Have you ever wondered why most star patterns are associated with specific seasons of the year? Just why, for instance, can evening sky watchers in the Northern Hemisphere enjoy Orion the Hunter only during the cold wintry months? During balmy summer evenings it is not Orion, but the stars of Scorpius, the Scorpion, that dominate the southern sky. Spring evenings provide us with a view of the sickle of Leo, the Lion. Yet on fall evenings, it's the Great Square of Pegasus that vies for the stargazer's attention.
The change is subtle. Were we to watch the night sky on any one night from dusk to dawn we would notice certain stars rising from above the eastern horizon in the evening hours. They would sweep across the sky during the night, finally setting beneath the western horizon by dawn. No big deal here, since, after all, the sun does the same thing during the daylight hours. It's caused by Earth's rotation.
But with the passage of time, we would notice something rather puzzling.
Those stars that were low over the western horizon during the early evening hours would, within a matter of a few weeks, disappear entirely from our view, their places being taken up 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 places.
But just why is this shift happening?
Four minutes a day
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 23 hours and 56 minutes, or four minutes shy of 24 hours. This would be a day based upon the apparent movement of the stars in our sky, which astronomers call a "sidereal" day from the Latin word for star.
While this is happening, all of us are being carried around the sun on an annual journey almost 600 million miles long. Our orbit is almost a circle and as seen from the sun the Earth would move about one degree each day, since we take about 365 days to go around a circle of 360 degrees. As seen from Earth — from our vantage point — the sun seems to move and it changes its place in the sky by that one-degree per day, as measured against the background of stars.
Of course, we can't see the stars in the daytime but astronomers can measure the position of the sun. The direction of the sun's apparent motion is eastward among the stars. Since the daily turning of the sky (caused by the Earth's rotation) appears to move westward, this slight motion of the sun is what makes a day as measured by the sun (called a solar day) longer: the Earth must turn about one degree (or about 4 minutes) more than a full circle to complete a 24-hour day as measured by the sun.
That slight shift each day is what makes the different stars and constellations appear at different times of the year. The sun slowly changes its position, but so slowly that the stars which are up when the sun is down also change.
If you want to try an experiment, look outside some clear evening from a location you can find again. Notice the exact time that a particular star is directly aligned with some object, like a telephone pole or a roof. Look the very next night; stand in the very same place and the star will be there four minutes before the time it was the previous night (of course your clock must be set accurately each night).
You are observing the effects of the Earth's motion around the sun.
Star time versus sun time
At this point you might be a bit confused. If the Earth takes 23 hours 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 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 other difference is that the sidereal clock 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 or setting at 6 a.m. (or something else strange). We're accustomed, of course, to being awake when it's light and asleep when it's dark, so astronomers also have developed a "mean" sun — which is fictitious and for most of the year deviates somewhat from the sun's actual position in the sky.
Yet, the mean sun governs our ordinary clocks and results in the 24-hour time scale of which we have become accustomed to all of our lives.