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Ancient Sky Observations and Calendar Development

Explore the ancient cultures that studied the sky and developed calendars based on celestial patterns. Discover the concepts of days, months, and years, and the evolution of early models of the universe. Learn about the contributions of Pythagoreans, Aristotle, Ptolemy, Copernicus, Tycho Brahe, Kepler, Galileo, and Isaac Newton.

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Ancient Sky Observations and Calendar Development

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  1. Ancient cultures studied the sky and developed calendars based upon the patterns they saw • Motion of the sun • Motion of the moon • Motion of the stars Ancient culture based their calendars on repeating cycles they observed in the sky.

  2. What is a day? What they saw: • The time it takes the sun to cross the sky and return to the same point What we know: • Time required for the Earth to rotate once on its axis

  3. What is a month? What they saw: • The amount of time required for the moon to move through a complete set of phases What we know: • The amount of time it takes the moon to revolve around the Earth

  4. What is a year? What they saw • The time it takes for a group of stars (constellation) to return to the same part of the sky at a certain time of day What we know: • Time required for the Earth to revolve around the sun.

  5. Early Models of the Universe

  6. Pythagoreans (500 B.C.) • Believed the stars, planets, sun, and moon were attached to crystalline spheres which rotated around the earth. • (geocentric)

  7. Aristotle (350 B.C.) • Believed the Earth is motionless at the center of the universe and all the stars and planets rotated around it. • Stars and planets moved in circular paths. (geocentric)

  8. Ptolemy (140 A.D.) • Expanded Aristotle’s theories to try to account for “retrograde motion” of the planets (Ptolemaic Theory) • Planets traveled in smaller circular paths as they traveled around the Earth (epicycles and deferents) • Popular model of universe for 1,500 years.

  9. Copernicus (1543) • Sun at center of universe and the planets orbit the sun (Heliocentric) • Solved the problem of “retrograde motion” • Theory did not immediately replace Ptolemy’s theory.

  10. Tycho Brahe (1600) • Favored an Earth-centered universe different from Ptolemy’s theory • Thought that other planets revolved around the sun, and that the sun and moon revolved around the Earth. • Theory incorrect, but made many precise observations of planets and stars.

  11. Johannes Kepler (1609) • Mathematician • Tycho’s assistant • Used Tycho’s observational data to develop laws of planetary motion • Kepler’s 3 Laws of Planetary Motion • Law of Ellipses • Law of Equal Areas • Law of Periods

  12. Galileo Galilei (1609) First person to use telescope for astronomical observations • Discoveries * Galilean Satellites (Jupiter’s moons) * Craters on the moon * Sunspots on the Sun * Phases of Venus Favored Copernicus’s theory over Ptolemy’s

  13. Isaac Newton • 1687 • Explained why planets orbit the Sun, and why moons orbit planets • Newton’s Law of Motion • Newton’s Law of Gravitation • Completed the work of Copernicus, Tycho, Kepler, and Galileo

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