EARTHSC 1121 Lecture Notes - Lecture 2: Retrograde And Prograde Motion, De Revolutionibus Orbium Coelestium, Deferent And Epicycle

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Origin of the Earth
CHAPTER 1: Cosmology and the Birth of the Earth
Day 2. Before we learn about the Earth, we have to understand where it came from.
Introduction (slide 2)
Cosmology: the scientific study of the Universe
Structure
History
Nature of Scientific Theory (3)
Theory: a set of statements or principles devised to explain a group of facts or phenomena, especially one
that has been repeatedly tested or is widely accepted and can be used to make predictions about natural
phenomena.
When Dr. Panero says things like “the Big Bang happened,” she says this statement because it has
been repeatedly tested, is widely accepted, and can be used to make predictions.
If she says something is NOT an open question (it’s not “up in the air” or “still being hypothesized”),
then it is a theory.
This scientific theory is definition 1.
Some assumptions and The RULES
Nature is self-consistent
Laws describing stuff shouldn’t conflict with one another. If they do, then something is wrong with at
least one of them.
Nature is repeatable
The laws don’t change arbitrarily
E.g. throwing a ball in the air--you ought to get the same results.
Logical reasoning can be applied to the hypothesized laws to make predictions
And the predictions must agree with observations; they ratify the original hypothesis.
The Scientific Method--simple version (5)
A hypothesis isn’t a question. You can test it with an
observation and see if the results match the
prediction.
The Scientific Method--in practice (6)
The theory (not a debate) of plate tectonics (which has only been around
for about 20 years)--this has happened due to happenstance. We ended up with a huge
amount of information about the Earth itself.
With the scientific method in mind…
How and why have our views of the heavens changed through time?
How can we describe the motions of bodies in the sky?
Why do the stars and planets move the way they do?
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What do we observe? (9)
They captured the sky as it was changing throughout the night- -we
see “streaks” (star trails).
We don’t get dizzy as the earth spins--original hypothesis was that
the sky moved and the Earth stayed still.
(10)
Constellations change throughout the year
Called the zodiacs. Migrate once zodiac is in sight.
Planets: Wanderers
Clearly different from the stars
Former definition: things that orbited in the night sky.
Names are (mostly) Roman in origin
Mercury, Venus (has been identified as a star a few times, an oncoming airplane without radar, and a UFO),
Mars, Jupiter, and Saturn are visible in the night sky.
Greek & Roman planets:
Sun, Mercury, Venus, Mars, Jupiter
Observation: Planetary motion in the sky
Appear to move relative to the background of stars. The planets move through the ecliptic which contains
the constellations of the zodiac (=circle of animals).
Mercury and Venus are always close to the horizon.
Mars, Jupiter, and Saturn can appear anywhere along the ecliptic, typically moving eastward from night to
night relative to the stars, but occasionally in the reverse direction.
Retrograde motion (13/14)
Means that it is headed back to the west.
Mercury frequently ends up in retrograde.
Hypothesis: the Earth is at the center of the universe, and the planets turn around the Earth. “The Immobile
Earth.” This is the GEOCENTRIC hypothesis.
Aristotle--Greek, 384-322 BC. Hypothesized a geocentric system (Earth at the center)
The Scientific Method--One of the First Hypotheses
Hypothesis: Epicycles
Ptolemy--Greek, 2nd century AD. Hypothesized a geocentric system which explained the retrograde motion
of the planets by placing them on epicycles (small circles on bigger ones)
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