earth is called the perigee（近地点）, and that point farthest from the earth is called the apogee（远地点）.
Comets And Meteors
Although comets are noted as great spectacles of nature, very few are visible without a telescope. Those that become widely visible do so because they develop long, glowing tails. Comets are swarms of relatively small solid bodies held together by gravity. The tail is directed away from the sun, so that it follows the head while the comet is approaching the sun. The total mass of a comet is very small, and the tail is so thin that stars can easily be seen through it. In 1910, the earth passed through the tail of Halley‘s
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comet without noticeable effect.
Compared to the well-ordered orbits of the planets, comets are erratic and inconsistent. Some travel east to west and some west to east, in highly eccentric orbits inclined at any angle to the ecliptic. Periods of revolution range from about 3 years to thousands of years.
Meteors, popularly called shooting stars, are tiny, solid bodies too small to be seen until heated to incandescence by air friction while passing through the earth‘s atmosphere.
A particularly bright meteor is called a fireball（火球，大流星）. One that explodes is called a bolide（火流星）. A meteor that survives its trip through the atmosphere and lands as a solid
particle is called a meteorite（陨星）. Vast numbers of meteors exist. It has been estimated that an average of about 1,000,000 bright enough to be seen enter the earth‘s atmosphere each hour, and many times this number undoubtedly enter, but are too small to attract attention.
Meteor showers（流星雨） occur at certain times of the year when the earth passes through meteor swarms（群）, the scattered remains of comets that have broken up. At these times the number of meteors observed is many times the usual number. Stars
Stars are distant suns, in many ways resembling the body which provides the earth with most of its light and
heat. Like the sun, stars are massive balls of gas that create their own energy through thermonuclear reactions. Although stars differ in size and temperature, these differences are apparent only through analysis by astronomers. Some differences in color are noticeable to the unaided eye. While most stars appear white, some
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(those of lower temperature) have a reddish hue.
The stars are not distributed uniformly around the sky. Striking configurations, known as constellations, were noted by ancient peoples, who supplied them with names and myths. Today astronomers use constellations—88 in all—to identify areas of the sky.
Under ideal viewing conditions, the dimmest star that can be seen with the unaided eye is of the sixth magnitude. In the entire sky there are about 6,000 stars of this magnitude or brighter. Half of these are below the horizon at any time. Because of the greater absorptionof light near the horizon, where the path of a ray travels for a greater distance through the atmosphere, not more than perhaps 2,500 stars are visible to the unaided eye at any time.
However, the average navigator seldom uses more than perhaps 20 or 30 of the brighter stars.
Stars which exhibit a noticeable change of magnitude are called variable stars（变星）. A star which suddenly becomes several magnitudes brighter and then gradually fades is called a nova（新星）. A particularly bright nova is called a supernova（超新星）. Galaxies
A galaxy is a vast collection of clusters of stars and clouds of gas. The earth is located in the Milky Way galaxy, a slowly spinning disk more than 100,000 light years in diameter. All the bright stars in the sky are in the Milky Way. However, the most dense portions of the galaxy are seen as the great, broad band that glows in the summer nighttime sky. When we look toward the constellation Sagittarius, we are looking toward the center of the Milky Way, 30,000 light years away.
Despite their size and luminance, almost all other galaxies are
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too far away to be seen with the unaided eye. 2.3.3 Elements Of The Celestial Sphere
天球：The celestial sphere is an imaginary sphere of infinite radius with the earth at its center. 天南极、天北极：The north and south celestial poles of this sphere are located by extension of the earth‘s axis. 天轴：
天赤道：The celestial equator (sometimes called equinoctial) is formed by projecting the plane of the earth‘s equator to the celestial sphere.
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天球子午线（测者子午圆）：A celestial meridian is formed by the intersection of the plane of a terrestrial meridian and the celestial sphere. It is the arc of a great circle through the poles of the celestial sphere.
测者天顶点、天底点：The point on the celestial sphere vertically overhead of an observer is the zenith, and the point on the opposite side of the sphere vertically below him is the nadir. The zenith and nadir are the extremities of a diameter of the celestialsphere through the observer and the common center of the earth and the celestial sphere. 测者子、午圆：The arc of a celestial meridian between the poles is called the upper branch（测者午半圆） if it contains the zenith and the lower branch（测者子半圆） if it contains the nadir. The upper branch is frequently used in navigation, and references to a celestial meridian are understood to mean only its upper branch unless otherwise stated. Celestial meridians take the names of their terrestrial counterparts, such as 65?west.
时圆：An hour circle is a great circle through the celestial poles and a point or body on the celestial sphere. It is similar to a
celestial meridian, but moves with the celestial sphere as it rotates about the earth, while a celestial meridian remains fixed with respect to the earth.
The location of a body on its hour circle is defined by the body‘s angular distance from the celestial equator. This distance, called declination（倾角）, is measured north or south of the celestial equator in degrees, from 0?through 90?, similar to latitude on the earth.
赤纬：A circle parallel to the celestial equator is called a parallel of declination, since it connects all points of equal declination. It
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