Why does the night sky appear to move slowly even though the earth is rotating at 1,000mph?


Why does the night sky appear to move slowly even though the earth is rotating at 1,000mph?

In: Earth Science

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 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).

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.

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.

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.


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.

look at a star with a fixed telescope and you will see how fast the star moves out of view.

Same reason why flying at 30,000 ft the earth below looks like it’s barely moving despite you travelling at 600mph.

Mph isn’t really relevant when your only frame of reference is the insanely distant stars/moon/sun.

You’re travelling 24,000 miles around the earth at 1000mph so the entire reference background of the sky only moves 1/4 of a degree above you every minute.

If you positioned a 100% fixed rigid telescope to a foundation you could only catch stars in your line of sight so briefly (depending on how powerful your zoom is)

Its like when you look out the side windows in a moving car. The stuff further away appears to move slower.

Ever watch a plane 5 miles in the sky whizz past hour field of vision going 300 mph? No? Me neither

Just like the view from one side of your car appears to be moving slower than the other when you look out of the window. Distance.

This has a lot to do with the relative frame of reference.

When you are in a vehicle you are in motion, however because you are inside it appears that everything around you is moving. This is why we don’t feel like we are hurling 1000’s of mph through space.

We also perceive closer objects to us, as moving fast than distant objects. Such as roadsigns vs mountains. Roadsigns seems to whip by on the highway, but restaurants and houses, other landmarks appear to pass us much slower.

Everything is miles and miles away. Thank a commercial aircraft flying between 460 – 575 mph airspeed through the air because of the distance from the viewer who is stood on the earth the aircraft would look slower due to the lack of reference points up in the atmosphere. Being an airspeed of 500 mph in the air would equate to a ground speed of around just 400 mph due to the curvature of the earth so there are more miles between point to point the higher up you go.

To get a reference stand on the edge of a merry go round and move it so that it makes a rotation once a day.
Very slow.

But make it more relatable.
Take a watch with a second hand put in on the floor and walk around the watch at the same rate as the second hand.
How far did you walk?
Now go back to the merry go round and spin it at 1 rotation a minute
Walk around it at the same rate.
How far did you walk.

The bigger it is the more you walk and the faster you have to walk even though it rotating at the same rate as the watch.

They are far? You really are 5

Think about the hands of a clock. The closer to the center, the less distance the arm travels compared to the point of the hand, we know this because the circumference at the center is smaller than the outer edge, but the hand moves the same overall degree. While we are spinning at 0.0007 rpm(or .252 degrees per minute, pull out the protractor), the stars do not. So every degree we spin, the stars will appear to move. The reality is we aren’t watching the stars move, but realizing the shift in our own point of origin in comparison to rest of the universe.