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.