Most star patterns in the night sky are associated with specific seasons of the year. Evening skywatchers in the Northern Hemisphere enjoy Orion the Hunter only during the cold wintry months, for example. Spring evenings provide a view of the Sickle of Leo, the Lion. In summer, the stars of Scorpius, the Scorpion dominate the southern sky. And the Great Square of Pegasus vies for the stargazer's attention on fall evenings.

What's going on? Armed with some facts, you can do a little observing and conduct a simple experiment in order to easily grasp this important celestial concept.

But with the passage of time, we would notice a more subtle change.

Stars that were low over the western horizon during the early evening hours would, within a matter a few weeks, disappear entirely from our view, their places being taken by groups which a few weeks earlier were previously higher up in the sky at sundown. In fact, it would seem that with the passage of time, all the stars gradually shift westward while new stars move up from the eastern horizon to take their place.

As our Earth whirls through space around the Sun, its motions cause night and day, the four seasons and the passage of the years. And if we were to synchronize our clocks using the motions of the stars as a reference we would discover that the Earth would complete a single turn on its axis not in 24 hours, but actually four minutes shy of that oft-quoted figure: 23 hours 56 minutes.

As a result, the stars appear to rise, cross the sky, and set 4 minutes earlier each night.

This amounts to a whole hour earlier in 15 days and two hours earlier in a month. A little more arithmetic shows that in one year the cycle will come full circle (12 months x 2 hours = 24 hours), since each star completes a full circle around the sky during the course of one year.

This can be made clearer by trying an experiment:

Look skyward on any night and pick out a bright star, then line it up with a nearby landmark (like a telephone pole or the peak of your neighbor's roof). Make sure you note the exact time and the exact spot when you lined up the star.

Then come back the next night at the exact same time and stand in the exact same place. You'll see that the star has apparently shifted slightly to the right (west) of the position that it was at the previous night. Had you arrived four minutes earlier, the star would have lined up exactly with the nearby landmark just as you had seen the previous night.

This apparent westward drift of the stars, is a motion that is in addition to the daily rising, circling and setting. For our Earth does not simply stand in the same spot in space and spins, but is constantly rushing eastward along in its orbit around the Sun. It carries us steadily toward and under the stars to the east and away from the stars that we are leaving in the west, until we make a complete circle around the Sun, bringing us back to our original position in one year.

And then the whole performance starts again.

Star time vs. Sun time (see graphic below)

All this raises a question: If the Earth takes 23 hours and 56 minutes to turn on its axis, why do we say that a day is 24-hours long?

Astronomers have devised special clocks adjusted to keep time solely by the stars. These astronomical clocks keep "sidereal" (star) time. There is no a.m. or p.m. in a sidereal day. With the clocks that we use every day, the hour hand goes completely around 12 hours twice a day.

But with a sidereal clock, there are 24 hourly numbers on the dial instead of 12, and the hour hand goes around only once in a sidereal day. The hours start at 00 hour (zero hour) and are numbered straight through to 23-hours and then starts at the zero hour again. The sidereal clock also differs in that it runs four minutes fast as compared to a regular clock.

Now, if our daily lives were governed by the sidereal clock, there would be times during the year when the Sun would appear highest in the sky at noontime, but at other times of the year it would appear highest at midnight; setting at 6 a.m. (or something else strange). We're accustomed, of course, to be awake when it's light and asleep when it's dark, so astronomers also developed a "mean" Sun, which governs our ordinary clocks and results in 24-hour (solar) time of which we are all accustomed to.

The Mean Sun however, is fictitious and for most of the year deviates somewhat from the true Sun's position in the sky. But the mean Sun was invented only to make 24-hour timekeeping by the Sun mathematically correct.