The Main Point. General Introduction to the Sky. Coordinates: Getting your bearings. Lecture #2: The Sky and Historical Astronomy

Transcription

1 Lecture #2: The Sky and Historical Astronomy General Introduction to the Sky. Coordinates: Getting your bearings. Earth's spin and orbital motion. Constellations & Astrology. Classical astronomy. Modern astronomy. The Main Point The movements of the Sun, Moon, planets, and stars that we observe every day and night are apparent, not actual motions, because we are moving as well! Astro 102/104 1 Astro 102/104 2 General Introduction to the Sky Coordinates: Getting your bearings Watching and knowing the sky is the key to understanding our place in the Cosmos. For most of human history, knowing the sky was critical for survival. Today, hardly anyone ever looks up... The sky moves! (or does it...) Actual vs. Apparent motion. Astro 102/104 3 Astro 102/

2 Key concepts: Zenith. Horizon. Celestial Sphere. Celestial Pole. Rotation Axis. Celestial Equator. Latitude. People living in different places on the Earth see different motions of the night sky... Astro 102/104 5 Astro 102/104 6 Earth's Spin and Orbital Motion Earth spins once per day on its axis. Not obvious! The sky appears to spin, not us! Earth orbits around the Sun once per year. Not obvious! The Sun appears to orbit us! Earth's orbital plane is called the ecliptic this is also the apparent path of the sun on the sky. Earth's axis is tilted relative to its orbit: The tilt angle (obliquity) is about The tilt is the reason for the seasons! (Lec. 4). Astro 102/104 7 Key Concepts: Earth's axis is tilted with respect to its orbital plane (ecliptic). Tilt angle changes the angle of sunlight striking the Earth's surface. At a fixed location on the Earth, the angle of the sunlight varies with time. Seasons! Other planets have different tilts, and thus different types of seasons. More details in Lecture 4 Astro 102/

3 Constellations & The Zodiac Constellations are patterns of stars in the sky, used by ancient peoples for navigation and folklore. They provide a "roadmap" to the sky, but no real scientific information (the stars happen to appear adjacent in the sky, but most are far apart in space). During one year, the Sun appears to travel through a group of constellations along the ecliptic: this zone is called the zodiac. The planets also move through the same zone since they are (essentially) in the same orbital plane as the Earth. Astro 102/104 9 The night sky: Late January sky at ~7 PM. Astro 102/ Orion The Winter Hexagon, looking ESE at ~8 PM. The Winter Triangle Looking SSE in late January at ~9 PM. Procyon Winter Canis Minor Triangle Betelgeuse Monoceros Rigel Sirius Canis Major Lepus Astro 102/ Astro 102/

4 Orion, The Hunter. Betelgeuse Bellatrix Looking west on early winter evening. Orion Nebula Saiph Rigel Astro 102/ Astro 102/ The Sun's apparent motion through the zodiac. Astrology ( Astronomy!) Astrology is an ancient system of beliefs that claimed that the positions of the Sun, Moon, and planets governed the actions and events in our lives. Astrology's ability to predict actions or personality traits has been repeatedly tested--and it fails. Yet astrology and horoscopes continue to pervade society and the media. Why? Therapeutic power of guidance and advice? Connection to a simpler, less technological time? Astrology is entertainment, not science. Astro 102/ Astro 102/

5 Classical Astronomy Many ancient civilizations had sophisticated concepts of astronomy and cosmology. Babylonians, Assyrians, Egyptians, Chinese, Mayans, Polynesians, Druids,... Calendars, navigation, crop & river cycles, comets, supernovae, Venus,... Much of modern astronomy derives from the Greeks and Romans (~ years ago). Astro 102/ What did the ancients know? The Earth is round. Pythagoras ( B.C.): "perfect spheres" Aristotle Stars rise higher or lower in elevation as one travels north or south. Lunar eclipse shadows. Masts of ships appear before hulls as they approach the shore. Earth's diameter ~12,700 km. Eratosthenes (~ 200 B.C.) Sticks, shadows, and pacing the distances. Derives diameter = 12,500 km (1.5% error!) Astro 102/ What did the ancients know? Precession of the equinoxes: Hipparchus' data (~ 150 B.C.) Earth's axis precesses like a top. Relative size of Earth and Moon (~ 4:1): Aristarchus ~( B.C.) From relative size of shadow of Earth on Moon during eclipses. Distance to the Moon (60 R earth = km): From geometry: angle subtended by the moon and its size. What did the ancients know? Distance to the sun - Aristarchus ~ ( B.C.) Look at the Sun, Earth, Moon system at quarter moon. Right angle formed. First Quarter Moon Distance to sun given by: α D sun =D moon /cos(θ ). D Moon θ D Measure θ ~ 87, Sun Earth D sun /D moon = 19. So that the Sun is much further away than the Moon. Actually θ ~ (a very difficult angle to measure). The real value is D sun /D moon = 386, (or D sun = 150 million km). Sun Astro 102/ Astro 102/

6 What did the ancients know? Relative size of the Sun and Moon Aristarchus ~ ( B.C.) Subtend the same angle (~ 1/2 ). Aristarchus deduces that since the Sun is much further away than the Moon, it is also much larger than the Moon (Actual: 386:1). Aristarchus concludes that the Sun is very massive, and therefore likely the central body of the solar system. This model was overlooked for many years because of an argument from authority. Aristotle ( B.C.): If Earth is spinning, why don t things fly off? Astro 102/ What did they think they knew? Earth is the center of the Universe. Simplest explanation for apparent motions! A comforting result. No stellar parallax observed. Sun, Moon, & Planets orbit Earth. Ptolemy (~ 140 A.D.) Complex circular paths. Model could predict positions! Ptolemaic (geocentric) cosmology would be the dominant paradigm for 1000 years! Astro 102/ "Modern" Astronomy Pioneers Nicholas Copernicus ( ) Proposed a heliocentric cosmology. Earth and planets circle the Sun. Only the Moon circles the Earth. A simple model; it fit the data. But a controversial model: Difficult to test at the time. Any experiments were widely mistrusted anyway. Inconsistent with contemporary philosophy, theology. Astro 102/ "Modern" Astronomy Pioneers Galileo Galilei ( ) Used math to describe acceleration. First astronomer to use a telescope! Discovered sunspots on the Sun. Discovered moons of Jupiter (1609). Explored features on the Moon. Discovered phases of Venus. Provided data and tests needed to validate the Copernican heliocentric model of the solar system. Dethroning of the Earth still met fierce resistance. Astro 102/

7 Summary Actual vs. Apparent motion: Earth spins on its axis. Earth's axis is tilted w.r.t. ecliptic: Seasons! Motion of the sky depends on latitude. Earth, planets orbit around the Sun. The need to know the sky was the foundation of astronomy. Greek, Roman astronomers provided the first pieces. Astrology is the historical relic of ancient astronomy: Astrology Astronomy!!! Copernicus and Galileo started modern astronomy. Astro 102/ Orbits and Gravity. Kepler and Newton. Next Lecture... Reading this week: Preface and Chapter 1. Chapters 2.1, 2.4, 3. Chapter 4. Astro 102/