Why does the night sky appear to move slowly even though the earth is rotating at 1,000mph?
In: Earth Science
Because the stars and moon and planets are very (very very very) far away. 1000mph is nothing when you’re dealing with distances well in excess of 250,000 miles (the closest object, by far).
Stars are blurred from the motion, 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.
Because mph is the wrong unit. The earth is rotating at 0.0007 rpm, so the stars appear to rotate at the same rate. The minute hand of a clock has an angular velocity 24 times faster than the stars (0.017 rpm), but it still isn’t fast enough to be able to see the movement.
Because the earth is so big. Even though we are rotating at a high speed, it still takes a decent amount of time to spin a few degrees. It takes 24 hours to spin 360 degrees, so 15 per hour, .25 per minute. Imagine spinning a basketball at a rate of once per day.
It’s 24 000 miles around the earth, so 1 000 mph is not that fast by comparison. The stars only have to move slowly across the sky to do one lap in just under a day.
The other answers are not wrong, but there is another aspect to this: our brains and sensory organs have evolved to perceive time, and thus motion, within a certain range. Things that are moving faster, beyond the upper limit of our perception, are blurry, invisible, or just happening too fast for us to react to or even notice at all. Likewise, things that are below the lower limit seem to be motionless and unchanging. We call those things “slow”, but it’s only slow to us; what we think of as a small, slow-moving creature may perceive the same phenomena as fast.
Anyway, if noticing the motion of the stars was important to our survival, we’d have that ability. It wasn’t, so we don’t.
I’ll explain it with a mama joke:
“Yo mama is soooo bigggg that when she spins through space at 1000MPH, the very edge of her belly seems to be moving much slower”
Parallaxing. It’s the same reason a town in the distance when looking out your car window doesn’t move as much unless it’s close up.
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If you go to a formula1 race and stand by the track, the cars fly by at 100s of MPH and you hardly see them, now, if you go stand on a hill and watch that same race, those cars are much easier to track.
With that way of thinking, think about how far those stars are away from us. That’s how I think if it.
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