Astronomy 110 Lecture 2. Fall, 2005 Astronomy 110 1 We each define a local horizon plane that is tangent to the Earth. It defines up and down and divides the sky into observable (above) and unobservable (below) halves Fall, 2005 Astronomy 110 2 Azimuth Zenith and Azimuth Angles Determine a Local Coordinate System Fall, 2005 Astronomy 110 3 The Earth’s Spin Defines Natural References For Location Fall, 2005 Astronomy 110 4 Fall, 2005 Astronomy 110 5 We project the naturally defined markers on the Earth (poles and the Equator) onto the sky to begin to establish a coordinate system on the celestial sphere. Fall, 2005 Astronomy 110 6 In the Northern hemisphere the star Polaris is currently (accidentally) very close to the North Celestial Pole. Fall, 2005 Astronomy 110 7 There is no such accidental alignment in the Southern hemisphere Fall, 2005 Astronomy 110 8 Why do stars rise and set? The Earth rotates east to west, so stars appear to circle from west to east. Fall, 2005 Astronomy 110 9 Coordinates on the Earth • Latitude: position north or south of equator • Longitude: position east or west of prime meridian (runs through Greenwich, England) Fall, 2005 Astronomy 110 10 The Night Sky Rotates as if stars were stuck on a rigid rotating sphere – The Celestial Sphere Fall, 2005 Astronomy 110 11 What are constellations? A constellation is a region of the sky. 88 constellations fill the entire sky. Fall, 2005 Astronomy 110 12 Fall, 2005 Astronomy 110 13 The Celestial Sphere Fall, 2005 Astronomy 110 14 Angle measurements: • Full circle = 360º • 1º = 60′ (arcminutes) •1′ = 60″ (arcseconds) Fall, 2005 Astronomy 110 15 How Long is a Day? • Solar day: The time it takes the Sun to return to the same position in the sky = 24 hours • Sidereal day: The time it takes a star to return to the same position in the sky = Earth’s rotation period = 23hr, 56min Fall, 2005 Astronomy 110 16 The Analemma The Solar day is not constant because the Earth’s motion with respect to the Sun is not constant Fall, 2005 Astronomy 110 17 Why don’t we see the same constellations throughout the year? 1. The Night Sky Varies With Latitude. 2. The Night Sky Varies With the Time of Year. Fall, 2005 Astronomy 110 18 The Altitude of the Celestial Pole is Your Latitude Fall, 2005 Astronomy 110 19 The sky varies as Earth orbits the Sun • As the Earth orbits the Sun, the Sun appears to move eastward along the ecliptic. • At midnight, the stars on our meridian are opposite the Sun the in the sky. Fall, 2005 Astronomy 110 20 What we see of the night sky is determined by the intersection of our local horizon with the celestial sphere. Fall, 2005 Astronomy 110 21 Some stars never cross the observer’s horizon and thus never appear to rise or set so they are always either above or below the horizon. If they are above the horizon they are called circumpolar and are always visible throughout the year. If they are below the horizon – they cannot be seen from that latitude. Fall, 2005 Astronomy 110 22 Our view from Earth: In general, for observers in the northern hemisphere: • Some stars are near enough to the north celestial pole that they are circumpolar and never set. • Some stars are near enough to the south celestial pole that they can never be observed from that latitude. • All other stars as well as the Sun, Moon and planets rise in east and set in west. Celestial Equator Your horizon Fall, 2005 Astronomy 110 23 Fall, 2005 Astronomy 110 24 How Can We find Our Latitude? 1. Paths of Stars as they Set 2. Altitude of the Pole 3. Region of the Sky that is Circumpolar Fall, 2005 Astronomy 110 25.