# Lesson 1-3 Classical Astronomy (500B.C. to 1400 A.D.)

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Lesson 1-3 Classical Astronomy (500B.C. to 1400 A.D.)

Lesson overview Who were the classical astronomers?
What is the Earth’s shape? How big are the Moon and Earth? How far is it to the Sun and Moon? What is the structure of the universe? What is Occam’s Razor?

Who were the classical astronomers?
Astronomers of ancient Greece and Egypt were the first to explain the heavens Used naked eye observations and geometry Some constructed idealized models to account for the motion of heavenly bodies Some famous Greek astronomers: Pythagoras Aristotle Aristarchus Eratosthenes Ptolemy

What is the Earth’s shape?
Pythagoras ( BC) said Earth was round: Sphere a perfect shape Gods made Earth Pythagoras was right about earth being spherical, but he couldn’t prove it. Aristotle made observations that the earth was indeed a sphere

Aristotle ( B.C.) Wanted to explain the movement of the sun, moon, 5 visible planets and the stars Geocentric (Earth-centered), geostatic (earth-fixed) universe Model used 55 concentric, “solid crystalline spheres” around Earth Like Pythagorus, he also argued the sphere was the perfect shape the Gods would have used to create Earth

Aristotle (384-322 B.C.) Divided universe into two realms:
Sublunar Realm Everything beneath the Moon’s sphere Imperfect area containing 4 elements: Earth and Water (move down) Air and Fire (move up) Superlunar Realm Everything outside the Moon’s sphere Perfect heavenly area containing “aether” where all elements move in circular motion

Aristotle (384-322 B.C.) Outermost sphere was domain of “Prime Mover”
Caused this sphere to move, imparting motion to other spheres Adjusting velocities of various spheres explained many features of planetary motion Model became the standard for almost 2000 years Weaknesses: Couldn’t explain varying planetary brightness Couldn’t explain “Retrograde” motion

Location of Mars among stars after several months of observation
Retrograde Motion Location of Mars among stars after several months of observation

What is the Earth’s shape? (cont’d)
Aristotle—two things “prove” Earth’s spherical shape: Earth’s shadow on the Moon What a traveler moving south sees in the sky

How big is the Moon? Aristarchus (310-230 BC)
Estimated moon was about 1/3 size of Earth (actually, it is 27%) Estimated sun’s diameter to be 7 times larger than earth (actually, it is 100 times larger) First to show the sun was much larger than the earth and proposed it should be at center of universe (heliocentric model) Stellar Parallax couldn’t be seen

Stellar Parallax

Eratosthenes (276 – 194 B.C.) First to accurately calculate the circumference of the earth Used simple geometry and a protractor Had to assume the earth was a sphere and that the sun was far enough away that light rays hitting earth at two different locations were parallel

Eratosthenes (276 – 194 B.C.) Heard that in Syene, Egypt, on the summer solstice, the sun left no shadow in a well Then, on that same day, he was in Alexandria, Egypt, and used a protractor to measure the angle of the sun’s shadow on a tall pole Then, he used simple geometry to compute the earth’s circumference

Distance from Syene to Alexandria = 500 miles
Angle measured with shadow: 7.2 degrees So, degrees / 7.2 degrees = Earth Circumference / 500 Therefore, Earth’s Circumference = (360 / 7.2) X 500 = 25,000 miles Actual Earth Circumference at Equator: 24,901 miles

You’ve been selected for a team that will travel to Mars
You’ve been selected for a team that will travel to Mars. One of your tasks is to accurately measure its circumference. You land at a spot where the next day at noon your ship will leave no shadow. So the next morning, you get up early, hop on your Mars rover and travel 150 miles to the south. Precisely at noon, you measure the length of your shadow and calculate it’s angle to be 4.1 degrees. What’s the circumference of Mars?

Ptolemy (circa 140 A.D.) Refined Aristotle’s solid-crystalline-sphere theory Continued three beliefs we now know were completely wrong All motion in the heavens is circular Objects in the heavens are perfect and cannot change their intrinsic properties (e.g., brightness) Earth is the center of the universe

Ptolemy (circa 140 A.D.) Ptolemy used complex combinations of circles (epicycles) to predict orbits for the Sun and Moon Helped explain retrograde motion Helped explain varying brightness as his model caused changes in the planets distance from Earth Results published in Almagest (“The Greatest”)

Ptolemy’s geocentric theory
Animation: Ptolemy’s Model of Motion of a Planet

Ptolemy (circa 140 A.D.) To try and explain some of the subtler motions, he included epicycles inside of epicycles His model survived until the 1500s It was adopted into Catholic Church doctrine But its complexity led to its eventual downfall One of the first to question was William of Occam (1285 – 1349), a British philosopher Occam’s Razor: Given two equally predictive theories, choose the simpler

Arabic Contributions Translated Ptolemy’s Almagest and wrote extensive commentaries on it These commentaries made their way to the west and became part of the Renaissance in Europe Later used by Copernicus who developed theory that put the sun at the center of our Solar System Arabic numbering (1, 2, 3) replaced Roman numbering (I, II, III) Gave us algebra and advanced trigonometry Perfected the astrolabe instrument Used to chart locations of stars Used in navigation for travelers

Astrolabe Instrument

Next: Lesson 1-4 Astronomy in the Renaissance (1400 – 1650 A.D.)