The History of Astronomy
Theories, People, and Discoveries of
the Past
Prehistory
• Early man recorded very little history.
• Left some clues in the form of petrographs.
• Stone drawings that show eclipses, comets, supernovae.
• The petrograph to the right represents a supernova.
– Found in a pueblo in southwest America.
Prehistory
• Early man was frightened
by the sky.
• Believed that the heavens
held power over earthly
existence.
– Psychology of the
Unknown
• Astrology born as an
attempt to understand,
predict, and influence
events.
History
• First written records
were astronomical
observations.
• Early Chinese, Central
American (Mayans),
and Northern Europe.
– Stonehenge in England
marks solstices.
1600 B.C.
• Babylonians
• First to write stuff
down.
• This early culture
recorded the positions
of the planets, the
times of eclipses, and
other celestial
phenomena.
~500 B.C.
• Hellenistic Culture
• Greeks acquired the records of the Babylonians.
• Conquest of Alexander the Great.
• Greeks applied data to construct a cosmological framework.
– Basically means – this is the way heaven is arranged.
~500 B.C.
• The Greeks used the star
charts for more than just
practical uses.
– Navigation on the seas.
• Began to develop new
ideas about the universe.
• Devised new experiments
to try to prove their
theories.
~ 480 B.C.
• Thales of Miletus
• Proposed that the
universe was rational and
could be understood by
humans.
• Used the data of the
Babylonians to predict
the occurrence of
eclipses.
400 B.C.
• Plato
• Made what seems like a
minor contribution.
• But his statement
dominated astronomical
thinking for centuries.
• Heaven is perfect and a
circle is the perfect form.
330 B.C.
• Heraclides
• Proposed the first model of the solar system.
• Geocentric – Earth at the center of the universe.
330 B.C.
• Orbits were perfect circles (Plato).
• Was the first round in the debate over geocentric
vs. heliocentric solar system.
270 B.C.
• Round 2
• Aristarchus of Samos
• Made several important contributions.
• He was the first person to propose a heliocentric model of the solar system.
• Mathematically proved the Sun is farther away from the Earth than the Moon.
270 B.C.
• This model put the Sun at the center (helios is
Greek for Sun, thus heliocentric model).
• Still used circles for the orbits (Plato).
270 B.C.
• Used trigonometry to prove that the Sun is farther
away from the Earth than the Moon.
• Idea was correct, just made mathematical errors.
270 B.C.
• Problems with Aristarchus’
model:
1. If the Earth is moving, why
couldn’t people feel it?
2. No parallax seen in the stars.
a. As Earth moves in orbit, the
apparent position of nearby stars
move in relation to stars farther
away.
3. Geocentric = Egocentric
a. More natural.
b. Highlights importance of man!
Parallax
100 B.C.
• Hipparchus
• Produced the first star
catalog showing
positions and
magnitude
(brightness).
• Recorded names of
constellations.
200 A.D.
• Ptolemy
• Librarian in the ancient city of Alexandria.
• Resurrected Heraclides geocentric model.
• Used the library resources containing centuries of data to formulate a complete description of the solar system.
200 A.D.
• Ptolemy explained and
predicted the apparent
motions of celestial
bodies (planets).
• He used perfect circular
orbits for the planets,
drawing on Plato’s idea
that all things in heaven
were perfect.
• Major problem was
retrograde motion.
Retrograde Motion
• Retrograde motion is movement in the opposite direction from the expected motion.
• Some planets would move “backwards” during their motion across the nighttime sky over the year.
• This became very difficult to explain in a geocentric model because the orbits were supposed to be perfect circles around the Earth.
Epicycle Motion
Ptolemaic Model
• Ptolemy developed a very complex system using circles
to explain the retrograde motion of the planets.
• He used circles on circles as seen below.
• The Sun and Moon did not need them because they did
not exhibit retrograde motion.
Ptolemaic Model
• The main circle around the Earth was called a deferent.
• The smaller circle on which the planet traveled around
the deferent was called an epicycle.
• To explain all the known motions, Ptolemy was forced
to use 28 circles.
Epicycle Motion
Ptolemaic Model
• As the planet moves
around the epicycle, it is
also traveling along the
deferent.
• This causes the retrograde
motion of the planets
(points 3 to 5 in the
diagram to the right).
• This is a way to diagram
how Ptolemy ‘s theory
would work.
200 A.D.
• Ptolemy realized that the planets were a lot closer to Earth than the stars, but he still believed in the existence of celestial spheres.
• Stars and planets were attached to these spheres.
• He also realized there might be other objects outside the sphere of fixed stars.
200 A.D. • So, Ptolemy proposed there were other spheres outside
those visible.
• Ended with the “Primum Mobile” (God), which provided the motions of the other spheres.
• This was the first mathematical attempt to explain nature.
~ 410 A.D.
• Alexandria burns and is destroyed along with most of the records in the library.
• Loss of most knowledge acquired by the Greeks and Romans.
• Roman culture collapses leading to the Dark Ages.
• Roman Catholic Church absorbs Aristotle’s scientific method and Ptolemy’s model of the universe. Ancient Lighthouse at
Alexandria
~ 1500’s
• The Renaissance
• People returned to scientific
values where new ideas
were more important than
religious beliefs.
• Art and science began to
flourish.
• Da Vinci, Copernicus,
Raphael, and others served
the church, but they began
to rebel (secretly).
~ 1500’s
• Nicholas Copernicus
• Reinvented the heliocentric
model.
• More than just an attempt to
solve retrograde motion.
• It had both social and
political consequences by
challenging the power of
the Catholic Church.
• Psalm 93 – “Thou hast
fixed the Earth immovable
and firm.”
~ 1500’s
• Copernicus’ model also
questioned the authority of
the most revered wise men
of the ancient world
(Ptolemy, Plato,
Heraclides).
• He forced a change in
humanity’s view of the
world and our importance
in it.
• Yet, even his model had
problems.
~ 1500’s
• Copernicus still used circular orbits.
• As a result, he still was forced to use epicycles and deferents.
• His model was even more complicated than Ptolemy’s.
• Would have failed our modern criteria that models be as simple as possible (Occam’s Razor).
1580
• Tycho Brahe
• 1st true observer.
• Built the Danish
Observatory.
• Measured the positions of
planets and the stars to a
very high degree of accuracy
for his time.
• He also used trigonometry to
measure the distance to the
Sun.
1600’s
• Johannes Kepler
• A student of Tycho
Brahe with access to
all of Brahe’s data.
• Used this data to
formulate the Laws of
Planetary Motion.
• He was the first to
realize that orbits were
elliptical.
– Theory fit the data, not
the other way around.
1600’s
• Solved one problem of heliocentric model.
• Eliminated the need for epicycles and deferents.
1600’s
• This highly accurate system determining all the
motions of the planets marked the beginning of the
“clockwork universe” concept.
First Law
• The orbits of celestial objects (planets) are ellipses.
• The Sun is not in the center, but at one of the two foci of the ellipse.
• No matter where you are on the ellipse, the sum of the distances from the foci to the object is constant. This ellipse is exaggerated, they
are very close to being circular.
Second Law • The planets sweep out
equal areas in equal times.
• If you look at the shaded areas to the right, those two areas are equal to each other.
• Which means the planet moves slightly faster in its closest pass to the Sun, slower farther away. Kepler's Laws with Animation
Third Law
• The square of a
planet’s orbital period
is proportional to the
cube of its average
distance from the Sun.
• The period is time of
revolution.
• The radius can be
miles, kilometers, or
astronomical units.
Examples of 3rd Law
Planet Period A.U. T² R³
Mercury 0.24 0.39 0.06 0.06
Venus 0.62 0.72 0.39 0.37
Earth 1.00 1.00 1.00 1.00
Mars 1.88 1.52 3.53 3.51
Jupiter 11.9 5.20 142 141
Saturn 29.5 9.54 870 868
Period = Earth Years T² = Period Squared
R³ = Average Radius of Orbit (A.U.) Cubed
You Do The Math
• The A.U. of a planet is 2.53.
• About how long will it take the planet to orbit the Sun?
• 2.53³ = 16.19
• The square root of 16.19 = 4.02 Earth Years
1620’s • Galileo
• Finished off the idea of a geocentric universe.
• Used a telescope to make the following discoveries:
1. Sunspots
2. Mountains on the Moon
3. Milky Way with Lots of Stars
4. Venus had Phases Like Our Moon
5. Jupiter’s Four Main Moons
1620’s • Most damaging discovery
was the four moons of Jupiter.
• Those moons were orbiting Jupiter instead of the Earth.
• This did not fit the idea of a geocentric model.
• At this point, the Catholic Church was forced to give in and accept a heliocentric model.
• Began the decline of religion.
1680’s
• Isaac Newton
• One of the most
intelligent men in
history.
• Two main applications
to Astronomy.
1. Law of Universal
Gravitation
2. Laws of Motion
Law of Universal Gravitation
• Simply put – every object in the universe has a
gravitational attraction for every other object.
• An atom in your fingernail has a gravitational pull
on the Moon!
Law of Universal Gravitation • Newton developed a formula to go with the Law.
• F = G M1M2 / r²
• F = force of attraction; G = gravitational constant; M1 = mass of 1st body; M2 = mass of 2nd body; r² = the distance between the two bodies squared.
Law of Universal Gravitation • The gravitational constant is a very small number.
• It is for this reason you can’t feel a gravitational pull between you and another person.
• Mass has a role as well, you may pull on the Moon, but your mass is so small there is no apparent change in its orbit.
Law of Universal Gravitation
• Lets look at an mathematical example.
• It will not be precise, because the gravitational constant is a lot smaller than the number we will use for this example.
• What is the attractive force (F) of two bodies in space with the following information:
• G = 0.00006
• M1 = 500 kg; M2 = 3000 kg
• R = 10 m
Law of Universal Gravitation
• Remember the formula: F = G M1M2 / r²
• 500 kg x 3000 kg = 1,500,000 kg
• 1,500,000 kg x 0.00006 = 90 kg
• (10m)² = 100m²
• 90 kg 100m² = 0.9 kg/m²
• This is a very small amount of force between two huge objects that are very close.
• Imagine if we used the real G or the objects were a lot farther apart!
• Try it at a distance of 100 m apart!
Law of Universal Gravitation
• Remember the formula: F = G M1M2 / r²
• 500 kg x 3000 kg = 1,500,000 kg
• 1,500,000 kg x 0.00006 = 90 kg
• (100m)² = 10,000m²
• 90 kg 10,000m² = 0.009 kg/m²
• The distance increased by a factor of 10,
the force decreased by a factor of 100!
• Try to imagine the force between the two
objects at astronomical distances!
• It is very tiny, but still there.
Law of Universal Gravitation
• This law also predicts that, in general, the orbit of an
object can be any of four conic sections as well as a
straight line.
• Conic sections are slices thru a cone at different angles.
Law of Universal Gravitation
• Scientists have discovered some comets that have
parabolic or hyperbolic orbits.
• In these cases, they do not return to the Sun after they
pass by it.
Laws of Motion
• We have looked at
the Laws of Motion
in Motion Unit.
• Will not go over
them again.
• Necessary for
spaceflight.