We described the movement of stars within the night sky, but what about during the daytime? The stars still circle during the day, but the brilliance of the Sun makes them difficult to ascertain . (The Moon can often be seen in the daylight, however.) On any given day, we can think of the Sun as being located at some position on the hypothetical celestial sphere. When the sun rises, that is, when the earth rotates to bring the sun above the horizon, the sunlight is scattered by our atmospheric molecules, making our sky full of light and hiding the stars above the horizon.
For thousands of years, astronomers are aware that the Sun does quite just rise and set. It changes position gradually on the sphere , moving every day about 1° to the east relative to the celebs . Very reasonably, the ancients thought this meant the Sun was slowly traveling Earth, taking a period of your time we call 1 year to form a full circle. Today, of course, we all know it’s Earth that’s going round the Sun, but the effect is that the same: the Sun’s position in our sky changes day to day. We have an identical experience once we walk around a campfire at night; we see the flames appear ahead of every person seated about the hearth successively .
The path the Sun appears to require round the sphere annually is named the ecliptic. Because of its motion on the ecliptic, the Sun rises about 4 minutes later every day with reference to the celebs . Earth must make just a touch quite one complete rotation (with reference to the stars) to bring the Sun up again.
As the months pass and that we check out the Sun from different places in our orbit, we see it projected against different places in our orbit, and thus against different stars in the background or we would, at least, if we could see the stars in the daytime. In practice, we must deduce which stars lie behind and beyond the Sun by observing the celebs visible within the other way in the dark . After a year, when Earth has completed one trip round the Sun, the Sun will appear to have completed one circuit of the sky along the ecliptic.
Fixed and Wandering Stars
The Sun isn’t the sole object that moves among the fixed stars. The Moon and each of the planets that are visible to the unaided eye—Mercury, Venus, Mars, Jupiter, Saturn, and Uranus (although just barely)—also change their positions slowly from day to day. During one day, the Moon and planets all rise and set as Earth turns, even as the Sun and stars do. But just like the Sun, they need independent motions among the celebs , superimposed on the daily rotation of the sphere. Noting these movements, the Greeks 2000 years ago distinguished them as fixed stars (stars that maintain a fixed pattern across many generations) from wandering stars or planets. The word “planet,” actually , means “wanderer” in Ancient Greek . Today, we don’t regard the Sun and Moon as planets, but the ancients applied the term to all or any seven of the moving objects within the sky. Much of ancient astronomy was dedicated to observing and predicting the motions of those celestial wanderers. They even dedicated a unit of your time , the week, to the seven objects that advance their own; that’s why there are 7 days during a week. The Moon, being Earth’s nearest celestial neighbor, has the fastest apparent motion; it completes a visit round the sky in about 1 month (or moonth). For this reason, the moon moves about 12° every day, which is 24 times its own apparent width in the sky.
The backdrop for the motions of the “wanderers” in the sky is the canopy of stars. If there have been no clouds within the sky and that we were on a flat plain with nothing to obstruct our view, we could see about 3000 stars with the unaided eye. To find their way around such a mess , the ancients found groupings of stars that made some familiar geometric pattern or (more rarely) resembled something they knew. Each civilization found its own patterns within the stars, very similar to a contemporary Rorschach during which you’re asked to discern patterns or pictures during a set of inkblots. The ancient Chinese, Egyptians, and Greeks, among others, found their own groupings—or constellations—of stars. These were helpful in navigating among the celebs and en passant their star lore on to their children. You may be conversant in a number of the old star patterns we still use today, like the large Dipper, Little Dipper , and Orion Orion , together with his distinctive belt of three stars. However, many of the celebs we see aren’t a part of a particular star pattern in the least , and a telescope reveals many stars too faint for the attention to ascertain . Therefore, during the first decades of the 20th century, astronomers from many countries decided to determine a more formal system for organizing the sky.