Transcript Lecture 02
The night sky? With the naked eye, we can see more than 2000 stars as well as the Milky Way. Copyright © 2012 Pearson Education, Inc. Constellations A constellation is a region of the sky. Eighty-eight constellations fill the entire sky. Copyright © 2012 Pearson Education, Inc. The Celestial Sphere Stars at different distances all appear to lie on the celestial sphere. The ecliptic is the Sun’s apparent path through the celestial sphere. Copyright © 2012 Pearson Education, Inc. The Celestial Sphere The 88 official constellations cover the celestial sphere. Copyright © 2012 Pearson Education, Inc. Question : The celestial equator is ? A The path of the Sun compared to the stars. B The path of the Moon compared to the stars. C Always directly overhead at the Earth's equator. D The average path of planets on a star chart. E Always along the horizon for people on Earth's equator. Copyright © 2012 Pearson Education, Inc. The Milky Way A band of light that makes a circle around the celestial sphere What is it? Our view into the plane of our galaxy Copyright © 2012 Pearson Education, Inc. The Milky Way Copyright © 2012 Pearson Education, Inc. The Local Sky An object’s altitude (above horizon) and direction (along horizon) specify its location in your local sky. Copyright © 2012 Pearson Education, Inc. The Local Sky Zenith: The point directly overhead Horizon: All points 90° away from zenith Meridian: Line passing through zenith and connecting N and S points on the horizon Copyright © 2012 Pearson Education, Inc. Angular Size An object’s angular size appears smaller if it is farther away. Copyright © 2012 Pearson Education, Inc. Apparent Brightness • A bright star in the sky. – Is really very bright? – Or is it very dim but very close? • A dim star in the sky – Is it really very dim? – Or is it very bright but very far away? Copyright © 2012 Pearson Education, Inc. Why do stars rise and set? Earth rotates west to east, so stars appear to circle from east to west. Copyright © 2012 Pearson Education, Inc. Question : In the northern hemisphere, the stars rise in the East, set in the West and revolve counter-clockwise around the North celestial pole. In the southern hemisphere the stars? A. Rise in the East, set in the West and revolve anti-clockwise around the South celestial pole. B. Rise in the East, set in the West and revolve clockwise around the South celestial pole. C. Rise in the West, set in the East and revolve clockwise around the South celestial pole. D. Rise in the West, set in the East and revolve anti-clockwise around the South celestial pole. Copyright © 2012 Pearson Education, Inc. Our view from Earth: • Stars near the north celestial pole are circumpolar and never set. • We cannot see stars near the south celestial pole. • All other stars (and Sun, Moon, planets) rise in east and set in west. A circumpolar star never sets. celestial equator This star never rises. Copyright © 2012 Pearson Education, Inc. your horizon Thought Question What is the arrow pointing to? A. the zenith B. the north celestial pole C. the celestial equator Copyright © 2012 Pearson Education, Inc. Question : You are standing at the north pole and you look at the stars. Which statement below describes what you would see? A B. C. D. There are no circumpolar stars. There are only circumpolar stars just after sunset. Some stars will be circumpolar and some will not. Every star you see is circumpolar. Copyright © 2012 Pearson Education, Inc. Question : You are standing on the Equator and you look at northern pole star on the horizon. Which statement below describes what you would see? A B. C. D. There are no circumpolar stars. There are only circumpolar stars just after sunset. Some stars will be circumpolar and some will not. Every star you see is circumpolar. Copyright © 2012 Pearson Education, Inc. Why do the constellations we see depend on latitude and time of year? • • They depend on latitude because your position on Earth determines which constellations remain below the horizon. They depend on time of year because Earth’s orbit changes the apparent location of the Sun among the stars. Copyright © 2012 Pearson Education, Inc. Review: Coordinates on the Earth • Latitude: position north or south of equator • Longitude: position east or west of prime meridian (runs through Greenwich, England) Copyright © 2012 Pearson Education, Inc. The sky varies with latitude but not longitude. Copyright © 2012 Pearson Education, Inc. Altitude of the celestial pole = your latitude Copyright © 2012 Pearson Education, Inc. Thought Question The North Star (Polaris) is 50° above your horizon, due north. Where are you? A. You are on the equator. B. You are at the North Pole. C. You are at latitude 50°N. D. You are at longitude 50°E. E. You are at latitude 50°N and longitude 50°E. Copyright © 2012 Pearson Education, Inc. The sky varies as Earth orbits the Sun • As Earth orbits the Sun, the Sun appears to move eastward along the ecliptic. • At midnight, the stars on our meridian are opposite the Sun in the sky. Copyright © 2012 Pearson Education, Inc. Special Topic: How Long Is a Day? • Solar day = 24 hours • Sidereal day (Earth’s rotation period) = 23 hours, 56 minutes Copyright © 2012 Pearson Education, Inc. Summary Of The Night Sky • Now work on Page 1-6 of the tutorial book. Copyright © 2012 Pearson Education, Inc.