Physics Question #1159

Jessia, a 14 year old n/a from Ontario asks on January 6, 2003,

How high in the sky would Polaris the North Star be, as seen from the North Pole?

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The answer

Donald J. Barry answered on January 6, 2003

As you may have guessed, the easy answer is "straight up" -- it's as if you were perched at the top of a spinning top and looking up along the axle of spin. Polaris is special merely because it is (closely) aligned with the axis of this particular planet's rotation.

Every planet's spin axis is differently pointed, though all of them, with the near miss of the planet Uranus, point in the same hemisphere of the plane in which the planets (again on average) orbit the Sun. The "North Star" of the planet Mars, for instance, lies somewhat near Deneb, in the constellation of Cygnus, the Swan. The average of the planets' spin axes is itself generally aligned with the plane of the Sun's own rotation. It all comes from the original "primordial" or original bit of spin that all the gas and dust had which formed the infant solar system.

Now unlike as in Julius Caesar ("But I am constant as the Northern Star, // Of whose true-fix'd and resting quality // there is no fellow in the firmament", Act III, Scene I), Polaris isn't -exactly- lined up with the Earth's spin axis -- it actually misses it by about half a degree, which is the apparent size of the Moon in the sky. And over time the Earth's spin axis changes due to tugs from other bodies in the solar system, very primarily our own Moon and secondly the Sun. This produces a wobble -- and over time various stars assume the mantle of "North Star" - to the ancient Egyptian cultures, the star Thuban in the constellation of Draco, the Dragon -- assumed this role. Every 24,000 years the spin axis describes a vast wobble around the sky, a phenomenon known as the precession of the equinoxes. And this has been known for a long time -- Hipparchus, the ancient Greek astronomer, may have been the first to note it (he was certainly the first that we know of to write about it).

There's much more to write about, but it becomes increasingly esoteric. The next level of complexity is a "little" wobble called the nutation, caused by shifts in the Moon's orbit, which causes a tinier circle of shift ot the North Point over a period of only 18 years. It takes special instruments, not just observant Greek astronomer/philosophers, to note these increasingly small effects.

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