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Constellations Patterns in the Sky • Stars which are “close” to each other (in angle) form patterns called constellations. – Not really close together • Constellations change are unchanging year after year. – Change over thousands of years is proper motion Greek Myth • Constellations are named for characters in Greek mythology. • Asterisms are features within a constellation. – Orion’s belt Polaris - The North Star • Polaris is a star very near the north celestial pole. – About halfway up from the north horizon in DeKalb – Not visible south of the equator • Stars in the northern hemisphere rotate around Polaris each night. • Circumpolar stars don’t cross the horizon. Big Dipper Polaris • The Big Dipper is an asterism in the constellation of Ursa Major. • Two stars in the Dipper are pointer stars to Polaris. – End of bucket – Handle of Little Dipper Big Dipper Seasonal Shifts • The Sun masks the light of the stars. • Constellations appear in certain seasons. • Stars peak overhead two hours earlier each month. Ecliptic • The earth is tilted 23º on its axis. • The celestial equator is not in line with the orbit of the sun, moon and planets. • The sun, moon and planets follow an imaginary curve called the ecliptic. Zodiac • The constellations that lie along the ecliptic are called the zodiac. – 12 constellations • The Sun passes through each one during the year. Stars and Planets • Stars are very numerous. • Stars are “fixed” relative to each other. • Planets are few in number (5 are visible to the unaided eye). • Planets “wander” along the ecliptic relative to the stars. • They produce their own light independent of the Sun’s location. • Their brightness does depend on the Sun’s location. • They are very far away - the Greeks understood this. • Planets have complicated, but predictable orbits when viewed from the Earth. Star Charts • Star charts are used to model the position of the stars. – Adjust for date and time • Websites can include planet positions, too. www.astronomy.com