides the apparent daily motion that's due to the rotation of earth.

Why don't we see any effects of this motion ? The answer is very simple, but still mind-boggling.

We don't see the effects of a star's proper motion during many human lifetimes because the stars are so far away. Any more than you could see a snail running for all he's worth across the road, 100 miles away.

Here are a few examples of the distances involved in space, coupled with the limitations of human perception:

-- The nearest natural body to the earth is the moon. Its distance from earth is about [1/4 of a percent] of the distance to the sun ! On an astronomical scale, we can practically reach out and grab the moon. Now, the moon is moving more than 2,000 miles per hour in its orbit. But we can look right at it and never notice that motion ... even though it's the nearest body in the universe to earth.

-- Now consider the nearest star to earth (aside from the sun). It's 4.4 light-years away from us ... more than 100 MILLION times as far from us as the moon is, and that's the NEAREST star to us.

If that star moved across our line of sight at the SPEED OF LIGHT, it would take 14 days to move the apparent size of the full moon.

If it moved across our line of sight at a speed of 1 million miles per hour, it would take almost 26 years to move the apparent size of the full moon.

And that's the NEAREST star to us !

That's why we don't notice the proper motion of the stars. If anybody is left in several thousand years from now, and if they have our records of how the sky used to look in the 21st Century, then they'll see the results of the motion of the stars. But we see essentially the same sky as the earliest humans who left us any records.

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15y ago

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