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-- Fist Quarter, November 2003 -- |
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| FRONT PAGE | Volume 1, Issue 1 | |
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by Alice Peiying Wang The celestial sphere was the topic of the first unit of Astronomy 361. Each member of the class constructed a 3 dimensional model of the celestial sphere. The models aided our understanding of phenomena such as the motion of the sun, the motion of the moon, the diurnal motion of the stars, the phases of the moon, eclipses and the seasons. The celestial sphere is an imaginary sphere of infinite radius that serves as a convenient model of the heavens. Earth lies at the center of this model. Ancient astronomers, in an attempt to describe the heavens, erroneously concluded from observation that each star is equidistant from earth and imbedded in an immense, hollow sphere. The diurnal motion (apparent east to west motion) of the stars was speculated to be the result of the celestial sphere rotating about a stationary earth. Quantities such as the number of hours per day, days per year and months per season were established based on the apparent motion of the sun. It was especially of consequence for farmers to be conscious of the boundaries betwixt seasons so that crops could be planted and harvested at the appropriate time. Before innovations such as the GPS, which determines latitude and longitude based on satellite transmission, people found their way by celestial navigation: the art and science of finding one's way by using the sun, moon, stars and planets as guides. It is now understood that no such sphere exists; stars are of all different sizes and distances from earth and the apparent motion of the stars is due to the rotation of earth on its axis. However, this fictional sphere is an elegant model for visualizing the positions of stars in the sky We identify different points on the celestial sphere for purposes of orientation. The north and south celestial poles are projections on the celestial sphere of the respective pole of earth. Similarly, the celestial equator is a projection of the earth's equator on the celestial sphere. During the course of the year the sun advances counterclockwise (with respect to a stationary earth) around the ecliptic, its apparent annual path on the celestial sphere, which is inclined to the celestial equator by 23o because of a corresponding tilt of the earth's axis of rotation. The northernmost point on the ecliptic is the Summer Solstice, which the sun reaches on June 21: the beginning of summer in the Northern Hemisphere and the beginning of winter in the Southern Hemisphere. The Southernmost point on the ecliptic is the Winter Solstice, which the sun reaches on December 21 marking the beginning of winter in the Northern Hemisphere and of summer in the Southern Hemisphere. The two points where the celestial equator intersects the ecliptic are the equinoxes. On March 21, the beginning of spring in the Northern Hemisphere and of autumn in the Southern Hemisphere, the sun passes the Vernal Equinox, traversing northward across the celestial equator. On September 21, the sun reaches the autumnal equinox, passing south across the celestial equator; this date marks the beginning of autumn in the Northern Hemisphere and of spring in the Southern Hemisphere. Notice that different seasons occur simultaneously in different hemispheres; thus seasons cannot be the result of earth being different distances from the sun. Instead, the seasons are the consequence of the sun's ray striking each hemisphere at different angles. The path of the moon on the celestial equator, the zodiac, always remains within 8o north or south of the ecliptic. The moon is generally viewed within one of the twelve constellations, Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius and Pisces, located along the Zodiac. Each of the zodiac constellations roughly corresponds with the position of the sun at local noon during a particular month. Although this is how the astrological signs were determined, due to the slight, year to year shift of stars, the astrological boundaries of the heavens are no longer accurate Ancient astronomers placed the star Polaris, also designated as the North Star or Pole Star, at the north celestial pole. It has since been determined that while Polaris is very near to the north celestial pole (less than 10o away) it is not actually on the pole. | ||
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