troChapter 1: Charting the Heavens (Astronomy Today)
The universe is the totality of all space, time, matter, and energy. Astronomy is the study of the universe. A widely used unit of distance in astronomy is the light year, the distance traveled by a beam of light in one year. Early observers grouped the thousands of stars visible to the naked eye into patterns called constellations. These patterns have no physical significance, although they are a very useful means of labeling regions of the sky. The nightly motion of the stars across the sky is the result of Earth's rotation on its axis. Early astronomers, however, imagined that the stars were attached to a vast celestial sphere centered on Earth and that the motions of the heavens were caused by the rotation of the celestial sphere about a fixed Earth. The points where Earth's rotation axis intersects the celestial sphere are called the north and south celestial poles. The line where Earth's equatorial plane cuts the celestial sphere is the celestial equator. The time from one sunrise to the next is called a solar day. The time between successive risings of any given star is one sidereal day. Because of Earth's revolution around the Sun, the solar day is a few minutes longer than the sidereal day. The Sun's yearly path around the celestial sphere, or, equivalently, the plane of Earth's orbit around the Sun, is called the ecliptic. Because Earth's axis is inclined to the ecliptic plane, we experienceseasons, depending on which hemisphere (Northern or Southern) happens to be "tipped" toward the Sun. At the summer solstice, the Sun is highest in the sky, and the length of the day is greatest. At the winter solstice, the Sun is lowest, and the day is shortest. At the vernal and autumnal equinoxes, Earth's rotation axis is perpendicular to the line joining Earth to the Sun, and so day and night are of equal length. The interval of time from one vernal equinox to the next is one tropical year. Because Earth orbits the Sun, we see different constellations at different times of the year. The constellations lying along the ecliptic are collectively called the zodiac. The time required for the same constellations to reappear at the same location in the sky, as viewed from a given point on Earth, is one sidereal year. In addition to its rotation about its axis and its revolution around the Sun, Earth has many other motions. One of the most important of these is precession, the slow "wobble" of Earth's axis due to the influence of the Moon. As a result, the particular constellations that happen to be visible on any given night change over the course of many years. The Moon emits no light of its own. It shines by reflected sunlight. As the Moon orbits Earth, we see lunar phases as the amount of the Moon's sunlit face visible to us varies. At full Moon, we can see the entire illuminated side. At quarter Moon, only half the sunlit side can be seen. At new Moon, the sunlit face points away from us, and the Moon is all but invisible from Earth. The time between successive full Moons is one synodic month. The time taken for the Moon to return to the same position in the sky, relative to the stars, is one sidereal month. Because of Earth's motion around the Sun, the synodic month is about 2 days longer than the sidereal month. A lunar eclipse occurs when the Moon enters Earth's shadow. The eclipse may be total, if the entire Moon is (temporarily) darkened, or partial, if only a portion of the Moon's surface is affected. A solar eclipse occurs when the Moon passes between Earth and the Sun, so that a small part of Earth's surface is plunged into shadow. For observers in the umbra, the entire Sun is obscured, and the solar eclipse is total. In the penumbra, a partial solar eclipse is seen. If the Moon happens to be too far from Earth for its disk to completely hide the Sun, an annular eclipse occurs. Because the Moon's orbit around Earth is slightly inclined with respect to the...
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