4.6 Stars. Star Patterns

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1 4.6 Stars Star Patterns If you have ever looked up at the night sky in winter, you probably would recognize the pattern of stars below. The Ancient Greeks named this pattern of stars Orion, after the giant hunter that Zeus placed among the stars. Today, Canadians and Americans still call it Orion. Others, like Australian Aborigines, call it "Julpan," which means canoe. Constellations and Asterisms A constellation is a large region of stars in the night sky. The stars in a constellation may be many light years from one another, but from Earth, they appear to be grouped closely together. A pattern of stars observed in the night sky is called an asterism. A constellation may contain several asterisms, or may itself be an asterism. The Big Dipper is an asterism within the constellation called Ursa Major, or Great Bear. Think of a constellation as a county and stars within it as the towns and villages. There are 88 known constellations. Well-known constellations that can be seen from the northern hemisphere include Leo (the Lion) and Scorpio (the Scorpion).

2 Changes in the Night Sky Ancient astronomers also wondered why different constellations were visible at different times of the year. Earth's revolution around the Sun changes our view of the night sky with each passing season. In the winter sky, we see stars such as Orion that are opposite the Sun. Other constellations are still there, but they are in our sky during the day. We cannot see them because the Sun's light is too bright. In the summer sky, we see constellations such as Lyra in the night sky. Orion is invisible to us because it is in the sky during the day. Polaris, the North Star, is different from most stars-it is always visible above the northern horizon. This means that we can see Polaris all year long in the northern hemisphere. The same is true for some constellations. A star or constellation that is always visible in the sky is said to be circumpolar. Circumpolar constellations in the northern hemisphere include Cassiopeia, Cepheus, Ursa Major, Ursa Minor, Camelopardalis, and Draco.

3 How People Use the Stars Many civilizations, both past and present, use the apparent motion of celestial objects as tools. For example, many cultures track the cyclical movements of the Sun, the Moon, and the stars to determine when to plant and harvest crops. Stars may be used to navigate while sailing in the oceans. Using the Stars on Land Ancient Egyptians noticed that the star Sirius always appeared in the summer sky. Its appearance coincided with the annual flooding of the Nile River. The Egyptians used their observations of the star Sirius to predict the annual floods of the Nile River. The flooding provided rich soil for growing crops. Therefore, the appearance of Sirius in the sky signalled the time to plant. Many of the most spectacular buildings in ancient cultures were built to keep track of important celestial events, such as the solstices and equinoxes. In Britain, the ancient Celts used Stonehenge to predict the timing of the seasons, which was important for their agriculture, and also for their religion. In Mexico, the ancient Mayans built the giant pyramid at Chichén ltzá to mark the equinoxes. On the equinox, the shadow of the serpent Kukulcan appears along the stairway.

4 Using the Stars at Sea Ancient sailors used the stars to find their way at sea. For example, sailors used their knowledge of the movement of specific stars, and where the stars would rise and set on the horizon, to determine where their ships were located during the journey. Azimuth and Altitude In geography, latitude and longitude are used to pinpoint a location on Earth's surface. In astronomy, two similar measurements are used to pinpoint the location of a celestial object, as viewed from Earth. Azimuth is the distance measured from north along the horizon to a point directly below the celestial object. North has an azimuth of 0, east has an azimuth of 90, south has an azimuth of 180, and west has an azimuth of 270. Altitude is the angular height of a celestial object, measured from the horizon. You can use your hands to determine the position of celestial objects using azimuth and altitude. lf you hold your hand out at arm's length, the width of your index finger is about equal to 1. The width of a fist at arm's length is about equal to 10. lf you extend the fingers of your hand, the width from the tip of your pinky finger to the tip of your thumb is about equal to 20. Using this method, you can estimate the azimuth and altitude coordinates of any celestial object in the sky relative to your position on Earth.

5 Worksheet What is the difference between an asterism and a constellation? 2. Why do most constellations appear to move from east to west across the sky each night? 3. Why do we see different constellations in the night sky during different seasons? 4. What is a circumpolar constellation? 5. What was the link between agriculture and astronomy in ancient cultures? 6. Why did ancient cultures build large structures such as Stonehenge? 7. Explain how sailors were able to find their way at sea before the invention of navigation devices. 8. Explain to a classmate how you can measure the azimuth of a celestial object.