SCI02 - CO4.2

• Ancient Beliefs about the Universe


  • Ancient Sumerians, Babylonians, and Egyptians documented their observations of the location of the sun, moon, the five visible planets, and the fixed stars
  • They believed that Earth is at the center of the Universe
• Anaximenes of Miletus


  • Described Earth as a flat disc
• Pythagoras of Samoa


  • Proposed that Earth is a globe located at the center of concentric spheres
• Plato, Eudoxus, and Aristotle


  • Used concentric spheres to show that the planets, the sun, and the moon revolve around the Earth in a circular orbit at constant motion
• Aristarchus


  • Introduced the idea of placing the sun at the center of the Universe, with Earth and the other planets moving around it
• Eudoxus' Model of the Universe: Spheres within spheres all rotate in perfect, uniform, circular motion and combine to produce retrograde motions
• Aristotle's Geocentric Model of the Universe: Earth is the center of the universe
• Aristotle believed that heavenly bodies move in a specific substance or medium called aether, through which light is transmitted and propagated. This was debunked in an experiment conducted by American physicists Albert Michelson and Edward Morley in 1887. The results of their experiments initiated a line of research that eventually led to the theory of special relativity, for which, Albert Einstein is most known for.
• Claudius Ptolemy of Alexandria


  • Author of the Amalgamest
  • Explained his theory of the Earth-centered universe (geocentric theory) using epicycles and deferent
• Ptolemaic model explained retrograde motion
• Nicolaus Copernicus


  • Improved Aristarchus' sun-centered theory of the universe (heliocentric theory)
  • Published On the Revolutions of the Celestial Heavenly Spheres
• Arrangement of the planets from the sun based on the difference in the lengths of time that the planets take to complete an astronomical cycle
• Tycho Brahe


  • Constructed the observatory Uraniborg
  • Invented different astronomical instruments such as sextants and quadrants, to determine the location of stars
  • Mentored Johannes Kepler
• Tychonic model of the solar system
• Johannes Kepler


  • Mathematically proved the Copernican theory of the universe using his three (3) laws of planetary motion
  • Presented in his books Astronomica Nova and Harmonice Mundi
• Kepler's own drawing of the various positions of Mars relative to the sun. Mars travel fastest at some point and slowest at another point. This peculiar motion is the consequence of having an elliptical orbit.
• Galileo Galilei


  • Made observations on heavenly bodies like the Moon, Venus, Jupiter, and the sun using his invented telescope
  • 1632- published the Dialogue Concerning the Two Chief World Systems
  • Presented evidence for Copernicus' heliocentric theory
• Isaac Newton


  • Explained his mathematical description of force and the influence of the inverse square law to Edmond Halley
  • Principia presented mathematical proofs- force that holds the planets in their respective orbits as they move around the sun as well as Kepler's claims on the elliptical orbits and his laws of planetary motion
• Law of Ellipses


  The path of each planet around the sun is an ellipse, with the sun at one focus
• Law of Equal Areas


  Each planet revolves around the sun and covers equal areas at equal time intervals. This law explains the variations in the orbital speed of planets.
• Law of Harmonies or Periods


  The square of the orbital periods of the planets is proportional to the cube of the distances of the planets from the sun.
• Earth's rotation


  • The period or amount of time that Earth takes to spin around its axis
  • Can be determined by measuring the time that Earth takes to turn on its axis and return to its original orientation relative to the rest of the universe (sidereal day - 23.9344696 hours)
  • Can also be determined by measuring the time that Earth takes to turn so that the sun returns to the same position in the sky (solar day - 24 hours)
• Diurnal motion


  The apparent daily motion of stars and other heavenly bodies across the sky
• Perihelion


  Point at which Earth is nearest the sun (147,095,000 km)
• Aphelion


  Point at which the Earth is farthest from the sun (152,100,000 km)
• These distances result from Earth's elliptical orbit, where the sun is one of the ellipse's foci. The same thing is true for other planets.
• Earth's axis inclination


  • 23.439° toward the ecliptic
  • This inclination and Earth's revolution explain the occurrence of different seasons
• Solstice


  An event occurring when the sun appears to reach its most northerly or southerly excursion relative to the celestial equator on the celestial sphere
• Summer Solstice (June 21)


  • Occurs when the Northern hemisphere faces the sun while the Southern hemisphere is turned away from the sun
  • Length of the day longer than that of the night in the Northern hemisphere, vice versa with Southern Hemisphere
• Winter Solstice (December 21)


  Occurs when the Southern hemisphere faces the sun and length of day shorter than that of the night. The opposite occurs in the Northern hemisphere because it is turned away from the sun.
• The apparent motion of the sun is observed against a backdrop of fixed stars at all days of the year. The apparent change in the sun's direction is a result of Earth's revolution around the twelve ancient constellations (Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, and Pisces)