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Astronomy 1021
Stacey Hallman

Chapter 3: The Origin of Modern Astronomy A Brief History of Ancient Astronomy • Our ancestors were excellent at observing and recording the world around them and recognizing patterns in what they discovered. • Careful observation and pattern recognition were also the first steps in what we now call the modern scientific method. • Lascaux, France where Paleolithic cave paintings dating back to 15 000 BCE were discovered in 1940 • Ancient people of centralAfrica could predict seasons from the orientation of the horns of the crescent Moon each month (once again a result of careful observation and pattern recognition). This also allowed them to determine when to plant seeds and grow their crops • Around 2500 BCE, the Egyptians used the first rising at dawn of the star Sirius to mark the beginning of a 365 day calendar • The Egyptians were the first to divide the night and day into 12 hours based on the rising of certain bright stars during summer nights • The motions of both the Sun and the Moon played an important role in the marking of time, with the Sun used for predicting seasonal changes and the lunar phases defining monthly changes • The Greeks were among the first to develop models to explain what they observed in the universe The Geocentric Model of the Universe • Pythagoras suggested that the Earth is a sphere and not flat, as had been previously assumed • Based on this spherical model, Eratosthenes was able to calculate the Earths circumference by observing the position of the Sun at noon in two different cities on the first day of summer • Based on the observations ofAristotle and later Ptolemy, ancient Greek philosophers believed that heavenly objects moved on circular paths at constant speed, with the Earth motionless at the centre of the universe. This geocentric model was championed by Aristotle. • As viewed from Earth, the planets seem to follow complicated paths in the sky, including episodes of “backward” motion that are difficult to explain in terms of motion on circular paths at constant speed • Earth is motionless at the centre of the universe • Aristotle believed as a first principle that the heavens were perfect. • Ptolemy set about making accurate mathematical descriptions for the motion of the planets • Ptolemy’s model could handle the complicated retrograde motion of the planets. Nicolaus Copernicus • Copernicus believed that the Sun and not Earth was the centre of the universe and the Earth rotated on its axis and revolved around the Sun • Copernicus was the first person to produce a detailed model of the heliocentric universe (sun-centered) • Earth moves faster along its orbit than the planets that lie farther from the Sun. Due to this, Earth periodically overtakes and passes these planets • The Copernican model could not predict the positions of the planets any more accurate than the Ptolemaic model could • The copernican model is inaccurate. The copernican hypothesis that the solar system is heliocentric is correct • The most important consequence of the Copernican model was not what it said about the Sun but what it said about the Earth. By placing the Sun at the centre, Copernicus made Earth move along an orbit like the other planets Tycho Brahe • in 1572, astronomers were startled to see a new star (now called Tycho’s supernova) • Tycho could not detect the new star’s parallax, meaning it had to be far beyond the Moon and that it was a change in the supposedly unchanging starry sphere Johannes Kepler • Kepler discovered the mystery of Mars • The orbit of Mars is an ellipse, not a circle • The planets do not move at uniform speeds along their elliptical orbits. Kepler recognized that they move faster when close to the Sun and slower when farther away Kepler’s Three Laws of Planetary Motion 1. The orbits of the planets are ellipses with the Sun at one focus 2. Aline from a planet to the Sun sweeps over equal areas in equal intervals of time 3. Aplanet’s orbital pe
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