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 only enjoy Orion the Hunter during the cold wintry months? And during summer evenings it’s 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. And on fall evenings, it’s the Great Square of Pegasus that vies for the stargazer’s attention.
The change is subtle to say the least. 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.
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 of stars 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.
But just why is this shift happening?
Four Minutes Per Day.
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 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 30 days. A little quick arithmetic shows that with a difference of two hours per month, 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.
Conduct Your Own Star Experiment
This can be made clearer by trying an experiment. Suppose you look skyward tonight 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, incidentally, 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.
Joe Rao is an esteemed astronomer who writes for Space.com, Sky & Telescope, and Natural History Magazine. Mr. Rao is a regular contributor to the Farmers' Almanac and serves as an associate lecturer for the Hayden Planetarium in New York City.