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Astronomers
All the “Dead White Guys”
of Astronomy
Aristotle (300 B.C)
• Believed in geocentric universe – Earth is
center
• Proved Earth is round rather than flat due
to eclipses and observing stars
• Influenced people for 1000 years
Ptolemy (140 A.D)
• Believed also in geocentric universe
• Developed geocentric model of the
solar system explaining motions of
planets using epicycles and deferents
• Deferent: planet’s orbital path around
Earth
• Epicycle: planet’s smaller orbital path
around the deferent
• All orbits are circular
Retrograde Motion:
Apparent backwards
motion of some planets
in the sky
Copernicus (1473-1543)
• Heliocentric Theory: all planets orbit the
sun, only the moon orbits Earth
• Assumed Earth rotated which explained
the seasons
• Knew of 6 planets orbiting the sun
• Made a new solar system model and
placed them in order
The closer a planet is to
the sun, the faster it must
be traveling…explained
retrograde motion
People did not believe him
at first because the idea
seemed so radical…
believe it or not the
universe does not revolve
around you!
Galileo (1564-1642)
• Pioneered modern science – observation,
experimentation, testing hypotheses
• Studied motion, friction, acceleration…able to
predict how far & fast bodies are moving in
various lengths of time
• Based on motion studies, adopted
Copernicus’ heliocentric theory
• 1st telescopic astronomer
• invented telescope
• Proved heliocentrism by observing the
phase of Venus and 4 moons of Jupiter
• Catholic church did not believe him and
his books were forbidden
Tycho Brahe (1546-1601)
• Made observations of night sky for over 20
years on the island of Hveen (near
Copenhagen) without use of telescope
• Recorded positions of Sun, Moon, Earth for
many years
• Designed many astronomical instruments,
calibrating and checking their accuracy often
• Johannes Kepler assisted him and later used
all of his data to work on his own theories
Did not believe in
heliocentrism, but
did not
completely agree
with geocentrism
either, so he
combined the two
ideas to form his
model
Kepler (1571-1630)
• Used data to come up with the theory
for planetary motion
• Mathematician who made advances in
calculus and geometry
• Lived in poverty and died in poverty
• Catholic church was trying to convert
him and forced him to move around a
lot
Laws of Planetary Motion
#1 Planets orbit the sun in elliptical orbits
with the sun at one focus
• Focus: point inside an ellipse
• Ellipse: a curve for which the sum of the
distances from any point on the ellipse to two
points inside the ellipse is always the same.
• Major Axis: widest diameter of the ellipse
• Semimajor Axis: distance from the center of
the ellipse to one end
• Eccentricity: roundness of the ellipse
Perihelion: closest point a planet gets to the sun
Aphelion: farthest point a planet gets from the sun
#2 A line from the sun to a planet
will sweep out equal areas in equal
times
• Planets move faster when closer to the sun
• Planets in circular orbits would always travel at
the same speed
1st and 2nd laws allow us
to calculate the speed of
a planet at any point in
the orbit
#3 Distance3 = Period2
• Distance is the semimajor axis
• Period is the time it takes for a planet to
revolve around the sun once
• Size of a planets orbit is related to its
length of time needed to orbit the sun
• 3rd Law proves a mathematical pattern for
laws 1 and 2
Newton (1643-1727)
• Found a conceptual framework
to completely explain the
observations/rules of Galileo,
Brahe, and Kepler
• Laws of Motion
• Help to explain why the
orbits are in elliptical paths
and why they stay within
those paths…GRAVITY
Laws of Motion
1) Object a rest will stay at rest and an
object in motion will stay in motion
until it is acted upon by an outside
force
2) Force = mass x acceleration
3) For every action there is an equal and
opposite reaction