1	Chapter 2 Discovering the Universe for Yourself Chapter Outline Patterns in the Sky The Circling Sky The Moon, Our Constant Companion The Ancient Mystery of the Planets
2	Stonehenge Stonehenge is late Neolithic stone structure located in southern part of Salisbury Plain in south-central Britain Building periods Stonehenge I: 2800 B.C. Stonehenge II: 2100 B.C. Stonehenge III: 2000 - 1500 B.C. Stonehenge IV: 1100 B.C. Ancient calendar
3	For how long and how do we know that the Earth is spherical? Aristotle (384-322 BC) refers to the knowledge that the Earth is spherical as “old” knowledge and offers three observations to prove the point Ships disappear by “sinking” over horizon when sailing away from the shore A new pattern of stars (i.e., constellations) is observed when traveling north and south The Earth projects a circular shadow when it eclipses the Moon, that is the cross-section of a sphere
4	Size of the Earth Eratosthenes (273-193 BC) knew Earth is a sphere (Aristarchus, ca. 320-ca. 250 BC) Sun at least 20 times farther than Moon (nearer 400) Assumed rays of sunlight parallel Procedure Observing stations at Alexandria and Syene Nearly on same meridian of longitude Syene - Sun passes through zenith approximately on summer solstice Alexandria - observed Sun to be about 7^o south of zenith Pace distance, about 4900 stadia (1 stadium = 0.16 km) Angle at Earth's center is equal to zenith angle Calculated circumference C/4900 stadia = 360^o/7^o C = 252,000 stadia, or about 40,320 km Present value 40,030 km
5	Celestial Sphere
6	Why do we have constellations in the night sky? Constellations serve as an aid to time keeping, i.e., a primitive calendar Constellations make nighttime navigation possible For modern astronomers, constellations are a first level of organization of the sky with definite boundaries
7	Constellation of Orion
8	View from Earth in Our Galaxy
9	Definitions in Your Local Sky
10	Common Cyclic Phenomena Diurnal motion – daily rising and setting Solar annual apparent motion – yearly motion of Sun from west to east relative to the background stars Lunar phases – monthly cycle of illumination for the Moon Solar and lunar eclipses – shadow effect Planetary configurations – changing position of the planets relative to the background stars
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12	Sun’s Annual Motion
13	Phases of the Moon
14	Why are eclipses important to astronomy? If one can predict an eclipse then we have an accurate time keeping system They can help in the study of the Sun (solar eclipse)
15	Conditions for Solar and Lunar Eclipse
16	Solar Eclipses
17	What observations did ancients make of planetary motion? They did recognize the five naked-eye planets as moving relative to background stars They observed planetary configurations and measured synodic periods They observed retrograde motions of planets, such as Mars
18	Inferior Planet Configurations
19	Conjunction
20	Mars Retrograde Motion in 2003
21	Explaining Retrograde Motion
22	The Big Picture You can enhance your enjoyment of learning astronomy by spending time outside observing the sky. The more you learn about the appearance and apparent motions of the sky, the more you will appreciate the universe of which you are a part. From Earth, one imagines that we are at the center of a great celestial sphere-even though we really are on a planet orbiting a star in a vast universe. We can understand what we see in the local sky by thinking about how the celestial sphere appears. Most of the phenomena of the sky are relatively easy to observe and understand. But the more complex phenomena, particularly eclipses and apparent planetary motion, challenged our ancestors for thousands of years and helped drive the development of science.