The stars and their apparent motion.

1) There are about 6,000 stars visible to the naked eye, and about a billion visible using the largest telescopes. We see the stars only at night, or, rarely, during a solar eclipse but, of course, they are always up there. It's sometimes useful to distinguish when a star is visible (only at night) from when it is up (above the horizon), which may be day or night or both.

There are stars in all parts of the sky, and they all move, and they all move together. They form fixed patterns (constellations), whose form is practically unchanged over centuries. To us on earth, their motion is as if there were holes punched into a very large (imaginary) sphere surrounding the earth. The arrangement of holes would be identical to the patterns of the constellations, and there would be lights behind the holes. If the sphere made a complete rotation every 23 hours and 56 minutes, the lights would move exactly as we see the stars move. Every star would appear to return to the same place in the sky, as seen from the earth's surface, after 23 hours and 56 minutes, as they actually do. These days we find it easier to think of the earth as spinning instead of the stars going around, but the idea of this "celestial sphere" is still useful.

Some stars rise and set. Others are up all the time. These latter are called ``circumpolar''. Those which rise and set, rise at the same place on the horizon night after night, year after year. (Where they set doesn't change either). Those which rise in the NE set in the NW. (Think about that statement and try to visualize its meaning.) Those which rise (exactly) in the east, set (exactly) in the west. Those that rise in the SE set in the SW. (Think about this one, too.) They all appear to travel in circles, and, depending on where they are in the sky and where we are on the earth's surface, we see all or part or none of their circles. We see all of the circles of the circumpolar stars. We see half of the circles of those stars on the equator of the celestial sphere, less than half for those which lie south of the celestial equator, and more than half for those which lie north of the equator. For example, looking north, as the stars come up and go down, they follow the paths shown:

(Just a bit of their paths is obscured by the earth. A star which doesn't set at all is circumpolar.) But looking south, the paths are:

and we see only the very top of the stars' "circles". The amount of circle cut off determines the fraction of time the star is not up. Stars near the southern horizon are never up for more than a few minutes at a time, stars on the equator are up for twelve hours, down for twelve, and stars in the northern sky are up for more than twelve.

2) The sun's path through the stars: The stars go around once every 23 hours 56 minutes, and the sun every 24 hours, so the stars are catching and overtaking the sun, and the sun appears to migrate eastward (to the left, as we look at the sky from the northern hemisphere) through the stars. After one year, the sun returns to the same place among the stars, and then travels through them again, repeating year after year and following the same path year after year. The path the sun takes is called the "ecliptic". This apparant motion of the sun through the stars happens because the earth moves around the sun. After one year when the earth has returned to the same place, the sun once again is in front of the same stars.