The skies reflect our spinning world

Bad Astronomy
The entire universe in blog form
Oct. 11 2011 9:57 AM

The skies reflect our spinning world

We live on a spinning ball, rotating madly as it moves through space. Once every day the surface of our planet makes a circuit around the imaginary line connecting its poles... well, imaginary it may be, but the effects are quite real, especially when you take long exposures of the night sky. That's what photographer Brad Goldpaint did, and created this lovely time lapse video he calls Breaking Point:

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  


[If you go to the Vimeo page for the video you can watch it in HD, which you really need to do, as well as make it full screen.]

Amazing, isn't it? The visual of the stars wheeling around the sky over our head invokes such a wonderful feeling, as if the whole Universe is spinning around us. But it can also be a little odd-looking too. For example, take a look at this picture Brad composed using some of the images he crafted into the video, which he has singled out and called Delineated:

[Click to siderealate.]

Strange, isn't it? For one thing, it isn't one long exposure, but instead composed of 60 short exposures added together. If you squint you might see streaks of light, but in reality those arcs are composed of individual dots, the images of stars frozen as they moved across the sky.

It's also a bit odd due to the fuzzy glow at the bottom. That's actually the smeared-out light from the Milky Way galaxy as it rose into the frame. Not being a point-like source of light like stars, it has a dreamier, fuzzier quality. Again, from the video, here's a single exposure from that series:

[Click to galactinate.]

See? Brad calls this picture Second Chances. There's the Milky Way, and all those stars. And the bright spot to the right is Venus, which made that brilliant trail in the first picture as it rose. If you have sharp eyes -- and know your sky -- you can spot a few other familiar objects in the sky: Deneb, Vega, Altair, and more.

But to me, in that first picture, perhaps the oddest thing is how on the left the stars appear to curve one way, and on the right they curve the other! Why is that?

It's because Brad was able to see quite a bit of the sky at once. He was facing East when he took this shot, so all the stars you see here are rising. If you look off to the left, toward the north, you see the stars are circling the north pole of the sky (marked closely by the position of the star Polaris, which is off-frame here). Those stars appear to be moving counterclockwise. If you look to the right (south), the stars are circling the south pole of the sky, and appear to be moving clockwise! Why is that?

Think of it this way: imagine you're in a car, driving on a road going through a forest. If you face left, looking out through the driver's window, the trees appear to pass you from your right (the front of the car) to your left (the back of the car). Now turn around and look out the passenger window: trees move from your left to your right! Directions reverse because you're facing the other way. The same is true for the sky, so while rising stars appear to move counterclockwise when you look to the north, they appear to move clockwise when you face the south.

For setting stars the directions are all reversed, but I'll leave that to you to straighten out in your head.

Perhaps the hardest thing to explain to people about astronomy are the motions of stars and other objects in the sky. It helps, I think, to go outside and experience it for yourself, picturing yourself clutching the surface of a spinning ball as it moves through space. If you do that a few times, the motions become apparent, and start to make sense (as they have to!).

There is something beautiful about seeing all these movements and understanding them. It's hard to put into words, but it's a sense of completeness, of having all the parts fit together seamlessly and utterly, like cogs in a vast and layered Victorian clockwork mechanism.

But it's better than that: it's nature, and it's all out there for you to see.

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