On an infinite-radius celestial sphere, all observers see the same things in the same direction. Objects which are relatively near to the observer for instance, the Moon will seem to change position against the distant celestial sphere if the observer moves far enough, say, from one side of the Earth to the other.
This effect, known as parallax, can be represented as a small offset from a mean position. 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. The direction of sufficiently distant objects is the same for all observers, and it is convenient to specify this direction with the same coordinates for all.
Telescopes equipped with equatorial mounts and setting circles employ the equatorial coordinate system to find objects. Setting circles in conjunction with a star chart or ephemeris allow the telescope to be easily pointed at known objects on the celestial sphere. Declination is analogous to terrestrial latitude. The vernal equinox point is one of the two where the ecliptic intersects the celestial equator. What would an observer in the latitudes of the United States or Europe see?
For those in the continental United States and Europe, the north celestial pole is neither overhead nor on the horizon, but in between. As Earth turns, the whole sky seems to pivot about the north celestial pole. They are always above the horizon, day and night.
This part of the sky is called the north circumpolar zone. For observers in the continental United States, the Big Dipper, Little Dipper, and Cassiopeia are examples of star groups in the north circumpolar zone.
That part of the sky is the south circumpolar zone. To most U. It is called Polaris , the pole star, and has the distinction of being the star that moves the least amount as the northern sky turns each day. Astronomers measure how far apart objects appear in the sky by using angles. To give you a sense of how big a degree is, the full Moon is about half a degree across.
We described the movement of stars in the night sky, but what about during the daytime? The stars continue to circle during the day, but the brilliance of the Sun makes them difficult to see.
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 rotation of Earth carries the Sun above the horizon—sunlight is scattered by the molecules of our atmosphere, filling our sky with light and hiding the stars above the horizon.
For thousands of years, astronomers have been aware that the Sun does more than just rise and set. Very reasonably, the ancients thought this meant the Sun was slowly moving around Earth, taking a period of time we call 1 year to make a full circle.
We have a similar experience when we walk around a campfire at night; we see the flames appear in front of each person seated about the fire in turn. The path the Sun appears to take around the celestial sphere each year is called the ecliptic Figure 4.
Because of its motion on the ecliptic, the Sun rises about 4 minutes later each day with respect to the stars. Earth must make just a bit more than one complete rotation with respect to the stars to bring the Sun up again.
Figure 4: Star Circles at Different Latitudes. The turning of the sky looks different depending on your latitude on Earth. Stars rise and set at an angle to the horizon. As the months go by and we look at 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 Figure 5 and Table 1 —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 stars visible in the opposite direction at night.
After a year, when Earth has completed one trip around the Sun, the Sun will appear to have completed one circuit of the sky along the ecliptic.
Figure 5: Constellations on the Ecliptic. The circle in the sky that the Sun appears to make around us in the course of a year is called the ecliptic. This circle like all circles in the sky goes through a set of constellations. The ancients thought these constellations, which the Sun and the Moon and planets visited, must be special and incorporated them into their system of astrology. Note that at any given time of the year, some of the constellations crossed by the ecliptic are visible in the night sky; others are in the day sky and are thus hidden by the brilliance of the Sun.
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. However, many of the stars we see are not part of a distinctive star pattern at all, and a telescope reveals millions of stars too faint for the eye to see.
Therefore, during the early decades of the twentieth century, astronomers from many countries decided to establish a more formal system for organizing the sky.
0コメント