How does Polaris (North Star) remain the center of star trail photos?

223 views

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?

In: 3

7 Answers

Anonymous 0 Comments

The axis wobbles a tiny amount. Polaris is the largish star closest to being in the right place, today. Over the centuries, it will wander off and another star will be best.

Polaris isn’t a special star, it’s just nearest to a special place.

Anonymous 0 Comments

The Earth’s axis is pointed directly at Polaris. If you stood at the North Pole, Polaris would be directly over your head, all the time.

The confusion you might have is that the tilt of Earth’s axis does *not* change over the course of a year. As the Earth orbits the Sun, the poles continue to point in the same direction. And that of course is why we have seasons, because in January the North Pole’s tilt is pointed away from the Sun and in July it’s pointed toward the Sun.

The axis does “wobble,” in that it spins around like a top losing its momentum, but this effect happens over many thousands of years, and is almost unnoticeable within a human lifetime.

Anonymous 0 Comments

When a wheel spins, what part of the wheel remains stationary?

The center.

To put this in more general terms, in a spinning perspective, anything sitting along the axis of rotation will not appear to move. We on Earth are a spinning perspective. The star happens to be in line with the axis of rotation.

Anonymous 0 Comments

Have a friend stand directly in front of you, and wave a stick back and forth. The tip of that stick is going to swing wildly from one edge of your vision all the way to the other.

Now have them stand at the opposite end of a football field from you, and wave the stick. The tip of the stick will barely be moving at all now, and will pretty much always be in the same area of your field of view.

Now have them stand approx 2,000,000,000,000,000 miles away, roughly the distance to Polaris. They could be swinging the stick back and forth miles at a time and it would still be absolutely stationary as far as you’re concerned, because it’s such a small portion of your field of vision that motion doesn’t even make a difference anymore.

Anonymous 0 Comments

It’s because earth’s axis (line through the center of the earth about which it rotates) from out of the North Pole is pointed at it (roughly….there is a small degree of error). If you were on the geographical North Pole and looked up, it would be almost exactly overhead.

Anonymous 0 Comments

The star is too far away to even notice even with all those orbital phenomena happening at the same time.

Instead of using a rotation motion example I will use a linear motion, but the principle is equivalent.

Watch [this video](https://www.youtube.com/watch?v=oHMTEdCEUG4) of a bullet train travelling at high speeds in Japan. Notice how close objects such as the railway walls and houses seem to pass by much faster than the mountains in the horizon. Near objects seem to move at much faster speeds than those far away. Now if there was a moon in the sky it would look as if is wasn’t moving at all.

Now going back to rotational motion.

The same concept applies when the Earth is rotating around its axis. Let’s make an analogy: Imagine an ant resting on the upper half of a spinning disco ball hanging by a thread from a ceiling. If the ant looked at the exact point where the thread is attached to the ceiling it would look as if it is stationary and the rest of the room is spinning around it. If the ant could take a long exposure picture of the room it would be the same as us taking a picture from the sky during night time, the point where the string attaches to the ceiling (Polaris) would look stationary and the rest of the room (the night sky) spins around.

The effects you mentioned don’t really matter because they are are much much slower than the rotation of the Earth. Earth’s rotation = 1 day, Earth’s orbit = 1 year, Earth’s wobble/tilt change = tens of thousands of years.

Anonymous 0 Comments

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.