Introduction to Classical Astronomy Mr Ross Brown Brooklyn

Introduction to Classical Astronomy Mr. Ross Brown Brooklyn School for Law and Technology

Aim: How did early astronomers study our universe? • 21 March 2016 • Do now: What evidence do you have that our Earth is round? Use “naked-eye” observations.

We knew the Earth was round • Pythagoras, 500 BC (Before Christ, or BCE, Before Common Era) suspected it • Aristotle (384 -322 BC) observed it • Naked-eye observations

Aim: How did early astronomers study our universe? 22 March 2016 Do now: Why does the Moon appear so much larger at the horizon? Why do the Sun and Moon appear similar in size in our sky?

Angular size

Ponzo Illusion • The human mind judges an object’s size based on its background.

Ebbinghaus Illusion

Angular size of Sun and Moon

Calculating the Earth’s Circumference • Eratosthenes (276 -195 BC) • Used measurement and geometry • Parallel lines and their transversal (a line that passes through 2 parallel lines)

Eratosthenes’ calculations

Relative sizes of Sun and Moon and Distances to Each • Aristarchus

Homework • Provide examples of observational evidence that the Earth is round.

How did classical astronomers explain planetary motion? • Do now: How does the Earth’s motion through space compare to Mercury? Or to Mars? Is the Earth moving faster or slower?

The Planets • Ancient Greeks noticed five bright “stars” in the night sky that didn’t stay fixed like stars • We named them planets from the Greek word for “wanderers” • Planets seemed to move independently, so they were named after gods

Where is the center of the Solar System? • Geocentric: Earth is the center • Heliocentric: Sun is the center

Retrograde Motion Worksheet • You can work with a partner • Please take your time and use care with this work.

A Flaw in the Geocentric Model • Retrograde Motion: planets beyond Earth relative to Sun sometimes appear to move backwards. This is due to slower paths around Sun and being “lapped” by Earth.

Homework #5 • What was Nicolaus Copernicus’ theory about the Solar System? • What did Tycho Brahe design and build? • What did Johannes Kepler learn about planetary orbits?

Aim: How did our study of the universe advance? • Do now: How did retrograde motion illustrate the likelihood of a heliocentric universe?

Nicolaus Copernicus (1473 -1543) • Used retrograde motion to show that planets closer to the Sun overtake planets further away, indicating heliocentric model • Calculated distances to other planets based on an astronomical unit (AU) • Showed that the universe was much, much larger than thought. People began thinking other stars might be suns with other planets orbiting them.

Distance from Sun to its planets (the universe is HUGE!!!)

Tycho Brahe (1546 -1601) • Danish astronomer, and wealthy. Used his power to design better telescopes and made great discoveries.

Johannes Kepler (1571 -1630) • Brahe’s assistant. Used Brahe’s data to create an accurate picture of the path of Mars. • Not a circle, but an ellipse (a flattened circle. ) • An ellipse has long and short dimensions, its major and minor axes. • ½ the major axis is referred to as the semimajor axis.

Kepler’s Three Laws 1. Planets move in elliptical orbits with the Sun at one focus of the ellipse 2. The planet’s orbital speed varies so a line joining the Sun and the planet will sweep equal areas in equal time intervals 3. There is a relationship between the time of a planet’s solar orbit and its orbit’s size, i. e. , the square of the time to orbit (p 2) is equal to the cube of the semimajor axis (a 3)

Aim: How did our understanding of the universe continue to develop? • Do now: What keeps the planets of our solar system from colliding?

Galileo Galilei (1564 -1642) • First person recorded to use telescope to study heavens • Saw mountains and features on Moon • Saw sunspots on the Sun • Saw objects orbiting Jupiter (which Kepler named ‘satellites’) • Got in trouble with the Church during the Inquisition. Forced to recant and held under house arrest for rest of his life.

Galileo Galilei (1564 -1642) https: //commons. wikimedia. org/w/index. php? title=File%3 AApollo_15_feather_and_hammer_drop. ogg

Isaac Newton (1642 -1727) • Contributions to math, physics, astronomy • First to truly understand importance of gravity as a force

Homework • If an asteroid has an average distance from the Sun of 4 AU, what is its orbital period?