# Why North Pole always points to the North Star?

Discussion in 'Astronomy, Exobiology, & Cosmology' started by kingwinner, Sep 18, 2005.

1. ### kingwinnerRegistered Senior Member

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796
Why North Pole always points to the North Star throughout the year?

Why earth's Geographic North Pole always points to the North Star throughout the year? The earth is revoluting around the sun so the Geogrpahic North Pole should point at different locations at times of summer and winter...it just can't be the some location...

For example, when you hold a pencil vertically and revoluting in big circles, each moment your pencil is pointing a differnt location...

Can someone briefly explain why? Thank you!

Last edited: Sep 19, 2005

3. ### PeteIt's not rocket surgeryModerator

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Hi Kingwinner,
The apparent motionlessness of the Pole Star tells us that the spin axis of the Earth always points in the same direction.

Grab a pencil, and draw some circles on the page. Notice how the pencil is always pointing in the same direction as you move it around? That's how the Earth's axis is.

5. ### kingwinnerRegistered Senior Member

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796
I understand that the earth's axis is pointing the same DIRECTION, but half year later, the earth has revoluted to the other side (and that is a huge distance from now, almost 300 million km away), so the earth's POSITION has changed and it should be pointing at another place while still pointing the same direction...

Then, why would the earth be pointing the North Star all the way throughout the year?

7. ### geodesic"The truth shall make ye fret"Registered Senior Member

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Polaris is approx. 390ly distant, as opposed to the relatively miniscule 16 light minutes which is the diameter of the Earth's orbit. Although there is some parallax, it is not noticeable night-to-night.

Image from APOD

Click for a larger image - the smallest circle is Polaris.

8. ### kingwinnerRegistered Senior Member

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796
Thanks for geodesic's explanation!
Another person explains that "The diameter of Earth's orbit around the Sun is very tiny compared to our distance from Polaris & so makes the position of Polaris relative to us appear stationary" which is the same meaning as yours!

9. ### kingwinnerRegistered Senior Member

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796
All stars, including Polaris and our Sun, are stationary, right? Then why would "red shift" occur, meaning that almost all galaxies (a system of stars) are MOVING away from the earth? Why are stars moving, which contradicts the first sentence?

10. ### geodesic"The truth shall make ye fret"Registered Senior Member

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Not quite. Firstly, our galaxy is rotating, so that even the stars in our galaxy are not really stationary with respect to each other.
Redshift is caused by the expansion of the Universe - on the local scale (ie. our galaxy), gravity can overcome this expansion. However, distant galaxies do separate from us due to this expansion. Redshift is also principally a large scale phenomenon.

11. ### kingwinnerRegistered Senior Member

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796
Red shift also means that systems of STARS are MOVING away from us...
How come stars are moving? So the Sun is moving? I was told that the Sun is stationary and the planets orbit around the Sun...

12. ### Prince_JamesPlutarch (Mickey's Dog)Registered Senior Member

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Kingwinner:

Stars are moving because they, like the galaxy itself, move around the gravitational centre of the galaxy. Over a course of hundreds of millions of years, the galaxy revolves completely around one time. Galaxies move because all energy/matter in the universe is in motion from the Big Bang and the inertia of the material has never been significantly slowed. Inertia is based on the fact that all things set in motion, tend to stay in motion, unless acted upon by another force. Inertia propels the bullet through the air until gravity forces it to collide into the ground. It is also why one cannot break a car, a bicycle, or even stop running immediatly.

13. ### James RJust this guy, you know?Staff Member

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Our sun makes a complete circle around the centre of our galaxy, the Milky Way, about once every 200,000 years at a speed of about 200 km per second. All the planets of our solar system orbit with it, but also have their own motions around our sun. For example, Earth orbits the Sun at a speed of about 30 km per second, and takes 1 year to make one circuit. Our galaxy itself is also moving relative to other galaxies, at a speed much higher than 200 km/s.

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15. ### James RJust this guy, you know?Staff Member

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I've just checked, and you're right. I remembered the number wrong. It ought to have been 200 million years rather than 200 thousand years. Sorry.

16. ### Prince_JamesPlutarch (Mickey's Dog)Registered Senior Member

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James R:

Not a problem.