_______________ is a slow, conical motion of the Earth's axis of rotation caused by the gravitational torque of
the moon and the sun on the Earth's equatorial bulge.
Precession. The Earth's rotation axis is not fixed in space; it is moving like the top of a wobbly top as illustrated
by the animation below. Of course by human standards, this conical motion is imperceptible--with a precession
period of 26,000 years, it takes 26,000 years for the Earth's axis to do a full rotation.
Another way to look at this is that right now the North Star is Polaris, but in 13,000 years the star Vega will be
approximately at the North Celestial Pole. Then in 26,000 years, the pole will be pointing in the same direction
again--back at Polaris.
The twenty brightest stars as seen from Earth are classified in that manner because they have high
______________ and/or are nearby.
Luminosity. Those stars are: Sun, Sirius, Canopus, Arcturus, Alpha Centauri, Vega, Rigel, Procyon, Achernar,
Betelgeuse, Hadar (Agena), Capella A, Altair, Aldebaran, Capella B, Spica, Antares, Pollux, Fomalhaut, and
Note that many people mistakenly think that Polaris, the North Star, is the brightest star in the sky, when it's