Just like we describe a point on the surface of the Earth with latitude and longitude, we describe a point on the celestial sphere with right ascension and declination.

These points are fixed with stars, not with our own view. So over a night, the right ascension (also called RA) and declination (also called DEC) of, say, the zenith, would change. But the RA and DEC of a star, say Betelgeuse, would stay the same.

Actually the coordinates of a star do change a bit--for one, stars are moving around the center of the Galaxy, although it takes a long time for us to notice that motion. Also the Earth's north pole is "precessing", so that it goes around in a circle every 23,000 years. Right now our North Pole points to Polaris, but it won't always point there. The celestial coordinates are based on our North Pole, so if our North Pole moves, the coordinates of a star will change, even if the star itself is not moving.

So we've already talked about declination. Just imagine lines of latitude from the Earth projected into the sky, and that's declination. Polaris is at about 90 degrees North declination (or +90 declination). All stars that are circumpolar from Claremont have declination of +56 degrees or more. The celestial equator has a declination of 0 degrees. The Sun appears in our sky at declinations between +23.5 degrees and -23.5 degrees.

The other coordinate, right ascension, is not measured in degrees, but in units of time!

Imagine you're looking at the part of your meridian from the north celestial pole to the Southern horizon. Every point on this imaginary line has a declination, and that declination won't change throughout the day's rotation of the Earth. The declination is just the angle above or below the celestial equator.

But the stars that are now on the meridian will move off as the Earth turns. And new stars will move onto it. Right ascension is defined as follows:

Look at your time, using a 24-hour clock. Whatever time it is, that's the coordinate of right ascension for anything on your meridian at that time.

Say the time is now 3:00. Any star on your meridian has a Right Ascension of 3h00m00s, for 3 hours and 0 minutes and 0 seconds. A star that will be on the meridian an hour from now has a RA of 4h00m. A star that was on your meridian an hour ago has an RA of 2h00m.

Does this depend at all on where you are on Earth? No, not at all! A star with RA of 3h0m for you also has an RA of 3h0m for someone who lives further to the East. That person will see the star cross the meridian, say, 1 hour earlier, but will also be in a time zone where the time is 1 hour earlier! So this person would see the star cross the meridian at 2:00 (your time), but because of the time zone difference would think it was 3:00, and would also agree that "at 3:00, this star crosses the meridian."

For one, the time has to be your local time, not just according to your time zone. Someone else halfway across your time zone to the East will see the star cross the meridian 30 minutes earlier. However, their official time, according to time zones, won't be 30 minutes earlier to compensate. So we'll use a local time that keeps track exactly what your longitude is, not just what time zone you're in.

Finally--and this is surprising--a day is NOT 24 hours!

It takes 24 hours for the Earth to rotate once on its axis--and THEN to rotate a little bit more, in order to face the Sun again. If a day were just the time it took for Earth to spin once in space, then 6 months later exactly at noon, after an even number of days had passed, you'd be pointing to the same spot in space--but it would be midnight!

The following diagram from the book makes clear the difference between "solar time" and "sidereal time":

So when we define RA, we should use "local sidereal time" instead of the time we normally use. A sidereal day is not 24 hours by 23 hours 56 minutes, and 4 seconds.

By the way, the 0 for Right Ascension in the sky is fixed to the position that the Sun has during the Vernal Equinox. (Remember the Vernal Equinox is in the spring, one of the two times of the year when the Sun's direct rays strike the equator, and the Sun appears on the Celestial Equator in the sky.)

A diagram from the book, showing three coordinate systems. One is based on our horizon, and measures altitude and angle from the N and S directions. Coordinates on Earth are based on Latitude and Longitude, and the 0 point of longitude is Greewich, England. Coordinates in the sky are based on RA and DEC, and the 0 point is the Vernal Equinox.