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Three Hundred and Sixty Five Starry Nights: An Introduction to Astronomy for Every Night of the Yearby Chet Raymo
1st: The map at the left shows all of the stars we will study during January — and more. To find them in the sky, it is best to start with Orion, one of the most conspicuous constellations. The stars of Orion vividly suggest the mythological figure they are supposed to represent — a bold hunter armed with a club and sword and faced by a charging bull. To observe Orion, find a place where you have a clear view of the sky. Turn so that you are facing south. During the evening hours this month you will find Orion about halfway up from the horizon to the zenith. The zenith is the point in the sky directly above your head. The most striking feature of the constellation is the alignment of three equally bright stars in the hunter's belt. If you are looking in the fight direction you can't miss it. As you stand facing giant Orion, the glittering yellow star almost directly over your head is Capella in the constellation Auriga the Charioteer. Capella, with Aldebaran in Taurus the Bull, Rigel in Orion, Sirius and Procyon in the Big and Little Dogs, and the Gemini twins Castor and Pollux, make up the Winter Hexagon. Betelgeuse, the brilliant red star in the arm of Orion, is near the center of the Hexagon. In January and February we will look closely at each of these stars and constellations in turn.
2nd: Once you have found and learned the stars of Orion's belt, you will never again have trouble recognizing this constellation. Look also for the bright stars of the shoulders and feet and the fainter stars of the head and sword. Orion rises in the early evening at the beginning of December and dominates the sky all winter long. The hunter's stars, which include two of the brightest in the heavens, outshine those of any other constellation. Because Orion stands above the earth's equator, it is visible from every inhabited place on earth. All human cultures in every time and place have given special note in story and myth to this wonderful array of stars. Sailors of old feared the sight of Orion, for his appearance on the eastern horizon forecast stormy winter weather. But the hunter has also long been associated in myth with the forces of goodness and light, and as such we welcome his appearance as a promise of sparkling starry nights to come.
3rd: It is important to acquire early a sense of your own place under the stars. Orion's belt will help. Point to the place on your horizon that is due east of your observing location (a map of your town will help you find the approximate compass points — later you will use the stars). Now swing your arm up in an arc through the stars of Orion's belt, and on to the point on your horizon that is due west. You have traced out the sky's equator, or celestial equator. The celestial equator is the imaginary line among the stars that lies directly above the equator of the earth.
4th: It is convenient to imagine, as the ancients believed, that all of the stars lie on one great skysphere that surrounds and encloses the earth. We call this imaginary sphere the celestial sphere. In fact, as we know, the stars are distributed in space at different distances from earth, and there is no "sky-sphere." The three stars of Orion's belt, however, do lie at about the same distance from the earth and are therefore part of a true cluster. Like many of the stars, the stars of the belt have Arabic names. These names derive from the time of Europe's "Dark Ages" when the Arabs were the keepers and developers of ancient Greek astronomy. When Europeans rediscovered astronomy during the late Middle Ages, it was often by way of Arabic translations of the Greek texts. Mintaka (MIN-tack-a) means "belt." Alnitak (Al-NYE-tack) also means "the belt." The meaning of Alnilam (Al-NILE-am) is less dear but is possibly "the belt of pearls," a beautiful name for this string of dazzling stars. As we go along, you may begin to wonder why so many star names begin with "Al-." The answer is a simple one. "Al-" is the Arabic prefix which means "the."
5th: We require a convenient way to describe the positions of the stars on the celestial sphere. This is accomplished through the use of an angular method of measurement, with the vertex of the measured angle at the eye of the observer. The full circle of the sky, all the way around the earth, is 360 degrees (360°). The angle from horizon to horizon passing over your head is 180°, and the angle from the horizon to the zenith is 90°. If you stretch your arm out in front of you and sight along it with one eye, the angle between the tips of your spread fingers is about 15°. This is the width of the constellation Orion. It should take about six handspans to measure the distance from the horizon to the zenith. Try it and see how closely your hand and arm fit the rule.
6th: Our custom of dividing a circle into 360° derives from ancient Babylonian astronomy. As seen from the earth, the sun appears to make a full circuit of the sky in a little over 365 days (see Jan. 8). A degree, then, as defined by the Babylonians, was about the distance the sun moved each day with respect to the background of stars. The three little stars of Orion's head occupy a circle of about 1° diameter. The angular size of the moon against the sky is 1/2°, or about half the width of your little finger held at arm's length. The moon would therefore fit nicely between the three stars of Orion's head. If you look for these stars on a clear night you might have the impression that the moon is much larger than the space they enclose. Hold out your little finger against these stars and then against the moon, and you will discover that the moon is smaller than you think. Some other useful guides for measuring angles in the sky are shown at the right.
7th: The earth turns on its axis under the stars once every 24 hours, and carries us around as it goes. The stars remain fixed in the deeps of space. The earth turns west to east on its axis. As a result, the stars — with sun, moon, and planets — seem to move from east to west, making one full circuit around the earth each day. Like the sun and moon, the stars of Orion rise in the east and set in the west about 12 hours later. If you watch Orion throughout the evening, you will see him move one handspan (15°) toward the west each hour. In 24 hours, Orion will set in the west, pass beneath the earth, and rise again from the east to regain his present position. The illusion that it is the stars, not us, which move is very powerful. The earth is near at hand and seems massive and stationary compared to the apparently tiny celestial objects. Only since the brilliant theoretical work of Nicholas Copernicus in the 16th century have we come to recognize that the "turning" of the stars is actually the turning of the earth.
8th: In addition to a daily spin on its axis, the earth makes a great annual journey around the sun. Since the stars we see at night are those on the side of the earth opposite the sun, the evening sky changes as our vantage point changes. As the earth carries the observer eastward around the sun, the stars seem to move night by night toward the west at a rate of about 1° per day. In 6 months' time, looking out into space from the other side of the sun, we shall find other stars in our starry night.
9th: The stars do not appear equally bright in the sky. This is due to two things: (1) the stars are at different distances from the earth, and (2) the stars are not all of the same intrinsic brightness. The scale that is used to describe the brightness of stars as they appear to earth observers is called the scale of a
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