why are the stars circumpolar?

Question

http://en.wikipedia.org/wiki/Circumpolar_star

Is it an illusion caused by the magnetism at the poles?

Answer 1

The stars stay stationary from the perspective of Earth, and earth rotates on it's axis (which extends more or less from pole to pole), creating the illusion that the stars are rotating around the poles.

Try looking straight up at the sky and turning in a circle. You'll notice that the things you can see seem to 'rotate' around you. The same thing happens with stars as the Earth goes through it's daily rotations.

Answer 2

Stars are circumpolar because they just happen to be near one of the poles. Those stars are "visible" 24 hours a day because they never set! (Although when the sun is up you can't see them!) Not everyone sees the same circumpolar stars. The further towards the Equator you get, the less you will see. If you lived at the North Pole, almost every star in the Northern Hemisphere would be circumpolar.

At times even our own Sun is circumpolar when you are near the poles, resulting in 24 hours of sunlight for months at a time!!!

Mostly in North America...we see the asterism the Big Dipper, the asterism the Little Dipper, Draco the Dragon, and of course, Polaris.

Edit: As an added note, the circumpolar stars change!! Over a 26,000 year cycle the Earth's poles shift in a circular motion due to the precession of the Earth. So, the circumpolar stars of today, where not the circumpolar stars of the ancient Egyptions!!

Answer 3

Saying a star is circumpolar is like saying a its "in front of you." If you turn around its no longer in front of you, but that doesn't make it an illusion.

edit: (if you think that the other stars are orbiting around the North Star, I suppose that could be considered an illusion...)

Circumpolar-ness is based on which way earth is facing, (edit: and where we're located on it) and which way it's rotating. It has nothing to do with any local property of the star, and certainly is not caused by earth magnetism.

Answer 4

A circumpolar star is a star that, as viewed from a given latitude on Earth, never sets (that is, never disappears below the horizon), due to its proximity to one of the celestial poles. Circumpolar stars are therefore visible (from said location) for the entire night on every night of the year (and would be continuously visible throughout the day too, were they not overwhelmed by the Sun's glare).

As the Earth spins daily on its axis, the stars appear to rotate in circular paths around one of the celestial poles (the north celestial pole for observers in the northern hemisphere, or the south celestial pole for observers in the southern hemisphere). Stars far from a celestial pole appear to rotate in large circles; stars located very close to a celestial pole rotate in small circles and hence hardly seem to engage in any diurnal motion at all. Depending on the observer's latitude on Earth, some stars – the circumpolar ones – are close enough to the celestial pole to remain continuously above the horizon, while other stars dip below the horizon for some portion of their daily circular path (and others remain permanently below the horizon).

The circumpolar stars appear to lie within a circle that is centred at the celestial pole and tangential to the horizon. At the Earth's North Pole, the north celestial pole is directly overhead, and all stars that are visible at all (that is, all stars in the northern celestial hemisphere) are circumpolar. As one travels south, the north celestial pole moves towards the northern horizon. More and more stars that are at a distance from it begin to disappear below the horizon for some portion of their daily "orbit", and the circle containing the remaining circumpolar stars becomes increasingly small. At the Earth's equator this circle vanishes to a single point – the celestial pole itself – which lies on the horizon, and there are therefore effectively no circumpolar stars at all.

As one travels south of the equator the opposite happens. The south celestial pole appears increasingly high in the sky, and all the stars lying within an increasingly large circle centred on that pole become circumpolar about it. This continues until one reaches the Earth's South Pole where, once again, all visible stars are circumpolar.

The north celestial pole is located very close to the the North Star (Polaris), so, from the northern hemisphere all circumpolar stars appear to rotate around Polaris. Polaris itself remains almost stationary, always at the north (i.e., the azimuth is 0°), and always at the same altitude (angle from the horizon), equal to the latitude of the point of observation on Earth.

Answer 5

Here's an experiment for you to try that will demonstrate circumpolar behavior. Find a room in your residence that has a ceiling light fixture. Turn on the light, stand underneath it, and tip your head back so that the light is in the middle of your field of view. Slowly turn round in a circle, keeping your head tipped back. You should always be able to see the light, no matter which way your body is facing. The light is like a pole star in this case; it's directly in line with your axis of rotation.

Now step about a foot to the right. Tip your head back and stare at the ceiling right up over your head. Slowly turn round in a circle while you're staring at the ceiling. No matter which way you're facing, you should still have the light in your field of view. Even though you are no longer staring directly at the light, it never completely passes out of sight. Now, the light is like a circumpolar star. It is positioned close to your axis of rotation, so from your perspective, it travels around the "sky" and never sets. Circumpolar stars are positioned close to the Earth's axis of rotation, so from the perspective of someone on the ground looking up, these stars proceed round the sky in a circle, and they do not set. From our perspective, they appear to circle (circum) around the pole star (polar), hence the name "circumpolar."

Just for grins, take a few more steps away from the ceiling light fixture so that you're 5 or 6 feet away. Stare at the ceiling directly over your head and turn around slowly. Now, when you're facing away from the light fixture, you probably won't be able to see it, but when you turn and face it, the light will be somewhere in your field of view. This is how most stars in the sky behave. We can see them when we're facing them, but when the earth rotates away from them, they "set", or pass out of our field of view.

I hope this helps demonstrate circumpolar stars for you.

Answer 6

There isn't any illusion. Circumpolar stars has no rise and set; they are visible (above the horizon) or invisible (below the horizon).