Introduction to Classical Astronomy

Introduction to ClassicalAstronomy

Mr. Ross Brown

Brooklyn School for Law andTechnology

Aim: How did early astronomers studyour universe?

•21 March 2016

•Do now: What evidence do you have that ourEarth 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 astronomersstudy our universe?

22 March 2016

Do now: Why does the Moon appear so muchlarger at the horizon? Why do the Sun andMoon appear similar in size in our sky?

Angular size

Ponzo Illusion

•The human mind judges an object’s size basedon 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 thatpasses through 2 parallel lines)

Eratosthenes’ calculations

Relative sizes of Sun and Moon andDistances to Each

•Aristarchus

Homework

•Provide examples of observational evidencethat the Earth is round.

How did classical astronomersexplain planetary motion?

•Do now: How does the Earth’s motion throughspace compare to Mercury? Or to Mars? Is theEarth moving faster or slower?

The Planets

•Ancient Greeks noticed five bright “stars” inthe night sky that didn’t stay fixed like stars

•We named them planets from the Greek wordfor “wanderers”

•Planets seemed to move independently, sothey were named after gods

Where is the center of the SolarSystem?

•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 thiswork.

A Flaw in the Geocentric Model

•Retrograde Motion: planets beyond Earthrelative to Sun sometimes appear to movebackwards. This is due to slower paths aroundSun and being “lapped” by Earth.

Homework #5

•What was Nicolaus Copernicus’ theory aboutthe Solar System?

•What did Tycho Brahe design and build?

•What did Johannes Kepler learn aboutplanetary orbits?

Aim: How did our study of theuniverse advance?

•Do now: How did retrograde motion illustratethe likelihood of a heliocentric universe?

Nicolaus Copernicus (1473-1543)

•Used retrograde motion to show that planetscloser to the Sun overtake planets furtheraway, indicating heliocentric model

•Calculated distances to other planets based onan astronomical unit (AU)

•Showed that the universe was much, muchlarger than thought. People began thinkingother stars might be suns with other planetsorbiting them.

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

Tycho Brahe (1546-1601)

•Danish astronomer, and wealthy. Used hispower to design better telescopes and madegreat discoveries.

Johannes Kepler (1571-1630)

•Brahe’s assistant. Used Brahe’s data to createan accurate picture of the path of Mars.

•Not a circle, but an ellipse (a flattened circle.)

•An ellipse has long and short dimensions, itsmajor and minor axes.

•½ the major axis is referred to as thesemimajor axis.

Kepler’s Three Laws

1.Planets move in elliptical orbits with the Sunat one focus of the ellipse

2.The planet’s orbital speed varies so a linejoining the Sun and the planet will sweepequal areas in equal time intervals

3.There is a relationship between the time of aplanet’s solar orbit and its orbit’s size, i.e.,the square of the time to orbit (p2) is equal tothe cube of the semimajor axis (a3)