Keep Looking Up

Part 1

``Astronomy compels the soul to look upward and leads us from this world to another.'' - Plato

Early Astronomy

Early Models

Big Dipper

Orion

Scorpio

Winter sky

The celestial sphere appears to rotate about the celestial poles (completing

one cycle in one day).

• Celestial north pole stays still (North star aka Polaris)

• Stars appear to move in counterclockwise fashion.

Question: What causes the observed circular motions of the stars, Sun, Moon,

and planets about the celestial pole?

• Hypothesis 1: The Earth is stationary, and the stars, Sun, Moon, and planets revolve around it.

• Hypothesis 2: The stars, Sun, Moon, and planets are not revolving about the Earth; it is the Earth which is rotating about its polar axis, once per day.

This or that?

The Sun moves west to east relative to the stars (completing one cycle in one

year).

Question: What causes this apparent motion of the Sun relative to the background stars?

• Hypothesis 1: The Earth is stationary, and the Sun revolves around it at a slightly slower rate than the celestial sphere does.

• Hypothesis 2: The Earth revolves around the Sun, once per year.

This or that?

The Moon moves west to east relative to the stars (completing one cycle in one month).

Question: What causes this apparent motion

of the Moon relative to the background stars?

• Hypothesis 1: The Earth is stationary at the center of the universe, and the Moon revolves around it a a slower rate than the celestial sphere does.

• Hypothesis 2: The Moon revolves around the rotating Earth, once per month.

The planets sometimes move west to east, sometimes east to west, relative to

the stars.

Wanderers

• Only 5 planets to the naked eye.

• Mercury, Venus, Mars, Jupiter, Saturn

This or that?