The ecliptic does not lie along the celestial equator but is inclined to it at an angle of about Figure 6: The Celestial Tilt. The celestial equator is tilted by As a result, North Americans and Europeans see the Sun north of the celestial equator and high in our sky in June, and south of the celestial equator and low in the sky in December.
The inclination of the ecliptic is the reason the Sun moves north and south in the sky as the seasons change. In Earth, Moon, and Sky , we discuss the progression of the seasons in more detail. The Sun is not the only 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 a single day, the Moon and planets all rise and set as Earth turns, just as the Sun and stars do.
But like the Sun, they have independent motions among the stars, superimposed on the daily rotation of the celestial sphere. Noticing these motions, the Greeks of years ago distinguished between what they called the fixed stars —those that maintain fixed patterns among themselves through many generations—and the wandering stars , or planets. Today, we do not regard the Sun and Moon as planets, but the ancients applied the term to all seven of the moving objects in the sky.
Much of ancient astronomy was devoted to observing and predicting the motions of these celestial wanderers. When we measure the angle in the sky that something moves, we can use this formula:.
This is true whether the motion is measured in kilometers per hour or degrees per hour; we just need to use consistent units. You note the time, and then later, you note the time that Sirius sets below the horizon. About how many hours will it take for Sirius to return to its original location? Rearranging the formula for speed we were originally given, we find:. Go outside at night and note the position of the Moon relative to nearby stars.
Repeat the observation a few hours later. How far has the Moon moved? For reference, the diameter of the Moon is about 0. Based on your estimate of its motion, how long will it take for the Moon to return to the position relative to the stars in which you first observed it? The individual paths of the Moon and planets in the sky all lie close to the ecliptic, although not exactly on it.
This is because the paths of the planets about the Sun, and of the Moon about Earth, are all in nearly the same plane, as if they were circles on a huge sheet of paper. The planets, the Sun, and the Moon are thus always found in the sky within a narrow degree-wide belt, centered on the ecliptic, called the zodiac Figure 5. How the planets appear to move in the sky as the months pass is a combination of their actual motions plus the motion of Earth about the Sun; consequently, their paths are somewhat complex.
As we will see, this complexity has fascinated and challenged astronomers for centuries. If there were no clouds in the sky and we were on a flat plain with nothing to obstruct our view, we could see about stars with the unaided eye.
To find their way around such a multitude, 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 in the stars, much like a modern Rorschach test in which you are asked to discern patterns or pictures in 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 stars and in passing their star lore on to their children. You may be familiar with some of the old star patterns we still use today, such as the Big Dipper, Little Dipper, and Orion the hunter, with his distinctive belt of three stars Figure 7.
In this way, astronomers can predict geocentric or heliocentric positions of objects on the celestial sphere, without the need to calculate the individual geometry of any particular observer, and the utility of the celestial sphere is maintained.
Individual observers can work out their own small offsets from the mean positions, if necessary. In many cases in astronomy, the offsets are insignificant. The equatorial coordinate system is a widely-used celestial coordinate system used to specify the positions of celestial objects. The origin at the center of the Earth means the coordinates are geocentric, that is, as seen from the center of the Earth as if it were transparent and nonrefracting. A right-handed convention means that coordinates are positive toward the north and toward the east in the fundamental plane.
Because of the great distances to most celestial objects, astronomers often have little or no information on their exact distances, and hence use only the direction. After a spectacular series of close passes lasting billions of years — and which will distort the structure of both galaxies — a final merger of the Andromeda Galaxy and the Milky Way galaxy will occur about 10 billion years from now.
Skip to content Space. Table of Contents. The stars appear to rise and set for the same reason the Sun does. As the Earth rotates on its axis, the spin carries the visible portion of the sky past us, moving from east to west.
Depending on your latitude, some stars never appear to set — only to fade out as the rising Sun turns the night sky into day. These stars are generally near the South Pole or North Pole and can be used to figure out your direction and time in the dark. Many ancient peoples thought that a dome enclosed the Earth and that the stars moved from east to west across the inside of the dome or sphere.
This old idea, however, is still useful. The idea of a celestial sphere provides a simple way of thinking about the appearance of the stars from Earth without the complication of a more realistic model of the universe.
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