Celestial pole - each of the two points at which the earth's axis, if extended, would touch the celestial sphere. Celestial sphere - the imaginary sphere which appears to enclose the universe. To an observer on the earth, the visible sky forms half of the celestial sphere. Circumpolar - stars that, due to their location with respect to the Earth's pole, never set. Comet - a bright heavenly body with a starlike center and often with a cloudy tail of light which always points away from the sun.
Comets move around the sun like planets, but in an oval course.
We can see comets only when they come close to the earth. The diameters of the heads of comets average 80,000 miles, approximately the size of Jupiter. Convex - curved out, like the outside of a circle or sphere. Crescent - the shape of the moon when it is small and thin. The moon is crescent in its first and last quarter.