Why does Polaris appear stationary on a rotating Earth?

Trailblazer

Moderator
Staff member
The thing with Polaris I wonder is how can I take timelapse with it being stationary from europe? If stars are spinning this makes sense, but if earth is spinning even tho camera is on a tripod it should be changing angles all the time. Stationary polaris timelapse makes sense to me personally only to be taken from the very north pole. I know the argument that polaris is very far away and thats why it's possible to do this.. but I don't buy this.. because earth spin changes the angles drastically and that movement of earth should be detected in a way that stationary polaris appears to be moving as angles change.. polaris should be all over the place if camera is the one thats moving.
I might be wrong but thats how I see it.

Polaris appears (almost) stationary because it is very close to the celestial north pole. And that is what makes it unique: it is the one point at which viewpoints converge, from all around the (northern half of the) globe, because it corresponds to the axis of rotation. In the northern hemisphere, if you point a camera northwards and angle it upwards at the same angle as your latitude, it will always point to the same spot in the sky. That's what the celestial north pole *is*! Get yourself a globe and you can confirm this for yourself - however the globe rotates, the camera will always be pointing "straight up" in this set-up. In other words, the camera direction is parallel no matter where in the northern hemisphere it is, or where in the rotation.
 
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Trailspotter

Senior Member.
Polaris appears (almost) stationary because it is very close to the celestial north pole. And that is what makes it unique: it is the one point at which viewpoints converge, from all around the globe, because it corresponds to the axis of rotation. If you point a camera northwards and angle it upwards at the same angle as your latitude, it will always point to the same spot in the sky. That's what the celestial north pole *is*!
Also, there is the celestial south pole right in the opposite direction (southwards and downward at the same angle as your latitude. Although you cannot see the patch of the sky around the celestial south pole from the northern hemisphere, you can readily deduce its position by taking a time lapse video or a long exposure photo of the southern sky at the starry night.

So you can readily verify the presence and identify location of the celestial poles with just a camera and some simple geometry. It will show you that the Earth does appear "enclosed in the celestial sphere" with the "fixed" stars on it. If you conduct the celestial poles' observations at different latitudes, you will find, that the Earth is a globe at the centre of the "celestial" sphere, the conclusion drawn by ancient astronomers many centuries ago. And they did not actually need a camera, they just measured the angular positions of the stars in the sky at different times with simple tools.
 
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Trailblazer

Moderator
Staff member
This is what I mean @Abishua.

At each location in the northern hemisphere the angle between the ground and the camera direction is equal to the latitude (red lines). You can see that the camera direction is always "straight up", in other words the lines are parallel to each other in space. That means the cameras are all looking in the same direction.

Technically speaking, the exact direction of the cameras are separated by a maximum distance of the Earth's diameter, on any given day, and the diameter of the Earth's orbit through the year, but that distance is entirely negligible compared with the distance to (and size of) Polaris itself.

(This is also another argument against the Earth being flat and stars being close by: if the diameter of the Earth was large compared to the distance to the star, then the angle wouldn't be the same for all observers, there would be a parallax error.)

polaris.jpg
 
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Mick West

Administrator
Staff member
The pole star is actually NOT stationary if you look close enough. It's about a degree off from perfect north in the celestial sphere. Here's a close up of the region. Polaris is the brghtest star, but there's also some faint stars closer to the center.
20170505-072519-mr0jy.jpg
Image source: http://www.emporia.edu/physci/planetarium/

And here's a much longer exposure, closer up. The bright arc is polaris:
20170505-072823-d0ibc.jpg
Image source: https://en.wikipedia.org/wiki/Circumpolar_star

This is something you could probably replicate with a camera like the Nikon P900, although you are limited to 15 second exposures, so you won't get much motion near the center - might be worth at go though.

You can also make star trail photos by taking multiple exposures - or even post-processing a time-lapse movie.
 
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Amber Robot

Active Member
(This is also another argument against the Earth being flat and stars being close by: if the diameter of the Earth was large compared to the distance to the star, then the angle wouldn't be the same for all observers, there would be a parallax error.)

On the round Earth, Polaris would be at the horizon when viewed from the equator. In the flat earth model typically shown, it would be quite a bit higher in the sky. It's nearly trivial to show this and completely disproves the flat earth model.

Check out the photo of startrails around the north celestial pole taken from the equator (see http://sguisard.astrosurf.com/Pagim/From_pole_to_pole.html).SGU-From-pole-to-pole-North-1200x800-cp8.jpeg
 

Henk001

Senior Member.
@Abishua: Imagine the Earth' rotating axis is a needle exiting the north pole and punching an imaginary sphere, on which the stars appear to be attached, in a place we call the North Celestial Pole. Now rotate the Earth and you will see that all stars will apparently rotate the other way round except for this NCP. Of course there is no such celestial sphere, it is an imaginary "sphere of directions". Every star has a different distance to the Earth, f.i. Polaris at 432.6 lightyears (as we know from its yearly parallax). That is more than 323 billion times the diameter of the Earth. But even if it was much closer you would still be looking at that stationary "punchhole".
 

Abishua

Member
Mick, I have one thing that's bothering me about polaris. If earth has a wobble how does polaris manage to stay directly above north pole trough the year?
 

Rory

Senior Member.
If earth has a wobble how does polaris manage to stay directly above north pole through the year?
It doesn't stay directly above the north pole. In fact, at some point in the future, Polaris won't be the north star at all.

A little googling should answer your questions.
 
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Spectrar Ghost

Senior Member.
Earth does indeed have a "wobble". Its axial precession has a period of about 26,000 years though, so the effect is minimal on human time scales.
 

tinkertailor

Senior Member.
I learned about this in elementary astronomy, and my star charts that I was required to use for the class even show the path the star will take. The large circle to the right is the path the North Star will take, with direction indicated.
 

Mick West

Administrator
Staff member
The large circle to the right is the path the North Star will take
Just to be clear, this is the path the north celestial pole with take. The current north star (Polaris) is not moving, but the point about which the stars will rotate is moving. It's kind of like a virtual North Star. In 13,000 years Vega will be the North Star, and all the stars will rotate around that.
 

tinkertailor

Senior Member.
Just to be clear, this is the path the north celestial pole with take. The current north star (Polaris) is not moving, but the point about which the stars will rotate is moving. It's kind of like a virtual North Star. In 13,000 years Vega will be the North Star, and all the stars will rotate around that.
Yeah, good catch! I always thought of it as a skewer stabbed through an orange, with the wide circle on the North celestial pole being what the skewer was pointing to as it wobbles. Now it coincidentally is pointing to the North Star but it won't be in the future.
 

Mick West

Administrator
Staff member
Can anyone explain why while the earth tilt and still Polaris remain in original north pole axis
The Earth rotates around the north-south axis, and this axis points at Polaris. The axis is tilted relative to the orbit around the sun, but not relative to Polaris.
(Image not to scale)


Same thing from a different perspective, just showing the directions


It's not perfectly fixed though and will drift over thousands of years
 

Mendel

Senior Member.
Can anyone explain why while the earth tilt and still Polaris remain in original north pole axis
when the Earth travels around the sun, its tilt does not change. It always looks in the same direction. (This is why we have summer and winter, because sometimes the north side of Earth is tilted towards the sun and sometimes the south side, as the Earth moves around the sun with unchanging tilt). If you walk a circle by taking a step forward, a step left, a step back, and a step right, you are also always looking in the same direction. That is what the axis is doing when travelling around the sun (except on a true circle, not just 4 steps).
 

Loki Thorson

New Member
Looking through a reticule of an equatorial mount would give a clue as to Polaris's movement through the day very near the celestial pole. The reticule will have a cross in its centre that represents the celestial rotational axis in the centre of a large circle. The position of Polaris will be in a smaller circle on the perimeter of the larger circle. The idea is to use the reticule to polar align your scope, the mount will have setting circles on the declination and right ascension axis, you set your date and time on these setting circles by moving both axis to match, look through your reticule and adjust your mount to get get Polaris in the small circle.

Here is a tutorial on how this is done and what the he reticule looks like


https://astrobackyard.com/polar-alignment/
 
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