How to see the rotation of the Earth

[Brian G Turner]

From:

Your world, unmoored

A time-lapse video set to follow the stars displays an unnerving view of the Earth moving, not the sky.

www.syfy.com

In most time-lapse videos of the night sky, the camera is fixed relative to the Earth, to the horizon, and — as they do to our senses, should we take notice for long enough — the stars in the sky move seemingly inexorably from east to west. Instead, though, Brummel turned the frame around, keeping the camera fixed on a spot in the sky, and, moreover, rotating the camera to match the sky’s orientation. Thus, as it more truly would be from outside our planet, we see the Earth’s motion, the horizon rotating as the Earth spins.

 

[-K2-]

Hmm... I'm not really sure what I think about that. I understand the viewpoint of it. The imagery is spectacular. But, it really isn't hitting home regarding showing our world turning vs. the universe around it. Perhaps an even wider/farther back view? Even still, the same image reversed (Earth static, sky moving)--knowing that the Earth is rotating--is enough for me. On the other hand, for something I'm fleshing out, I need to think in terms of the universe revolving around the Earth to convey the protagonists limited beliefs.

So, it helps give me something else to consider in that regard. Thanks for posting it!

K2

 

[tinkerdan]

Although that does demonstrate rotation.
I've always found that the time-lapse images exposed on a single frame from a fixed camera seem to do so just as well, by stretching the light of the stars reminding me a bit of Van Gogh's Starry Night.

 

[tegeus-Cromis]

Well, actually that doesn't really show rotation. It shows the Earth as a tilting plane. The image is rotating around the center of the screen -- so around a point on the surface of the Earth, and not around its central axis at all. The video doesn't actually show what it claims to be showing.

 

[J Riff]

Yep, what's hard to grasp is the speed, one thou mph. or whatever it exactly is. When orbiting you can go with it, against it, or acrost it and get a different look each way.

 

[StilLearning]

Well, actually that doesn't really show rotation. It shows the Earth as a tilting plane. The image is rotating around the center of the screen -- so around a point on the surface of the Earth, and not around its central axis at all. The video doesn't actually show what it claims to be showing.

It's a 2d representation of a 3d reality, but it is 3d, regardless of how it's presented. In 3d rotations don't happen around a point, they always happen around an axis, by definition. And motions are always relative, so it doesn't change the nature of the motion if you keep the ground steady in the frame or the sky steady in the frame.

If you pick a point on the sky, in the video, and follow it, it's path is curved wrt the ground (and visa versa, and I just did this on my tablet with a dry Wie marker) - so either the ground or sky is rotating. So the video is showing rotation, which must be happening around an axis - and that doesn't change whether you take the Earth or sky as still.

Where that axis runs through I'll take on faith, as I don't relish hunting down the photographer and taking detailed location and pointing information from them to work it out properly myself.

But I will point out that if you really wanted you could pick two widely separated locations, repeat this experiment simultaneously in both places, work out where the axis of rotation was precisely and check the results against each other.

 

[StilLearning]

The image is rotating around the center of the screen -- so around a point on the surface of the Earth, and not around its central axis at all

I should also point out that if you do trace the motion with a pen you'll definitely see it's not centred on the centre of the screen - sorry if that was a bit of a patronising response, being a former physics teacher I have a certain knee jerk reaction.

 

[Elckerlyc]

The motion may not be centered on the center of the screen, but not by a very great distance. And no where near the axis on which the Earth actually is spinning. What is missing in this video is the suggestion of movement caused by the rotation.
When I'm sitting in a train and look out of the window I see the landscape moving by. No one needs to show me a film of a train to tell me it it is the train (and I) that is moving, not the scenery. This shot is nice but pointless.
Perhaps the proper and only point on Earth's surface to get a proper idea of it's rotation is on the South- or North Pole and looking straight up.

 

[StilLearning]

Really? Because when I did this the centre of the rotation was not only a long way from the centre of shot, I'm fairly sure it was outside the shot entirely Edit: On reviewing and using a ruler (yes I'm that kind of person) it actually looks like it's inside the shot, but only just End Edit. Have we misunderstood each other? I'm tracing the motion of one of the distant house lights. The arc it describes is definitely not centred anywhere near the centre of shot. It looks like it's centre is off or near the bottom of the shot, which makes sense as its centre is the direction the Earth's axis points in, and Fonts point is in the Anza-Borrego Desert near the equator.

Point being - for a simple geometric problem like a spinning ball surrounded by arbitrarily distant stars it doesn't matter which is spinning, the axis of movement is the same. So as long as the camera is still wrt the ground or the sky (either one) the shot has to be spinning around the axis of the Earth. And that's what this video does show: It's still with respect to one point in the sky, and taking that as your reference it's the Earth that's spinning. But the axis, and the centre of rotation if you look at it in a 2d format like any screen (and the human eye generally) is the same both ways.

It's only when you get into details that this video doesn't really show - like the motion of planets - that it becomes much simpler to use a model of a spinning Earth rather than a spinning sky. For these purposes - finding the axis - it doesn't make much difference.

 

[StilLearning]

I suppose what I'm saying is: As long as its been set up to keep the same patch of stars in shot, in the same orientation WRT the frame, the shot is still WRT the stars. That's as valid a definition of still as still WRT to the ground. But either way - whether you see the ground or the stars as spinning - the axis of rotation is the same. So on the video the spot around which things spin will be the same. If you trace out the motion on the screen to get a segment of circle, and then work out where that centre is with a ruler the centre of rotation is right down the bottom of the frame, where you'd expect it to be when viewed at that latitude. So I'm happy the video shows the Earth's rotation, and only that.

WRT the train: It's simpler to assume the train is moving, yes. But when you take you seat on the train, and eat a sandwich or a drink a drink (assuming the train's speed is constant and its motion is fairly straight) you instinctively assume that you eat and drink sat still, and it works. Both the reference frame where you're doing a steady 70 to get to Wigan station, and the reference frame where you're sat still in your seat to eat are vali even though, in the one where you're sat still to eat, the ground is moving at 70.

The camera is set up to approximate a still reference frame WRT the stars. Hence the Earth seems to move. And if you trace its motion you can see on the screen it is a rotation, centred on a point well below the horizon, as you'd expect for a shot at that latitude.

Clear as mud?
.