When you take a long exposure photo of the sky at night, the result is many circular trails of light surrounding a central star (Polaris). I still can’t wrap my head around how this seemingly remains a constant with all the different orbital motions (spin, rotation, tilt, wobble) that earth is continously going through.
I know it’s said that Polaris hasn’t *always* been the North Star or center of rotation in the sky, and that it supposedly shifts over many hundreds of years. But, how is it possible to remain constant for *any amoung of time* with us spinning around a wobbling axis at around 1000 MPH, while we rotate around the sun, and the entire solar system is rotating? Shouldn’t that mean there are three different axises of rotation and the center of any star trail photo would be changing daily if not minute-by-minute?
I have also heard that this phenomena remains constant because the stars are just *too far* away (trillions of miles) for the movement to be discernable/ noticable.. which makes even less sense to me. -If you attached a laser pointer to a gently rotating, wobbling object, and aim it at a very close surface, the amount of movent of the beam on the surface might be very minimal/ negligable. But, if you aim it at a very distant surface (trillions of miles away) the amount of movent will be exponentially more significant. The same should be true for a fixed camera lens perspective, especially over the course of hundreds of years.
So I guess what I’m saying is; how does our axis of spin continuously align with Polaris while that axis is also on a wobble, and that wobbling axis of earth is rotating within an also-rotating solar system, AND while everything in the cosmos is constantly expanding?
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So First: Why does it appear stationary. It isessentially above the north pole. The earth spins, so when you look north, you’re looking along the axis it spins.
It’s like standing in your house under a ceiling light, and spinning. The light will appear stationary while the rest of the room spins around it. Same reasoning here.
Now, as for why it doesn’t wobble a lot: The earth is like a top. As a top spins, it’s fairly stable. It doesn’t really wobble that much. And the larger the top, the more smooth the spin, the less the wobble.
A top with a mass of 0.3 kg (~1 pounds) mass spinning at 20 revolutions per second will “precess” or wobble, a full circle every 2 seconds. Double the mass, and the wobble takes 4 seconds. Halve the spin rate, and the wobble decreases (from 2s, down to 1s). The faster you spin, the less precession (this is why tops wobble more when they slow down).
The earth doesn’t spin fast, only 0.00001 rotations per second. So 2s is reduced to… 0.00002 seconds of precession. THat’s really unstable… but the earth is also insanely massive. The earth has 6,000,000,000,000,000,000,000,000 kg of mass…
End result (with some other factors like angle of tilt etc) is it takes 23,000 years for the wobble to go full circle, and as it does it sweeps past and between many many other stars. And it does cover a pretty wide swath of sky as shown on the wikipedia supplied map: [https://en.wikipedia.org/wiki/Pole_star#/media/File:Precession_N.gif](https://en.wikipedia.org/wiki/Pole_star#/media/File:Precession_N.gif)
So it’s not fixed to Polaris at all. That star just happens to be where our spin axis is pointed right now. And the wobble you worry about is present…it’s just spanning thousands, not hundreds, of years. Other stars were used as pole stars during recorded history (two more minor stars in the big dipper served this role up till year 300.
But for the duration of a single night’s photos…not really an issue.
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