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Chapter 1 Observing the Sky: The Birth of Astronomy What do we see when we look at the sky? Why did people look at the sky? Where did astronomy begin? January 10, 2006 Astronomy 2010 1 1.1 The Sky Above Many of the ways we think about the sky come from antiquity. The celestial sphere zenith, horizon celestial poles, celestial equator Ecliptic, zodiac, constellations January 10, 2006 Astronomy 2010 2 The Celestial Sphere An imaginary sphere on which the stars are fixed. The Earth is at the center. January 10, 2006 Astronomy 2010 3 On the Celestial Sphere Useful locations: Celestial equator Celestial poles (North and South) Zenith Horizon The celestial sphere appears to rotate, about the axis passing through the celestial North and South Poles, once per day due to the Earth’s rotation. January 10, 2006 Astronomy 2010 4 Objects Rotate Around the Poles The stars appear to rotate about the celestial pole. Polaris, the North star, is near the North celestial pole, and appears stationary. January 10, 2006 Astronomy 2010 5 Stars from Different Latitudes January 10, 2006 Astronomy 2010 6 Planets and Constellations The planets move relative to the stars “Planet” is ancient Greek for “wanderer” Stars appear “fixed” to the celestial sphere The sky is divided into 88 constellations Think of constellations like counties in the sky. The borders of the constellations are irregular The 12 constellations along the Sun’s path (the ecliptic) make up the zodiac. January 10, 2006 Astronomy 2010 7 1.2 Ancient Astronomy Many (all?) ancient civilizations made astronomical observations. Our astronomy comes from the Greeks and Romans via the Arabic culture names. Greeks believed the Earth a sphere: 1. The Moon is, so the Earth should be as well. 2. The Earth casts a round shadow on the Moon during lunar eclipses. 3. Different stars visible, and height of North star changed between Greece and Egypt. January 10, 2006 Astronomy 2010 8 Eratosthenes and Earth’s Diameter About 200 B.C., Eratosthenes used shadows cast in vertical wells to determine the diameter of the Earth. How do we know the Earth is round? January 10, 2006 Astronomy 2010 9 Hipparchus and Precession Hipparchus: - catalogued about 850 stars - noted that the position of the North celestial pole had shifted over a period of 150 years. The Earth spins and wobbles, like a top. The wobble is the precession of the Earth’s axis. Takes about 26,000 years for the axis to complete 1 “wobble”. January 10, 2006 Astronomy 2010 10 Ptolemy and Geocentric Model Ptolemy created a model of the solar system which predicted the positions of the planets. 1. 2. 3. 4. Stationary Earth near the center of the solar system Planets, Sun, Moon, and stars orbit the Earth All orbits are circles (perfect, god-like shape) Epicycles to account for retrograde motion (movie) The Ptolemaic model was used for more than a thousand years. January 10, 2006 Astronomy 2010 11 Epicycles Epicycles were required to account for “retrograde” motion of the planets. January 10, 2006 Astronomy 2010 12 Astrology and Astronomy Astrology is the belief that our lives are governed by the motions of stars and planets. Began as an ancient religion. No evidence supporting this claim! Astronomers appreciate astrology only to the extent that it increases the public’s awareness of constellations, planets, and astronomical phenomena. But it has given us the line “Hi, what’s your sign”. January 10, 2006 Astronomy 2010 13 1.4 Birth of Modern Astronomy Nicolaus Copernicus (1473 – 1543) Published his heliocentric model of the solar system in 1543 (shortly before his death). Placed Sun at the center of the solar system with planets orbiting on circles. How does one choose between heliocentric and geocentric models? Galileo Galilei (1564 – 1642) Galileo strongly advanced the scientific method. January 10, 2006 Astronomy 2010 14 Galileo (cont’d) First to use a telescope for astronomy. Saw craters and “seas” on the Moon Saw four moons orbiting Jupiter Saw that the Milky Way contained many more stars than visible with the naked eye Saw phases of Venus predicted by heliocentric model. January 10, 2006 Astronomy 2010 15 Phases of Venus Venus exhibits phases (like the Moon) that depend on the relative orientation of the Sun, the Earth, and Venus. January 10, 2006 Astronomy 2010 16