"Shooting stars" are small, solid bodies of the solar system, usually no larger than a grain of sand, which enter the earth's atmosphere. They are observed only when they enter the atmosphere. Most meteors are completely vaporized as they travel through the atmosphere. The small percentage which are not completely destroyed and strike the surface as solid particles are called meteorites. Most are composed largely of nickel and iron; the rest are stone.
Some meteors are small asteroids, which were drawn out of their elliptical orbits about the sun by the earth's gravity. Comets, seem to travel in orbits; these are believed to cause the "showers of shooting stars" which occur periodically. Most meteors are believed to weigh only a small fraction of an ounce, but some can be of great size.
Meteors enter the atmosphere at an estimated rate of 100 million a day. The dates of prominent annual meteor showers are listed in most astronomical texts. At such times, the observer may see ten or more, and on rare occasions sometimes hundreds, in an hour; the hours between midnight and dawn are the best for observation.
The earth revolves about the sun in a slightly elliptical orbit; it is about 91,400,000 miles from the sun in January, and 94,500,000 miles in July. It rotates 360° about its axis once in 23 hours 56 minutes; this is termed the sidereal day, and differs from the solar day, which averages 24 full hours, because of the earth's motion in its orbit.
The earth's equator is inclined about 23.5° to its orbit, the north pole being inclined towards the sun from March to September. During the balance of the year, the south pole is inclined towards the sun. The resultant apparent annual path of the sun among the stars is called the eclipic. This inclination of the equator causes the change of seasons. The earth remains inclined in space due to rotation of the mass. spinning gyroscope's axis is rigid.
The atmosphere of the earth is a great blanket of air. mostly 78 percent nitrogen and 21 percent oxygen, with small amounts and traces of other gases and contaminants. Half the atmosphere is concentrated within about 3 1/2 miles of the surface the remainder thins out to an altitude of roughly 1000 miles.
Without the diffusing effect of the atmosphere, the stars and the sun would be visible at the same time. Molecules make up the atmosphere, aided by suspended dust, scatter the sun all directions, and make it difficult to see the stars. Astronauts report that at altitudes of over 100 miles they are still unable to see most stars in daytime, but that from true "outer space" The short wavelength blue light from the sun is affected by this scattering, giving the sky its characteristic blue color.
When a celestial body is near the horizon, its light must pass through a greater volume of air than when it is overhead. This causes additional scattering, and permits very little light to reach the observer, leaving only the long wavelength This causes the reddish-orange appearance of the sun and moon near the horizon.
The atmosphere also causes light rays to be refracted, or bent as they enter it from space; this refraction of the sun's rays prolong twilight. Except when a celestial body is directly overhead refraction affects its apparent altitude, causing it to appear higher than it actually is. The entire disc of the sun can be visible after the upper limb has passed below the horizon. The atmosphere also reduces the apparent brightness of celestial bodies.Atmospheric turbulence causes the light from a star to twinkle,planets usually do not because they are closer to earth and are bigger.
The earth with the entire solar system revolves around the axis of our galaxy. This motion has very little effect upon the apparent motion of the celestial bodies across the heavens. But there are three major and two minor types of earth motion or changes which affect the apparent paths of these bodies. The three major motions of the earth are rotation about its axis, revolution around the sun, and precession. The two minor motions are wandering of the terrestrial poles, and variations in the speed of rotation.
The daily rotation of the earth on its axis causes the principal apparent motion of the heavenly bodies across the sky from east to west. This motion is: parallel to the plane of the earth's equator; occurs in circles whose centers are on the earth's axis or its extension; and is at an almost constant rate. These circles are called diurnal or daily circles. To be visible to an observer a body must be above his celestial horizon, which is considered as a plane passing through the center of the earth, and perpendicular to a line connecting the observer's position and the earth's center. The plane of his horizon changes as he changes latitude. If he located at one of the poles his horizon is parallel to the equator. If the body's brightness and atmospheric conditions are ignored its visibility depends both upon the position of the body's diurnal circle, relative to the observer's latitude, and its location on that circle. The declination of a body on the celestial sphere is identical to the latitude of the point on earth directly under the celestial body. This is referred to as the geographical position (GP) of the body. Declination and GP are the apparent motion of the celestial bodies caused by the rotation of the earth on its axis which means the GP of the body is moving westward along a parallel of latitude.
