Physical Science Astronomy Week One

• Astronomy


  Study of celestial objects and phenomena
• Objectives are the most essential learning competencies for the session
• At the end of the session, you will be able to:

  1. Explain how the Greeks knew the Earth is spherical

  2. Site examples of astronomical phenomena known before telescopes

  3. Explain how Brahe's data collection paved the way for Kepler's laws
• Ancient Greek philosophers and scientists

  • They were great contributors in various fields, not just philosophy
• Ptolemaic system


  Model where planets move in a complicated system of circles, with the Earth at the center
• Shape of the Earth

  • Bulging at the equator, squeezed at the poles
• The North Star appears closer to the horizon when observed near the equator
• Anaxagoras


  A student of Plato, considered one of the greatest philosophers, proposed the Earth-centered view that dominated for 2000 years
• The ancient Greeks thought the Earth was the center of the universe
• The ancient Greeks proposed the Earth is spherical, based on observations like ships disappearing over the horizon
• Eratosthenes' method to measure Earth's circumference

  1. Measured angle of sun at noon on June 21

  2. Knew distance between Alexandria and Syene

  3. Calculated Earth's circumference as 40,000 km
• Heliocentric model


  Model where planets orbit the Sun, proposed by Aristarchus
• The Ptolemaic geocentric model was the accepted model before the heliocentric model
• Telescopes allowed more detailed observations of celestial bodies
• Astronomical phenomena known before telescopes

  • Phases of the Moon
  • Lunar and solar eclipses
  • Seasons
  • Procession of equinoxes
• Planets known before telescopes

  • Mercury
  • Venus
  • Mars
  • Jupiter
  • Saturn
• Tycho Brahe


  • Danish astronomer, made accurate observations of celestial bodies
  • Had a working relationship with Johannes Kepler
• Johannes Kepler


  • German astronomer, Tycho Brahe's assistant
  • Discovered 3 laws of planetary motion
• Kepler's 3 laws of planetary motion

  1. Law of Ellipses: Planets orbit in ellipses

  2. Law of Equal Areas: Imaginary line from Sun to planet sweeps out equal areas in equal times

  3. Law of Harmonies: Square of orbital period proportional to cube of average distance from Sun
• Kepler's laws played a vital role in understanding natural body motions and placing artificial satellites
• Modern understanding of celestial motion was made possible through discoveries of astronomers like Brahe and Kepler