Three important ideas developed by the ancient Greeks

- models of nature

- mathematics

- theories pertaining to observations

Thales (624-546 BC)

Philosophized about what is the universe made of. Believed the Earth is a flat disk floating in an infinite ocean.

Anaximander (610-546 BC)

Invented the idea of the celestial sphere

Pythagoras (560-480 BC)

First said the Earth is round

Democritus (470-380 BC)

Proposed that the world is built from individual atoms.

Plato (428-348 BC)

Proposed that heavenly objects move in perfect circles at constant speeds.

Aristotle (384-322 BC) and Ptolemy (100-170 BC)

Proposed an Earth-centered universe (geocentric modeling)

Heraclides (388-315 BC)

First to suggest Earth rotates

Aristarchus (310-230 BC)

First to suggest Earth orbits the Sun.

Eratosthenes (240BC)

Measured the size of the Earth using a heliocentric model.

Hipparchus (190-120 BC)

Predicted planetary motion, discovered precession, and invented the magnitude system.

Copernicus (1473-1543)

Proposed a heliocentric model of the universe, but still in perfect circles.

Brache (1543-1601)

Developed Naked-eye observatories

Kepler (1571-1630)

First to figure out that planetary orbits are ellipses, matching Brache's data.

Semi-Major axis (Ellipse)

Half of the long axis of the Ellipse

Semi-Minor axis (Ellipses)

Half of the short axis of the ellipse

Eccentricity (Ellipses)

Describes how stretched an ellipse is relative to a perfect circle

Kepler's first law of planetary motion

The orbit of each planet about the sun is an ellipse, with the sun at one focus

Kepler's second law of planetary motion

A planet moves faster on the part of it's orbit nearer to the sun and slower when it moves farther away, sweeping equal areas within the ellipse in equal amounts of time

Kepler's third law of planetary motion

More distant planets orbit the sun at slower average speeds.

P² = a³

P is the orbital period in years

a is the average distance from the sun in AU

Perihelion

The point on an eclipse when a planet is closest to the star.

a(1 - e)

Aphelion

the point on an Ellipse when a planet is farthest from the star.

a(1 + e)

Earth is closer to the sun in January than July. Which of the following is true?

• A. Earth travels faster in July than January
• B. Earth travels faster in January than July
• C. It is summer in January and winter in July
• D. None of the above

• B. Earth travels faster in January than July