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Chapter 22

Chapter 22. “The Origin of Modern Astronomy”. Chapter 22.1 “Early Astronomy”.  Astronomy. Astronomy is the science that studies the universe. It includes the observation and interpretation of celestial bodies and related phenomena. Ancient Greeks.  Golden Age of Astronomy.

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Chapter 22

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  1. Chapter22 “The Origin of Modern Astronomy”

  2. Chapter 22.1 “Early Astronomy”  Astronomy • Astronomy is the science that studies the universe. • It includes the observation and interpretation of celestial bodies and related phenomena.

  3. Ancient Greeks  Golden Age of Astronomy • The “Golden Age” of early astronomy centered in Greece from 600 – 150 B.C. • The Greeks used philosophical arguments to explain natural phenomena, however they also relied on observations of the night sky.

  4. Ancient Greeks  Golden Age of Astronomy • The Greeks used instruments to make these observations such as the astrolabe. • They also developed the basics of geometry and trigonometry and applied them to astronomical measurements. An astrolabe was made of brass and used for determining the time and position of the Sun and stars.

  5. Ancient Greeks  Aristotle 384 – 322 B.C. • Aristotle was a famous Greek philosopher that concluded that the Earth was round because it always casts a curved shadow when it passes between the sun and the moon. • This idea was popular until the Middle Ages.

  6. Ancient Greeks  Aristotle 384 – 322 B.C. • Aristotle also believed the Earth was at the center of the universe and surrounded by spheres. • He thought that the closest spheres contained the four elements: earth, air, fire, and water.

  7. Ancient Greeks  Eratosthenes 276 – 194 B.C. • Eratosthenes made the first successful attempt to establish the size of the Earth. • He observed the angles of the noonday sun in two Egyptian cities that were roughly north and south of each other – Syene and Alexandria.

  8. Calculating Earth’s Circumference Eratosthenes figured the circumference of Earth must be 50 times the distance between these two cities by finding that the angles of the noonday sun between the two cities differed by 7 degrees or 1/50th of a circle. • Eratosthenes calculation of the Earth’s circumference was 39,400 km., which is very close to the actual circumference of 40,075 km.

  9. Ancient Greeks  Hipparchus 190 – 120 B.C. • Hipparchus was best known for his star catalogue in which he carefully located and plotted the positions and brightness of almost 850 stars. • He divided the stars into 6 groups based on brightness.

  10. Ancient Greeks  Hipparchus 190 – 120 B.C. • Hipparchus also measured the length of the year to within minutes of the modern year and developed a method for predicting the times of lunar eclipses to within a few hours.

  11. Ancient Greeks  Geocentric Model (Earth-Centered) • In the ancient Greeks’ geocentric model of the universe, the moon, sun, and the known planets: Mercury, Venus, Mars, and Jupiter orbit the Earth.

  12. Ancient Greeks  Geocentric Model • The Greeks believed the Earth was a sphere that remained motionless at the center of the universe. • Beyond the planets, they believed there was a transparent, hollow sphere of fixed stars which traveled daily around the Earth called the celestial sphere. • The Greeks also believed that all heavenly bodies remained in the same relative position to one another, except the seven “wanderers” which were: the sun, the moon, Mercury, Venus, Mars, Jupiter, and Saturn.

  13. Ancient Greeks  Aristarchus 312 – 230 B.C. • Aristarchus was the first Greek to believe in a sun-centered universe. • He used geometry to calculate the relative distances between the Earth, sun, and moon. • He later used these distances to calculate the size of the sun and the moon, but his measurements were much too small.

  14. Ancient Greeks  Heliocentric Model (Sun-Centered) • In the heliocentric model, Earth and the other 5 planets orbit the sun. Celestial Sphere of fixed stars.

  15. Geocentric and Heliocentric Models Picture B Picture A • Despite the fact that there was enough evidence to support the heliocentric model, the geocentric model dominated Western thought for nearly 2000 years.

  16. Ancient Greeks  Claudius Ptolemy 85 – 165 A.D. • Much of our knowledge of Greek astronomy comes from Claudius Ptolemy. • Ptolemy created a model of the universe that accounted for the movement of the planets, which he published in a 13-volume work in 141 A.D.

  17. Ancient Greeks  Ptolemaic System • Ptolemy noticed that the planets moved slightly eastward among the stars each night and even appeared to stop and reverse direction for a time before resuming eastward. • He created a model of the universe that accounted for this movement of the planets called the Ptolemaic System.

  18. Retrograde Motion  Ptolemaic System • This movement, called retrograde motion, is the apparent westward motion of the planets with respect to the stars. • This apparent motion occurs because the Earth has a faster orbital speed than the other inner planets as they all revolve around the sun. • Ptolemy used small circles called epicycles, on which the planets traveled as they revolved around the Earth, in order to explain the retrograde motion of the planets.

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