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ASTR100 (Spring 2008) Introduction to Astronomy The Science of Astronomy Prof. D.C. Richardson Sections 0101-0106 How did astronomical observations benefit ancient societies? Keeping track of time and seasons. For practical purposes, including agriculture. For religious and ceremonial purposes. Aid to navigation. Ancient people of central Africa (6500 BC) could predict seasons from the orientation of the crescent moon. Days of week were named for Sun, Moon, and visible planets Planets Known in Ancient Times The “Wanderers” • Mercury – difficult to see; always close to Sun in sky • Venus – very bright when visible — morning or evening “star” • Mars – noticeably red • Jupiter – very bright • Saturn – moderately bright What did ancient civilizations achieve in astronomy? • • • • • • daily timekeeping tracking the seasons and calendar monitoring lunar cycles monitoring planets and stars predicting eclipses and more… Egyptian obelisk: shadows tell time of day, like a modern-day sundial. England: Stonehenge (completed around 1550 B.C.) SW United States: “Sun Dagger” marks summer solstice Wyoming: Big Horn Medicine Wheel South Pacific: Polynesians were very skilled in art of celestial navigation Modern science traces its roots to the Greeks • Greeks were the first people known to make models of nature. • They tried to explain patterns in nature without resorting to myth or the supernatural. Greek geocentric model (c. 400 BC) How did the Greeks explain planetary motion? Underpinnings of the Greek geocentric model: • Earth at center of Universe. • Heavens must be “perfect”: objects moving on perfect spheres or in perfect circles. Plato Aristotle But this made it difficult to explain apparent retrograde motion of planets… Over a period of 10 weeks, Mars appears to stop, back up, then go forward again… Explaining Apparent Retrograde Motion Easy for us to explain: occurs when we “lap” another planet (or when Mercury or Venus lap us). But very difficult to explain if you think that Earth is the center of the universe! In fact, ancients considered but rejected the correct explanation… The most sophisticated geocentric model was that of Ptolemy (A.D. 100-170) — the Ptolemaic model: • Sufficiently accurate to remain in use for 1,500 years. Ptolemy • Copernicus (1473-1543) proposed a Sun-centered model, but still based on perfect circles. Copernicus (1473–1543) • Copernicus (1473-1543) proposed a Sun-centered model, but still based on perfect circles. • Tycho Brahe (1546-1601) made careful observations but could not detect Earth’s motion. • Copernicus (1473-1543) proposed a Sun-centered model, but still based on perfect circles. • Tycho Brahe (1546-1601) “If I had believed that we could made carefulignore observations these eight minutes but [of arc], would could Inot detect have patched up myEarth’s hypothesis motion. accordingly. But, since it was not • Kepler Brahe’s data permissible to ignore, thoseused eight show Copernican minutes pointed theto road to the a complete model could work, but only reformation in astronomy.” if planetary orbits are Johannes Kepler ellipses, not circles! (1571-1630) Kepler’s three laws of planetary motion Kepler’s First Law: The orbit of each planet around the Sun is an ellipse with the Sun at one focus. Kepler’s Second Law: As a planet moves around its orbit, it sweeps out equal areas in equal times. This means that a planet travels faster when it is nearer to the Sun and slower when it is farther from the Sun. Kepler’s Third Law More distant planets orbit the Sun at slower average speeds, obeying the relationship p2 = a3 p = orbital period in years a = avg. distance from Sun in AU Kepler’s 3rd Law Thought Question An asteroid orbits the Sun at an average distance of a = 4 AU. How long does it take to orbit the Sun? A. B. C. D. 4 years. 8 years. 16 years. 64 years. (Hint: remember that p2 = a3.) Thought Question An asteroid orbits the Sun at an average distance of a = 4 AU. How long does it take to orbit the Sun? A. B. C. D. 4 years. 8 years. 16 years. 64 years. (Hint: remember that p2 = a3.) How did Galileo solidify the Copernican revolution? Galileo (1564-1642) overcame major objections to the Copernican view. • Using his telescope, Galileo saw: sunspots on Sun (“imperfections”) mountains and valleys on the Moon (proving it is not a perfect sphere) Galileo also saw four moons orbiting Jupiter, proving that not all objects orbit the Earth… Ptolemaic View Copernican View … and his observations of phases of Venus proved that it orbits the Sun and not Earth. ASTR100 (Spring 2008) Introduction to Astronomy Newton’s Laws of Motion Prof. D.C. Richardson Sections 0101-0106 How do we describe motion? Precise definitions to describe motion: Speed: rate at which object moves. speed = distance/time (units: m/s) Example: speed of 10 m/s. Velocity: speed and direction. Example: 10 m/s due east. Acceleration: any change in velocity. Units: speed/time (m/s2). The Acceleration of Gravity All falling objects accelerate at the same rate (not counting friction of air resistance). On Earth, g ≈ 10 m/s2: speed increases 10 m/s with each second of falling. The Acceleration of Gravity (g) Galileo showed that g is the same for all falling objects, regardless of their mass. Apollo 15 demonstration Momentum and Force Momentum = mass × velocity. A net force changes momentum. Often only velocity changes (not mass). The rotational momentum of a spinning or orbiting object is known as angular momentum. Is there a net force? Y/N 1. 2. 3. 4. 5. A car coming to a stop. A bus speeding up. An elevator moving at constant speed. A bicycle going around a curve. A moon orbiting Jupiter. How is mass different from weight? mass – amount of matter in an object. weight – force that acts upon an object. You are weightless in free-fall! On the Moon… A. B. C. D. My My My My weight weight weight weight is is is is the same, my mass is less. less, my mass is the same. more, my mass is the same. more, my mass is less. On the Moon… A. B. C. D. My weight is the same, my mass is less. My weight is less, my mass is the same. My weight is more, my mass is the same. My weight is more, my mass is less. Why are astronauts weightless in space? • There IS gravity in space… • Weightlessness is due to a constant state of free-fall… How did Newton change our view of the Universe? Realized the same physical laws that operate on Earth also operate in the heavens… one universe. Discovered laws of motion and gravity. Much more: experiments with light; first reflecting telescope; calculus… Sir Isaac Newton (1642-1727)