Pole Star

[tansy]

I just read something interesting (to me) about the pole star.

It hadn't occurred to me, though perhaps if I'd thought about it, it should have done, that the pole star changes every so often. I'd just assumed it was always the one that we can go by and easily find with the naked eye today. I think the article said that we will have a new one in 2040 or thereabouts.

Anyhow, this got me wondering, how did they find out thousands of years ago which was the pole star? How did they know what was north? Obviously I know that one can figure out roughly north, south, east, west etc from the position of the sun in the daytime, so did they just remember which direction the sun was at a certain time of day and then see which star closely correlated to that at night? Does anyone have an answer? Also did they realise that magnetic north (at least at the moment is more or less lined up with true north)...though I know that at some stage the magnetic poles flipped.

I think the ancient astronomers were amazing, figuring everything out like they did, especially without the aid of telescopes

Star of the week: Thuban | EarthSky.org

 

[USincognito]

The ancients didn't know Polaris was "north", just that it was fixed in the heavens (as far as they knew) and thus was useful for navigation.

We won't have a new north pole star for more like 15,000 years and the last time the magnetic pole shifted (which is different from the celestial pole) as like 700,000 years ago.

 

[tansy]

The ancients didn't know Polaris was "north", just that it was fixed in the heavens (as far as they knew) and thus was useful for navigation.

We won't have a new north pole star for more like 15,000 years and the last time the magnetic pole shifted (which is different from the celestial pole) as like 700,000 years ago.

Interesting though that the pole star is more or less magnetic north. Is that co-incidence or is it somehthing to do with the way all the heavenly bodies are aligned, having an influence on each other? (I don't know, perhaps magnetically or whatever other things might influence the positioning, orbit and rotation of stars, moons, planets etc.)

 

[Larniavc]

Interesting though that the pole star is more or less magnetic north. Is that co-incidence or is it somehthing to do with the way all the heavenly bodies are aligned, having an influence on each other? (I don't know, perhaps magnetically or whatever other things might influence the positioning, orbit and rotation of stars, moons, planets etc.)

It's coincidence.

Stars influence each other by gravity. But this influence falls off dramatically (what with it being inversely proportional to the square of the distance between them.

But it is cumulative.

 

[tansy]

It's coincidence.

Stars influence each other by gravity. But this influence falls off dramatically (what with it being inversely proportional to the square of the distance between them.

But it is cumulative.

Thanks, but am not quite understanding what you mean by 'cumulative'? I understand what you mean by influence falling off inversely proportionately, but not the cumulative bit.

 

[Larniavc]

cumulative

It means that you can normally add up all the mass to work out the total gravity (unless there is too much mass and you have to use general relativity to work it out).

This might help.

The gravity of gravity — Einstein Online

 

[tansy]

It means that you can normally add up all the mass to work out the total gravity (unless there is too much mass and you have to use general relativity to work it out).

This might help.

The gravity of gravity — Einstein Online

Ah, thank you. I see what you mean now, makes sense. Thanks for the link also, skimmed through, but will look at it more thoroughly when it's not half eleven at night and my brain's functioning better!

 

[Larniavc]

Ah, thank you. I see what you mean now, makes sense. Thanks for the link also, skimmed through, but will look at it more thoroughly when it's not half eleven at night and my brain's functioning better!

I know what you mean. It's 11:34 here too so as soon as I finish my glass of wine I shall be retiring.

 

[JackRT]

The earth rotates on its axis just like a gyroscope, and exactly like a gyroscope it precesses. That is to say, the axis of rotation does not always point in the same direction but this direction of point changes in a regular way over a period of time. If I remember correctly, for the earth that period is about 23,000 years. This has been confirmed by numerous astronomical and historical observations.

 

[USincognito]

Interesting though that the pole star is more or less magnetic north. Is that co-incidence or is it somehthing to do with the way all the heavenly bodies are aligned, having an influence on each other? (I don't know, perhaps magnetically or whatever other things might influence the positioning, orbit and rotation of stars, moons, planets etc.)

Mostly coincidence that a star is in position relative to the earths axis and magnetic north. There could just as easily be the same for the south pole, but the southern polar star is too faint to be useful for navigation. It's all pretty cool though.

Fun fact - Polaris has a visual magnitude of 1.97 (46th brightest) but has an absolute magnitude of -3.64. Sirius, the brightest star other than the sun, is just the opposite with -1.46 visual magnitude and 1.4 absolute.

 

[Radrook]

The earth rotates on its axis just like a gyroscope, and exactly like a gyroscope it precesses. That is to say, the axis of rotation does not always point in the same direction but this direction of point changes in a regular way over a period of time. If I remember correctly, for the earth that period is about 23,000 years. This has been confirmed by numerous astronomical and historical observations.

Here is an illustration about precession

Precessional movement of Earth. Earth rotates (white arrows) once a day around its rotational axis (red); this axis itself rotates slowly (white circle), completing a rotation in approximately 26,000 years
By NASA, Mysid - Vectorized by Mysid in Inkscape after a NASA Earth Observatory image in Milutin
Milankovitch Precession., Public Domain, File:Earth precession.svg - Wikimedia Commons

 

[Astrophile]

I just read something interesting (to me) about the pole star.

It hadn't occurred to me, though perhaps if I'd thought about it, it should have done, that the pole star changes every so often. I'd just assumed it was always the one that we can go by and easily find with the naked eye today. I think the article said that we will have a new one in 2040 or thereabouts.

This is completely wrong. Polaris will be the nearest bright star to the pole for more than a thousand years. In fact it will be at its closest to the true celestial pole in September 2101. By the year 3150 the star gamma Cephei (Errai) will be nearer than Polaris to the north celestial pole, but gamma Cephei is only third magnitude, not as bright at Polaris.

Anyhow, this got me wondering, how did they find out thousands of years ago which was the pole star? How did they know what was north? Obviously I know that one can figure out roughly north, south, east, west etc from the position of the sun in the daytime, so did they just remember which direction the sun was at a certain time of day and then see which star closely correlated to that at night? Does anyone have an answer?[

They probably knew that most of the stars rise and set in the course of a night, and that constellations near the pole, such as the Great Bear, Cassiopeia and the Dragon, move in circles without rising or setting. It would then be fairly easy, by observing over several nights, to judge the position of the centre around which all the stars were moving, and to see which star was the nearest to this central point.

Also did they realise that magnetic north (at least at the moment is more or less lined up with true north)...though I know that at some stage the magnetic poles flipped.

I don't know. I think that the Chinese had magnetic compasses during the Han dynasty (206 BC to 220 AD), so they should have known that the compass pointed approximately towards the north pole.

I think the ancient astronomers were amazing, figuring everything out like they did, especially without the aid of telescopes.

Yes, I agree with you; they were amazing.

Star of the week: Thuban | EarthSky.org[/QUOTE]

 

[Radrook]

Polaris is the central star in this time-laps photo. Our Earths northern pole is tilted towards it and it seems to remain stationary as earth rotates on its axis' while the other stars appears to circle it.

A typical Northern Hemisphere star trail with Polaris in the center
By Ashley Dace, CC BY-SA 2.0, File:Star Trail above Beccles - geograph.org.uk - 1855505.jpg - Wikimedia Commons

Another thing about Polaris is that it isn't one star.

Polaris is a multiple star, comprising the main star (Polaris Aa, a yellow supergiant) in orbit with a smaller companion (Polaris Ab); the pair in orbit with Polaris B (discovered in 1780 by William Herschel). There were once thought to be two more distant components—Polaris C and Polaris D—but these have been shown not to be physically associated with the Polaris system.

This artist's concept shows: supergiant Polaris A, dwarf Polaris Ab, and the distant dwarf companion Polaris B.
By NASA/ESA/HST, G. Bacon (STScI) - http://hubblesite.org/newscenter/newsdesk/archive/releases/2006/02/image/fTransferred from en.wikipedia to Commons., Public Domain, File:Polaris system.jpg - Wikimedia Commons

 

[essentialsaltes]

Interesting though that the pole star is more or less magnetic north. Is that co-incidence or is it somehthing to do with the way all the heavenly bodies are aligned, having an influence on each other?

There's a connection, but not so grand as all that. Both the celestial pole and the magnetic pole are really artifacts of the earth. The earth spins on its axis, and if you project the line of the axis out into space, it points fairly close to Polaris, the pole star.

Although there is still a little mystery about the origin of earth's magnetism, it would seemingly have to do with the giant molten iron core of the earth, which spins more or less the same way the earth as a whole does. Like a giant bar magnet, the iron core has a 'north' and 'south' pole. And because it rotates similarly to the way the earth as a whole does, the magnetic poles line up reasonably well with the geographic poles.

 

[morse86]

Polaris proves a stationary (and non globe) earth especially when you have contradictions in the red shift. It puts a dent in the big bang theory. Also, some planets spin the opposite direction. Another dent.

I think it's all bad science propped up by $$$ funding. You see a light in a sky, that is all. You cannot even accurately measure the distance using trig. All of the "telescopes" are digital...CGI renderings is all you see.

 

[essentialsaltes]

Polaris proves a stationary (and non globe) earth

The reason why people in Australia can't see Polaris, and people in Norway can't see Canopus, is that their heads are pointing different directions when they look up. Because the thing they stand on -- the earth -- is not flat.

especially when you have contradictions in the red shift.

Two crackpot theories does not add up to one solid theory.

 

[Shemjaza]

Polaris proves a stationary (and non globe) earth especially when you have contradictions in the red shift. It puts a dent in the big bang theory. Also, some planets spin the opposite direction. Another dent.

I think it's all bad science propped up by $$$ funding. You see a light in a sky, that is all. You cannot even accurately measure the distance using trig. All of the "telescopes" are digital...CGI renderings is all you see.

Then why can't we see it in the southern hemisphere?

 

[tansy]

Thanks for everyone's replies....for some reason I haven't been getting notification of them. No time to read them properly now as it's late at night. Will look at them properly when I've more time.

 

[Radrook]

Then why can't we see it in the southern hemisphere?

So how do they usually respond to this very poignant question?

 

[Steve Petersen]

The earth rotates on its axis just like a gyroscope, and exactly like a gyroscope it precesses. That is to say, the axis of rotation does not always point in the same direction but this direction of point changes in a regular way over a period of time. If I remember correctly, for the earth that period is about 23,000 years. This has been confirmed by numerous astronomical and historical observations.

Yes, we spin on our axis, but our axis also slowly wobbles. This is why we have the precession of the equinoxes.