LESSON 1

• During Ancient times, Earth was perceived to be flat.
• Early Babylonian, Chinese, Egyptian and Hebrew civilization believed that Earth had corners.
• It was then believed that if you set sail straight in the ocean, you would get to the edge of Earth and fall. However, many philosophers theorized that Earth was round and not flat.
• Navigators such as Ferdinand Magellan and Juan Sebastian Elcano were given credit for circumnavigating the world and proving that Earth is not flat.
• But how did the idea of the spherical Earth evolve?

  • Greek philosopher also investigated on the shape of Earth.
  • Philosophers like Plato and Aristotle described Earth and the heavens as spherical.

  Then around 6BCE, these Greek philosophers speculated that Earth was actually spherical.
• The timeline below shows the ideas of some Greek philosophers, including Pythagoras (570 BCE – 490 BCE) and Eratosthenes (276 BCE – 194 BCE), about the sphericity of the Earth.
• Pythagoras -Started the idea of the spherical Earth
• Plato - Educated his students on the sphericity of Earth but made no justifications
• Aristotle - Noted that there were stars in Egypt that could not be seen on other parts of Earth This phenomenon was only possible if Earth had a curved surface
• Eratosthenes - Estimated the circumference of Earth
• Before the telescope was invented, ancient astronomers only used their unaided eyes to observe the sky and the stars. Eventually, they created models of the universe. Some appeared similar, while others were different.
• Eudoxus of Cnidus  (400 BCE – 355 BCE) - He was considered as the first astronomer to explain the retrograde motion of the planets in the sky. He found out the differences in the motion of each planet that should be considered to give an accurate description of the universe.
• Retrograde motion is an apparent change in the movement of the planet through the sky.
• According to Eudoxus’s model, the universe was composed of Earth, five other planets (that are visible with the unaided eye, namely Mercury, Venus, Mars, Jupiter, and Saturn)
• Aristotle somehow patterned his model to the model of Eudoxus. Aristotle’s model showed that the universe was spherical and finite. Just like Eudoxus, he perceived Earth was at the center of the universe and was stationary. He believed so because Earth is too big to move; thus, it could not rotate.
• Aristotle believed that Earth was composed of four elements: Earth, Water, Air, and Fire.
• Both models of Eudoxus and Aristotle were considered Geocentric. This means that both astronomers believed that Earth was at the center of the universe.
• Aristarchus of samos (310 BCE – 230 BCE) - He was a Greek astronomer who made the first attempt to create a Heliocentric Model, which places the sun at the center of the Universe. He proposed that the sun and the fixed stars were at rest, while Earth revolves around the sun in a circular path.
• The only work of Aristarchus that survived was entitled On The Sizes And Distances Of The Sun And The Moon. In this book, Aristarchus calculated the sizes of the sun and the moon and their distances from Earth by estimating the relative angles of the moon and the sun from the Earth.
• He had three assumptions:

  a.) Earth was Spherical.

  b.) It is far from the sun.

  c.) Moon passes through Earth’s shadows when they align.
• Claudius Ptolemaeus (Ptolemy) (100 AD – 178 AD)

  Ptolemaic universe- In this model, it is assumed that Earth was at the center of the universe, while the other celestial bodies revolved around the Earth in perfect circles with constant velocity.
• Ptolemy’s model was considered more refined than previous geocentric models because his model could explain the motion of the celestial bodies accurately.
• Ptolemy assumed that planets revolved on epicycles (small spheres) which moved around the deferent (large sphere). He added that the stars belonged to the celestial sphere which was located beyond the planetary spheres. The center of the deferent is called the eccentric. What Ptolemy did next was to “move” Earth from its original position to a position below the center of the system that is still inside the deferent.
• Nicolaus Copernicus (1473-1543) - During the 16th century, a polish astronomer revived the heliocentric model of Aristarchus. He was hesitant to publish his findings because he was afraid of condemnation. But it was published a year before his death.
• Copernicus strongly believed in the heliocentric model because there were loopholes in the Ptolemaic model in terms of predicting the positions of the planets.
• In Copernicus’s model, it could be concluded that:

  a.) Heavenly bodies exhibited constant circular and perpetual motion along their epicycles.

  b.) The sun was at the center of the universe.

  c.) The order of planets from the sun is Mercury, Venus, Earth, Mars, Jupiter, Saturn and beyond the planets were fixed stars.
• Tycho brahe (1546-1601) - He was a Danish astronomer who also made planetary observation. Unlike Copernicus, Brahe believed in a geocentric universe, but his idea of the geocentric universe is slightly different from Ptolemy’s.
• In Brahe’s model of the universe, called the Tychonic System, Earth was at the center and the sun and the moon revolve around it, and all the other planets orbited the sun. Such a model was a type of the geoheliocentric system.