Chapter 2 The Rise of Astronomy Copyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
2.1: Early Ideas of the Heavens: Classical Astronomy As far as we know, the ancient Greeks were the first to use logic, mathemaecs, and geometry to study the world around us. The ancient Greeks knew the world was round.
Early Ideas: Pythagoras Pythagoras taught as early as 500 B.C. that the Earth was round, based on the perfeceon of the sphere.
Early Ideas: Aristotle By 300 B.C., Aristotle based it on the curved shape of Earth s shadow on the Moon during a lunar eclipse.
Early Ideas: Aristotle He also noted that a traveler moving south will see stars previously hidden by the southern horizon
Early Ideas: The Size of the Earth Eratosthenes (276-195 B.C.) Head of the library at at Alexandria, Egypt, calculated the circumference of the Earth to be about 25,000 miles. a value very close to today s value
Early Ideas: The Size of the Earth At Alexandria at noon on 6/21, the Sun cast a shadow on the obelisk, but at Syene (500 miles south) it shone straight down a well. The angle b/t Sun and verecal was 7 degrees or about 1/50 th of a circle. 50 x 500 miles gave him 25,000 miles circumference.
Early Ideas: Distance and Size of the Sun Aristarchus calculated the relaeve size of the Sun, Moon, and Earth He calculated the Moon to be 1/3 the Earth s diameter by looking at the Earth s shadow on the Moon and Moon
Early Ideas: Distance and Size of the Sun and Moon Aristarchus calculated the diameter of the Sun based on 1 st and 3 rd quarter Moon phase observaeons. He thought the Sun was only 7X the Earth s diameter (it s really 100X).
Early Ideas: Distance and Size of the Sun and Moon This helped establish that the Sun was much larger, pu[ng it at the center of the Universe. Other Greeks did not believe Aristarchus (Sun at center) because they could not observe parallax.
The reason they could not observe parallax ( apparent shi^ in a stars posieon from season to season) is because the stars are so far away and they had no telescopes.
Measuring the Diameter of Astronomical Objects l linear size of object d distance to object α angular size of object
2.2 The Planets
Planets and the Zodiac The word planet comes from the Greek word for wanderers. The planets always remain close to the eclipec, within the constellaeons of the zodiac. All the planets orbit the Sun on roughly the same plane. The planets normally move eastward through the backdrop of stars as a result of their orbital moeon.
Apparent moeon of planets is usually from west to east relaeve to the stars, although on a daily basis, the planets always rise in the east Planets and the Zodiac
Retrograde MoEon Retrograde moeon occurs when Earth overtakes another planet and it appears to move backward (E to W) against the backdrop of stars. Explaining retrograde moeon was one of the main reasons astronomers ulemately rejected the idea of the Earth being located at the center of the solar system
Early Ideas: The Geocentric Model Early observers had a geocentric model of the universe (Earth at center).
Early Ideas: The Geocentric Model
Ptolemy created a model in which planets moved on small wheels aeached to a larger wheel. The small circle is called an epicycle. Ptolemy of Alexandria
Ptolemy of Alexandria Ptolemy s model was able to predict planetary moeon with fair precision This model became to complex to be plausible. Occam s Razor is a principle which states that simplicity is an important part of scienefic theory.
2.3 Astronomy in the Renaissance
Non- Western ContribuEons Islamic ContribuEons Relied on celeseal phenomena to set its religious calendar Created a large vocabulary sell evident today (e.g., zenith, Betelgeuse) Developed algebra and Arabic numerals Asian ContribuEons Devised constellaeons based on Asian mythologies Kept detailed records of unusual celeseal events (e.g., eclipses, comets, supernova, and sunspots) Eclipse prediceons
Astronomy in the Renaissance Nicolaus Copernicus (1473-1543) was a Polish physician. His model was good but flawed because he put the planets in circular orbits.
Astronomy in the Renaissance Tycho Brahe (1546-1601) He was a Danish nobleman. He made meeculous, accurate measurements of planetary posieons.
Astronomy in the Renaissance Johannes Kepler (1571-1630) Upon Tycho s death, his data passed to Kepler, his young assistant Using the very precise Mars data, Kepler showed the orbit to be an ellipse
Planets move in ellipecal orbits with the Sun at one focus of the ellipse Kepler s 1 st Law
The orbital speed of a planet varies so that a line joining the Sun and the planet will sweep out equal areas in equal Eme intervals The closer a planet is to the Sun, the faster it moves Kepler s 2nd Law
Kepler s 3 rd Law The amount of Eme a planet takes to orbit the Sun is related to its orbit s size The square of the period, P, is proporeonal to the cube of the semimajor axis, a
2.4 The Birth of Astrophysics
Astronomy in the Renaissance Galileo (1564-1642) Contemporary of Kepler First person to use the telescope to study the heavens and offer interpretaeons The Moon s surface has features similar to that of the Earth The Moon is a ball of rock
Astronomy in the Renaissance The Sun has spots The Sun is not perfect, changes its appearance, and rotates Jupiter has four objects orbieng it The objects are moons and they are not circling Earth Milky Way is populated by uncountable number of stars Earth- centered universe is too simple
Evidence for the Heliocentric Model
Astronomy in the Renaissance Deduced the first correct laws of moeon Was brought before the InquisiEon and put under house arrest for the remainder of his life
Isaac Newton (1642-1727) was born the year Galileo died He deduced the Law of gravity and invented calculus to calculate it. Isaac Newton