History of Astronomy

Prehistoric and Classic Astronomy

Lesson Overview  Celestial Sphere  Greek Earth-Centered Model  Ptolemy’s Model Chapter 1, Lesson 1

The Celestial Sphere  Celestial sphere refers to an imaginary sphere of heavenly objects that seems to center on the observer  The north celestial pole and the south celestial pole refer to the points on the celestial sphere directly above Earth’s poles. A constellation is an area of the sky containing a pattern of stars.A constellation is an area of the sky containing a pattern of stars. Courtesy of NOAO/AURA/ASF

The Celestial Sphere  It takes the Sun about days to revolve around the celestial sphere  The ecliptic is the name for the Sun’s apparent path among the stars around the Earth  The zodiac is the group of constellations the Sun passes through on its apparent path along the ecliptic

The Moon’s Phases  The Moon does orbit the Earth When the Moon is behind the Earth, it gets a full splash of sunshine and appears as a full moon. When it is between the Earth and the Sun, the Earth- facing part of the Moon gets no light, and so the palest sliver of moonlight is all that’s visible on Earth.When the Moon is behind the Earth, it gets a full splash of sunshine and appears as a full moon. When it is between the Earth and the Sun, the Earth- facing part of the Moon gets no light, and so the palest sliver of moonlight is all that’s visible on Earth.

The Moon’s Phases © J. Gatherum/ShutterStock, Inc.

Observing Planetary Motion The planets are another major class of objects in the sky. Planet comes from a Greek word meaning “wanderer.”The planets are another major class of objects in the sky. Planet comes from a Greek word meaning “wanderer.” They were first observed in retrograde motion— backward motionThey were first observed in retrograde motion— backward motion  Planets move in elliptical, not circular orbits  Revolve around the Sun, not Earth

The Greek Earth-Centered Model  The ancient Greeks studied astronomy out of a pure desire to understand how the universe works.  They looked for symmetry, order, and unity in the cosmos.  Thales of Mileus  Stars are not gods but balls of fire  Aristotle – 322 BC  Placed the Earth at the center of things because there was no parallax  Parallax is the apparent shifting of nearby objects with respect to distant ones as the position of the observer changes Chapter 1, Lesson 1

Ptolemy’s Model  The Greek model of the universe that won the widest acceptance was that of Claudius Ptolemy.  Claudius Ptolemy – AD 150  Ptolemy believed that heavenly bodies (planets and stars) were perfect, in contrast to earthbound objects  The universe was based on perfect circles Chapter 1, Lesson 1

Ptolemy’s Model  In Ptolemy’s view, a planet revolved in a small orbit around a central point that was part of a perfect circle around the Earth.  Epicycles were Ptolemy’s way of explaining the retrograde motion of planets.  An epicycle is the circular orbit of a planet, the center of which revolves around the Earth in another circle Chapter 1, Lesson 1

Evaluating Ptolemy’s Model Strengths  Fits evidence available during his lifetime  Includes testable predictions Weaknesses  Assumed that Earth was stationary  Theory of epicycles was a departure from simplicity and symmetry Chapter 1, Lesson 1

Geocentric theory fails the test of “Occam’s razor”  Science has a rule called “Occam’s razor,”  Named after the British philosopher William of Ockham (about 1285−1349)  Occam’s razor says “If you can choose between a very complicated theory and a simple theory, the simple one probably is better... and is more likely to be correct.”  Because of “Occam’s razor” astronomers dropped geocentric models of our solar system and replaced them with a simpler model 13

Activity 2: Testing Parallax  Follow the step-by-step directions on your worksheet in order to test the concept of parallax.  Then answer the questions about parallax on the worksheet.  Be prepared to share your answers with the class. Chapter 1, Lesson 1

Review  Celestial sphere refers to an imaginary sphere of heavenly objects that seems to center on the observer  Earth’s gravity holds the Moon in such a firm grip that it always keeps the same face turned toward the Earth  The ancient Greeks looked for symmetry, order, and unity in the cosmos  Thales of Miletus reasoned that the Sun and other stars were not gods but balls of fire Chapter 1, Lesson 1

Review, cont.  Pythagoras proposed that the Earth is spherical  Aristotle placed the Earth at the center of things  The Greek model of the universe that won the widest acceptance was that of Claudius Ptolemy; for 1,300 years, his model was the conventional wisdom of the scientific world Chapter 1, Lesson 1

Summary  The Celestial Sphere  Greek Earth-Centered Models  Ptolemy’s Model Chapter 1, Lesson 1

Next…  Done – Prehistoric and Classical Astronomy  Next – Astronomy and the Renaissance Chapter 1, Lesson 1 © Photos.com