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, mathemacs, 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 perfecon 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 vercal was 7 degrees or about 1/50th of a circle. • 50 x 500 miles gave him 25,000 miles circumference. Early Ideas: Distance and Size of the Sun and Moon • Aristarchus calculated the relave 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 Early Ideas: Distance and Size of the Sun and Moon

• Aristarchus calculated the diameter of the Sun based on 1st and 3rd quarter Moon phase observaons. • 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, pung 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 posion 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 • All the planets orbit the Sun the Greek word for on roughly the same plane. “wanderers.” • The planets normally move • The planets always remain eastward through the close to the eclipc, within the constellaons of the backdrop of stars as a result zodiac. of their orbital moon. Planets and the Zodiac

• Apparent moon of planets is usually from west to east relave to the stars, although on a daily basis, the planets always rise in the east Retrograde Moon

• Retrograde moon occurs when Earth overtakes another planet and it appears to move backward (E to W) against the backdrop of stars. • Explaining retrograde moon was one of the main reasons astronomers ulmately 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 of Alexandria

• Ptolemy created a model in which planets moved on small wheels aached to a larger wheel. • The small circle is called an epicycle. Ptolemy of Alexandria • Ptolemy’s model was able to predict planetary moon 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 scienfic theory. 2.3 Astronomy in the Renaissance Non-Western Contribuons • Islamic Contribuons – Relied on celesal phenomena to set its religious calendar – Created a large vocabulary sll evident today (e.g., zenith, Betelgeuse) – Developed algebra and Arabic numerals • Asian Contribuons – Devised constellaons based on Asian mythologies – Kept detailed records of unusual celesal events (e.g., eclipses, comets, supernova, and sunspots) – Eclipse predicons 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 meculous, accurate measurements of planetary posions. 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 Kepler’s 1st Law

• Planets move in ellipcal orbits with the Sun at one focus of the ellipse Kepler’s 2nd 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 me intervals • The closer a planet is to the Sun, the faster it moves Kepler’s 3rd Law

• The amount of me a planet takes to orbit the Sun is related to its orbit’s size • The square of the period, P, is proporonal 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 interpretaons • 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 orbing 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 moon” • Was brought before the Inquision and put under house arrest for the remainder of his life Isaac Newton • Isaac Newton (1642-1727) was born the year Galileo died • He deduced the Law of gravity and invented calculus to calculate it.