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Keep Looking Up
Topic 2
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?