How the Greeks Knew the Earth is Spherical - Physics Lesson PDF

PHYSCI – Q4 – LESSON 1 # HOW THE GREEKS NEW THAT THE EARTH IS ## Activity 1. And the Shape Is The activity is a demonstration where a light source, such as a flashlight, is pointed at a ball. The light casts a shadow on a second ball behind it. #### Procedure: 1. Align the flashlight and the b...

PHYSCI – Q4 – LESSON 1 # HOW THE GREEKS NEW THAT THE EARTH IS ## Activity 1. And the Shape Is The activity is a demonstration where a light source, such as a flashlight, is pointed at a ball. The light casts a shadow on a second ball behind it. #### Procedure: 1. Align the flashlight and the balls horizontally. 2. Point the flashlight to one of the balls. 3. Observe the shadow casts on the ball behind it. #### Questions: 1. What is the shape of the shadow cast by Ball 1? 2. Which of the two balls represents the Earth? the Moon? 3. Supposed that the moon or Earth is a heart shape, what shadow would it form when sunlight is aligned with it? Why? The Mesopotamians as accounted in their mythology around 6th century BC described Earth as a flat disk floating in the ocean and bounded by a spherical sky. This idea has prevailed for hundreds of years until a new view was presented. As the quest for knowledge continues and civilization prospers, several models of the universe were presented. And it is categorized as a **geocentric model** or Earth-centered and **heliocentric model** or Sun-centered. They were convinced that the shape of the Earth was not flat instead, it was spherical. This idea was already entertained by Pythagoras and Plato however they do not have concrete evidence to support their claim. Until Aristotle (around 320 BC), a student of Plato presented his arguments that established the claim during their time. Aristotle argued that: * the sphere is perfectly solid and the heavens are a region of perfection * the Earth's component pieces, falling naturally towards the center, would press into a round form * in an eclipse of the Moon, the Earth's shadow is always circular thus, if Earth is a flat disc, it would cast an oval shadow. * even in short travels northwards the Pole Star is higher in the sky. If it is not a sphere then why measure its circumference. One of the astronomical events that fascinated early civilization was the eclipse. And the recorded information about eclipses was used by Eratosthenes (about 235 BC) to approximate the circumference of the Earth. This was another proof presented about the spherical shape of the Earth. As you know an eclipse is a shadow formation. This happens when the path of light rays is blocked by an object. Story Time – Learning Chime! While working as a librarian at the University of Alexandria in Egypt, he came across information that during summer solstice especially at noontime, sunlight shines directly down a deep well in Seyene, a city south of Alexandria, and reflected up again and no shadow is cast by any object on a noontime. But in Alexandria at the same date and time, a vertical pillar and other structures cast a shadow. And so, Eratosthenes measured the shadow cast by a vertical pillar in Alexandria and he found out that it is 1/8 of the height of the pillar. This is roughly equivalent to a 7.20 angle between the sun's rays and the vertical pillar while in Syene it's 0 degrees. Based on his calculation, 7.20 is equivalent to 1/50 of a circle. Thus, it follows that the distance between Alexandria and Syene must be 1/50 the circumference of the Earth. The illustration shows the orientation between Alexandria and Syene relative to the sun. When the rays shine directly above Syene, it is not directly overhead at Alexandria which around 800 km north. The pillar in Alexandria cast a shadow, while the water in the deep well at Syene directly reflected the sunlight. Circumference – is the distance around the boundary of a circle or other closed figure. PHYSCI – Q4 – LESSON 1 # Astronomical Phenomena Known to Astronomers Before In this modern time, we come to know of the things around us because there are appropriate instruments or gadgets used to study or analyze such an event or phenomenon. Take for example the telescope, it is one of the instruments invented by mankind that is very useful in studying the cosmos. But despite the absence of the telescope in ancient times, still they were able to discover some astronomical phenomena. List of other pre-telescope astronomical events studied by ancient people. **1. Phases of the Moon** The appearance and path of the moon were observed by ancient people to change within 29.5 days. They observe that the moon changes appearance from a thin semi-circular disk to a full circular disk. The periodic change of the moon's phases was the basis of the ancient calendar. **2. Lunar Eclipse** A phenomenon such as this is known as a lunar eclipse. A lunar eclipse occurs when Earth is between the moon and the Sun, Earth casts a shadow on the moon. **3. Solar Eclipse** A solar eclipse happens when the moon is in between the Sun and the Earth and the moon partially or completely blocks out the Sun. This caused temporary darkness in the daytime, thus, ancient people feared the occurrence of a solar eclipse since they associate it with the wrath of God. **4. The Motion of the Stars** The astronomers noticed that the constellations' positions in the night sky vary depending on the time of the year. It was also observed that the stars seem to be attached to a celestial sphere that rotates around an axis in one day. **5. Visibility of Planets** They noticed a few stars in heaven were relatively brighter than the rest of the stars. The distant stars seemed to be fixed in their position but these stars change positions periodically, thus the Greeks called them "wanderers" or planets. These wandering stars are named Mercury, Venus, Mars, Jupiter, and Saturn which were later discovered to be planets, not stars. Now that you have learned about the different astronomical events even before an instrument like a telescope was invented. The information that they gathered, has resulted in innovation and invention. One of these is the calendar. With the use of a calendar at home, how about you try tracking the change of phase of the moon without necessarily looking at the sky at night time. In most modern calendars, the moon's movement is indicated.