Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: greensleeves on 03/09/2014 20:47:10

Title: Why does the axis of Earth's rotation always point towards the Pole Star?
Post by: greensleeves on 03/09/2014 20:47:10

I wonder if anyone can help with an explanation of this. Summer and winter, the stars in the northern night sky always rotate around the Pole Star, Polaris. This, as I understand it, is because the Earth's north-south axis is by coincidence pointing more or less directly at that star. But given that the tilt of the Earth's axis in relation to the Sun varies quite considerably (giving us our seasons) why is it that the tilt of the Earth's axis does not apparently vary in relation to Polaris? I would have expected that the axis of Earth's rotation/spin would change with the seasons, altering the point in the sky around which other stars appear to revolve.

Presumably the reason is that the tilt is always pointed in the same direction and it only differs in respect of the Sun because at different times of the year we are on different sides of the Sun, but why in that case does the Earth's tilt effectively never change?

I hope I have made my question clear enough to understand. Equally, I hope someone can provide an explanation simple enough for me to understand!  [:)] Thanks.

Title: Re: Why does the axis of Earth's rotation always point towards the Pole Star?
Post by: CliffordK on 03/09/2014 21:40:01

You've answered some of your questions.

Take a pencil with a fixed tilt axis (with respect to the room), and slowly move it around a basket ball and you'll see essentially the effect of the axial tilt and the seasons. 

Now, if the lead of the pencil is pointed towards the ceiling, then as you move around the basketball, the point on the ceiling that your pencil is pointing to will vary by a few inches, or the diameter of your circle around the ball.

However, in the case with Earth and Polaris, Earth's rotation around the sun has a diameter of about 17 lightminutes.  On the other hand, Polaris is about 434 lightyears away, much further, and the actual angle is very small.

Over longer periods of time (decades, centuries, or millenia), there is a slight wobble of earth's axis and we may eventually have a different "North Star".