Phases of the Moon
What Can You See
http://kids.niehs.nih.gov/lunar/home.htm
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Phases of the Moon
When Can You See It
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Giants of Science
• Our understanding of the Universe developed slowly over
centuries.
• Most of the breakthroughs came through careful study of the
positions and motions of the Sun, Moon, planets and stars.
• The careful analysis of these large sets of measurements and
the use of mathematics led to the creation of physical models
that were used to make predictions on the future position of
planets, Sun, Moon, etc.
• When these models failed to explain observed phenomena
they were abandoned or modified. (eventually!)
• The following is a short summary of several notable
philosopher/scientists and their contributions.
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Aristotle (c. 384 - 322 B.C.)
• Taught that the
Earth was the
unmoving center
of the Universe.
• Taught that the
Earth was
spherical
– Lunar eclipses due
to Earth’s shadow
are always round.
– Different stars are
visible the farther
South you go.
Two examples Aristotle used to prove the Earth is a sphere.
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Aristarchus (c. 310 - 230 B.C.)
• Used geometry to measure the size and distance of the Moon
& Sun. Moon results were good. Sun results were bad.
• Showed that the Sun was bigger than Earth and much farther
than the Moon.
• Taught that the Earth went around the Sun. (Not much proof
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and not widely accepted).
Aristarchus (c. 310 - 230 B.C.)
Relative Distances
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Stellar Parallax
Stellar Parallax application
• Parallax - the shift in an
object’s position caused
by the observer’s
motion.
• Since the Earth moves
we should see parallax
but we don’t.
• No one realized that
stars are very far away
so parallax is a very
small effect.
• Only with powerful
telescopes can they be
observed. 300 years
later the first parallax
was observed.
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Eratosthenes (c. 276 - 196 B.C.)
• Used shadows and geometry to
measure the size of the Earth!
• On Summer Solstice Sun was
directly overhead in Syene,
Egypt.(no shadows at noon)
• On the same day there were
shadows at noon in Alexandria,
Egypt.
• Using shadow length and
distance between Syene and
Alexandria and geometry found
Earth’s circumference.
• His value probably around
42,000 kilometers.
• Actual value about 40,000 km.
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Ptolemy (100 - 170 A.D.)
• Last of the great Greek
astronomers
• Used discoveries and ideas
of others plus his own
observations to develop a
model of the Universe.
• His work survived from
Arabic translation of
Almagest (means the
greatest compilation)
• His model of the Universe
was undisputed for almost
1,400 years!
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Ptolemy’s Model of the Universe
• A geocentric (Earthcentric) model
• All the stars, planets, Sun
and Moon moved around
the Earth
• Motions were perfectly
circular and perfectly
uniform
• Fastest objects were
closest to Earth.
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Retrograde Motion
• It was well known that
the planets occasionally
appear to stop and change
direction in orbit, but
why?
• It is an illusion based on
the motion of the Earth
and the planet.
• Ptolemy explained this
effect without moving
Earth. He used epicycles
instead.
Retrograde Motion Application
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Epicycles
The large circle is called a deferent
the smaller circle is an epicycle.
• In a Ptolemy’s model
these are small circles to
which the planets are
fixed.
• These epicycles are
attached to the larger
circles which move
around Earth.
• Ptolemy adjusted the
sizes of the circles and
their rate of motion to
replicate the motion of
the planets including
retrograde motion.
• Planets maintained
perfect uniform, circular
motion.
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Nicholas Copernicus (1473 - 1543)
• Recognized that the predicted
planetary positions using
Ptolemy’s model were not
always very accurate.
• Tried to fix Ptolemy’s model
but eventually abandoned it.
• Returned to the idea of
Aristarchus 1800 years before.
• Created a heliocentric (Suncentric) model of the Universe.
Finding the distance of the planets
from the Sun using Copernicus’
heliocentric model.
– Made retrograde motion easier to
explain.
– Distances of the planets from the
Sun could be determined using
geometry
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Problems with Copernicus’ Model
• Copernicus maintained that all planets move in
perfectly circular orbits at constant rates (not correct).
• Didn’t really do much better than Ptolemy at
predicting planetary positions. He actually needed to
keep epicycles to make the model work out right.
• Several objections
– We don’t feel the Earth moving. Went against “commonsense”. Went against philosophical (Aristotle) and religious
beliefs.
– Why don’t we see stellar parallax?
– For these reasons it was not widely accepted at first
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