Imagine looking up at the night sky in spring, as the winter ice and snows are still melting, and seeing the familiar summer constellations of Scorpius and Sagittarius, the sparkling summer-time Milky Way crossing the sky, and the three bright stars of the "Summer Triangle". Imagine in autumn, while the days are still warm and somewhat long, seeing the dazzling and beautiful, icy stars and constellations of winter - the famous Belt of Orion, the red supergiant Betelgeuse, Sirius the Dog Star, the Gemini Twins, and the Big Dipper in its upright, question mark position.Would you like to see a preview of the prime time stars of the next season - how the evening sky will look like exactly three months from now? If you would, look today, but look six hours later! Look at the sky at 3:00 am and you'll get a preview of what the sky will look like exactly three months from now at 9:00 pm. Here’s why.As the Earth speeds along on its approximately 600 million mile, year long path around the Sun, the direction we face in at a specific time of night slowly changes. To help visualize this, think of a race car driver on a large round track, with a control tower in the center and bleachers filled with people surrounding the track. When the driver looks toward the center, he always sees the control tower. However, when he looks toward the bleachers, he sees different people passing by. Each time he finishes one lap and returns to the starting point, he begins seeing the same people over again.
At the center of the Earth’s track, or orbit, is the Sun. When the part of the Earth we are on faces the center, it is daytime and we always see the Sun, no matter where on the track we are. Surrounding the Earth’s track are the stars. We see them at night when the part of the Earth we are on faces away from the center. During the course of each year, or lap, the stars we see, at a specific time of night, slowly changes, just as the people the race car driver sees changes as he drives around each lap. When the Earth has gone a quarter of the way around the Sun, three months have passed, one season has changed to the next, and the stars that are visible at a given time are dramatically different from those visible three months earlier at the same time. At the end of each year, or lap, the Earth returns to the same place on the track and we begin seeing the same stars over again.
Unlike the race car, the Earth not only speeds around on its year long track around the Sun, it also spins around its own center, or axis, once each twenty four hours. So the stars we see each night are also changing during the night itself as the Earth spins. Since the Earth spins one quarter of the way around each six hours, the nightly change of direction during each six hour period is the same as the yearly change of direction, at a specific time, during each three month period.
Therefore, if it is a spring evening at 9:00 pm, and we want to see how the sky will look like exactly three months later, we can either wait three months and look at the sky at 9:00 pm, or we can wait six hours and look at the sky at 3:00 am. The part of the sky we see will be the same.
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