Origins of Modern Astronomy

Astronomy of the Ancients Archaeoastronomy – is the study of astronomy ancient peoples. Ancient peoples were curious about the sky, sun, moon and stars. Their observations often included mythology and religion in there explanations.

Astronomy of the Ancients Some of these structures have survived and point to astronomical alignments Stonehenge in England (3000 B.C) Temple if Isis in Egypt (250 B.C) Sun Dagger in New Mexico (1200 A.D) Mayan Temples and Books (1500 A.D.) Unfortunately, many of these cultures either had no language or their records were lost or destroyed.

Greek Astronomy Greek Philosophers took a different approach to astronomy and nature. They suggested the universe was rational and could be understood using logic and reason. Thales of Miletus (624-547 B.C) – said the Universe was rational and could be understood. Pythagoras (570-500 B.C) – discovered how many things in nature could be described with geometry and mathematical relationships.

Greek Astronomy Plato (428-347 B.C.) – said the earth was imperfect and the heavens were perfect. To study the motion of the heavenly bodies was to focus not on human observation but on logic and thought of the ideal forms that underline nature. Plato said the most perfect shape was a sphere and the perfect motion was Uniform Circular Motion (U.C.M.) Plato’s Influence on those that followed him was significant. (Aristotle and Eudoxus)

Greek Astronomy Eudoxus of Cnidus (409-356 B.C.) – student of Plato who devised a system of 27 nested spheres that rotated at different rates around the earth. Mathematical model of motion of the universe. Some philosophers just saw these spheres as mathematical constructs. Others saw them as actual objects that went around the Earth.

Greek Astronomy Aristotle (384-322 B.C) – Also a student of Plato and he seemed to think the spheres suggested by Eudoxus were real. Believed in a geocentric universe (earth centered) Added spheres to Eudoxus’ system. His had 55 spheres turning at different rates.

Greek Astronomy Aristarchus – (About 280 B.C.) proposed the earth rotated on its axis and said it traveled around the sun. His idea’s lacked support because most of his writings were lost and it went against Aristotle’s ideas. Eratosthenes (About 200 B.C.) determined how to get an accurate approximation for the radius of the earth using geometry.

Greek Astronomy Ptolemy – (About 140 A.D.) based his work on Aristotle’s system. Set up a geocentric model that addressed the problem with planetary motion. Model was geocentric using U.C.M. Used retrograde motion to explain the motion of the planets. Had the planets travel in epicycles Mathematical Syntaxis or Al Magisti later updated to be the Alfosine Tables

Uniform Circular Motion Uniform Circular Motion (UCM) describes an object traveling in a circular path at a constant speed Does an object in U.C.M. accelerate? YES!! Because there is a change in Direction!!

Centripetal Acceleration ac = centripetal acceleration ac = (circular speed)2 radius of the circle ac = vc2/r S.I. Unit = m/s2

Centripetal Acceleration Acceleration that causes a change in direction. An object in circular motion has a centripetal acceleration. Centripetal (means “center-seeking”) Acceleration always acts towards the center of the circle.

Circular Speed Circular speed (vc) is the speed an object travels in a circle with. vc = distance = circumference time time vc = 2pr/t

Centripetal Force Centripetal (Fc) Force is the Force that causes a change in direction We can use Newton’s 2nd Law to calculate the Centripetal Force (Fc) Fc = mac

The Copernican Revolution Nicolaus Copernicus – (1473-1543) was the first to set up a heliocentric system with the earth as one of the planets around the sun. Challenged assumptions of Aristotle and Greek Astronomy (and now the Church) Simplified the motion of the planets Assumed Uniform Circular Motion (Incorrect) De Revolutionibus started a huge debate

Planetary Motion While the Copernican system did put the sun in the right place it did not improve the prediction of the path of the planets. Copernicus used Circular Motion for the orbits His tables for the path of the planets were off This started a debate over which model (geocentric or heliocentric) was correct

Kepler’s Three Laws of Planetary Motion Tycho Brahe (1546-1601) – Danish astronomer who dedicated much of his life to accurately collecting astronomical data Built the finest observatory at the time on the island of Hven and charted the movements of the planets and stars for 20 years

Kepler’s Three Laws of Planetary Motion In 1597 Johannes Kepler(1571-1630) became an assistant of Brahe in Prague Kepler attempted to use Geometry and Mathematics to describe a heliocentric (sun-centered) system that would agree with Brahe’s Data Result: Kepler’s Three Laws of Planetary Motion

Kepler’s Three Laws of Planetary Motion The path of the planets around the sun are ellipses with the sun at one of the focus points Consequence: The planets traveling in their orbits are sometimes closer to the sun and other times further away Note: The orbits are only slightly elliptical. They are close to circular.

Kepler’s Three Laws of Planetary Motion The planets sweep out equal areas in equal time intervals no matter how close or far away a planet is from the sun Consequence: Planets speed up as they move closer to the sun and slow down as they move further away

Kepler’s Three Laws of Planetary Motion 3. The ratio of the squares of the Periods of any two planets going around the sun is equal to the ratio of the cubes of their Average Distances away from the sun. Consequence: Planets that are farther away from the sun have a longer Period of Revolution

Equation for Kepler’s Third Law Kepler’s 3rd Law can be written in an equation form. (Ta)2 = (Ra)3 (Tb)2 (Rb)3 Kepler’s 3rd law applies to any two objects going around a third central object! (2 planets around the sun or 2 moons around a planet)

Galileo Galilei Galileo – (1564-1642) built his own telescope and set out to observe the night sky. His discoveries lead him to agree with the Copernican Model. Jupiter’s Moons Craters on the Moon Sun Spots

Galileo Galilei Galileo was confident his discoveries pointed to a the earth moving around the sun like the other planets. Unfortunately the Church at the time saw his theories as a threat and he was tried as heretic and forced to abandon his views Dialogo and Trial

Modern Astronomy Modern astronomy is said to have started during the 99 years between the deaths of Copernicus (1543) and Galileo (1642). It was an age of transition from the Ptolemaic Model to the Copernican Model. The Discovery of the telescope. Part of the Renaissance Lead to more questions about the universe