Right now, the Earth's rotation axis happens to be pointing almost exactly at Polaris. But in the year B. Don't feel bad for Polaris, however, because in 26, more years it will once again be the Pole Star! By the way, there is not currently a star in the direction of the southern hemisphere spin axis. So we do not now have a "South Star". Laura A. Whitlock Curator: J. Polaris has a magnitude of 1. Although it appears to the naked eye as a single star, the Polaris star system consists of at least three and more likely four or five separate stars.
The main star called Polaris A is actually two separate stars so close to each other that they cannot be resolved with Earth-based optical telescopes. The brightest component of this pair Polaris Aa is a bright variable supergiant star which is on average times more luminous than the Sun. Its close companion is called Polaris Ab and these revolve around each other once every 29 years. They have a more distant companion called Polaris B which takes years to do a single orbit around Polaris A.
In the American astronomer Burham discovered two faint stars in the neighbourhood of Polaris which are called Polaris C and Polaris D.
It is still unclear whether or not these are part of the Polaris system. It is possible that, although they lie close to Polaris in the sky, they may be foreground or background objects. However, this is unlikely. If they are associated with Polaris, then they are so far away that they will take around years to complete one orbit.
I hope you have enjoyed this post. If we look again at the simpler case of the spinning top, discussed in the main article, the torque caused by the force of gravity causes it to precess. The rate of precession about the line OZ is given by the formula:. The derivation of this formula is normally covered in the first year of an undergraduate physics course and I will not repeat it here. However, for any readers wishing to find out more, the following links are useful.
There is also an interesting video which describes in simple terms how precession works without using any mathematics. If we consider a top having: diameter of 5 cm, mass 80g, rotating at a speed of times a second and the distance between the bottom of its spindle and the centre of mass of 15 cm.
In the ideal case, if there were zero friction at the bottom of the spindle and zero air resistance, then the top would continue to spin and precess at the same rate indefinitely. In reality, this is not the case, the top will gradually slow down as it loses rotational energy, start to become unstable and then eventually fall over. As stated before, in the case of the Earth the torque is exerted mainly by the Sun and the Moon and arises because the Earth is not a perfect sphere and is slightly flattened having an equatorial bulge.
Because the Earth is spinning this torque will cause its axis to precess for the same reason that the spinning top precesses. It can be shown mathematically that the strength of the torque of the Sun on the Earth varies as the inverse cube of the distance between the Earth and the Sun.
Because the Earth moves in an elliptical orbit and is closest to the Sun in early January and furthest away in early July, this also causes an additional variation in the torque resulting in the torque at the December solstice being stronger than it is at the June solstice.
This means that the Moon also exerts a torque on the Earth. Because the Moon moves in an elliptical orbit, this also causes a variation in torque. The eccentricity, which is usually given the symbol e is a measure of how elliptical an ellipse is. In particular, the torques from the planets, especially Venus which can approach as close as 38 million km to Earth. Because of the complexity and number of other factors contributing to the total torque on Earth it is not possible to calculate the rate of precession exactly and, in any case, it fluctuates over time.
My blog explainingscience. It is written in a style that it is easily understandable to the non scientist. Publications and videos For links to my books and videos please visit www. Was there a clear North Star at that time? This is the imaginary line that extends through the planet and out of the north and south poles. Earth rotates around this line, like a spinning top.
Polaris is located quite close to the point in the sky where the north rotational axis points — a spot called the north celestial pole. As our planet rotates through the night, the stars around the pole appear to rotate around the sky.
Over the hours, these stars each sweep out a circle around the celestial pole. The farther a star is from the pole, the larger the circle it travels around the sky. This one, a white dwarf, lies only Its extreme closeness to the far more brilliant Polaris A explains why it went unseen for so long. Exactly where you see Polaris in your northern sky depends on your latitude. From New York it stands 41 degrees above the northern horizon, which also corresponds to the latitude of New York.
Since 10 degrees is roughly equal to your clenched fist held at arm's length, from New York Polaris would appear to stand about "four fists" above the northern horizon. At the North Pole, you would find it overhead. At the equator, Polaris would appear to sit right on the horizon.
So if you travel to the north, the North Star climbs progressively higher the farther north you go. When you head south, the star drops lower and ultimately disappears once you cross the equator and head into the Southern Hemisphere. And always keep this fact in mind: Polaris is more accurate than any compass. A compass is subject to periodic variations and can only show you the direction of the lines of the strongest magnetic force for a particular spot and for a particular time.
But even Polaris isn't positioned exactly due north. Only about 0. In case you're wondering, 0.
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