Early History of Astronomy
What is astronomy?
Astronomy is the science that studies matter in
outer space, especially the positions,
dimensions, distribution, motion, composition,
energy, and evolution of celestial bodies and
What is astrology?
Astrology is the study of the movements and
relative positions of celestial bodies interpreted
as having an influence on human affairs.
Until the ancient Greeks, astronomy was the same
as astrology.
Babylonian Astronomers
• 1200 BC - 60 BC
• Compiled star
• First to divide circle
into 360 degrees
• Made calculations
of daylength
changes, planet
motions and lunar
Chinese Astronomers
• 600 BC onward
• Compiled star
• Used for
• Observed and
predicted comets
and eclipses
• First to record a
‘guest star’, a
supernova, in
185 AD.
Greek Astronomy
• In the 4th century BC, Greeks treated astronomy
as a branch of mathematics and developed
geometric models to explain the motion of the
known planets—Mercury, Venus, Mars and
• Later Greek astronomers used philosophical
arguments and reason to explain natural
phenomena, while also using some
observational data to support their explanations.
• Some Greek astronomers believed that the sun,
moon, and the known planets followed a
geocentric model, others a heliocentric model.
One of the objections to the heliocentric
model was that, if the Earth was moving, why
didn’t it leave the air and the moon behind?
• Greek mathematician and
• 384 BC – 322 BC
• He taught that the universe
was a complex system of
55 concentric spheres
with a stationary Earth at
the center
• Taught that the heavens
were immutable
(unchanging), made of a
different (perfect) material
than was Earth.
Aristarchus of Stamos
• Greek astronomer and
• 310 BC - 230 BC
• First heliocentric model
of the solar system
• Understood that the
moon was much closer
than the sun
• Had the right geometric
method to estimate the
distance to the sun but
had poor data so his
calculation was incorrect
• Greek mathematician,
head librarian at the
Great Library in
• 276 BC – 195 BC
• Calculated the
circumference of
Earth, and therefore its
size, very accurately
• Also measured the tilt
of Earth’s axis
• Founder of
geography; developed
system of latitude and
longitude and created
first map of the world
• May have measured
the distance to the sun
• May have invented the
leap day
Eratosthenes (cont.)
Measuring the sun’s
angle at noon on the
solstice at Alexandria
(angle of a post’s
shadow) and knowing
that the sun’s angle at
Syene (due north) was
0°, he calculated that
this angle as 1/50th of
a circle (Greeks knew
Earth was a sphere).
Knowing the distance
between Alexandria
and Syene, he
multiplied by 50 to get
the Earth’s
Hipparchus of Greece
• c. 190 BC – c. 120 BC
• Mathematician, developed
• Discovered the precession
of Earth’s axis (26,000
year cycle), leading to
changes in which star is
the pole star and dates of
the equinoxes.
• He calculated the distance
to the moon and a method
for predicting solar eclipses
• Greek mathematician,
astronomer, geographer
• c. AD 90 to c. AD 168
• Wrote a treatise on
astronomy that was
translated into many
• Using Aristotle’s model, he
developed an epicycle
model of a geocentric
solar system used until
the Middle Ages
Ptolemy’s Model
• Geocentric
• Each planet
moved in a circle
(epicycle) along a
circular orbit about
the Earth
• Order was: moon,
Mercury, Venus,
Sun, Mars, Jupiter
and Saturn
Retrograde Motion
Ptolemy’s model of epicycles explained the
retrograde motion of a planet, when it appears
to move backward (westward) with respect to the
stars. This apparent motion is caused by the
different orbital speeds of Earth and the other
Let’s take a break: Write your
answers on a piece of paper
• What is the difference between astrology
and astronomy?
• Why don’t scientists accept astrological
beliefs as scientific facts?
• What’s the difference between a
geocentric and a heliocentric model?
• What is retrograde motion?
• Why does Mars appear to have retrograde
Western Europe in the
4th through 12th centuries
• After the Roman Empire fell, scientific
studies declined. Almost no one could read
ancient Greek, and the only writings
available were simplified summaries.
• In the 7th Century, an English monk named
Bede wrote a text about how to compute the
proper date of Easter. This was used until
the 12th Century.
• Under the Emperor Charlemagne in the 8th
Century, there was a revival in learning and
ancient texts were studied.
• By the 10th Century word got around (after
knights returned from Crusades) that the
Arabic world had knowledge about
astronomy. Western European scholars
traveled to Spain and Sicily and translated
the writings they found there from Greek
and Arabic into Latin.
• The strong control of the RC Church
meant, however, that these writings had to
be examined to see whether they agreed
with Church theology. If they did, they
were ‘reinterpreted’ through the Church’s
Aristotle’s Model
Aristotle’s and Ptolemy’s model of the universe was
reintroduced to scholars, given the Church’s
stamp of approval after tweaking it to agree with
Church theology, and therefore accepted as
completely true and supported by scripture:
1. The stationary Earth is at the center of the
2. The planets and other celestial bodies travel in
perfect circles around it.
3. The heavens are made of a perfect, unchanging
substance different from substances on Earth.
The Birth of Modern Astronomy
• In a break from these philosophical and
religious views, in the 16th Century,
scientists began to develop more modern
ideas about the universe.
• They investigated and discovered the
natural laws that govern the universe.
• Major scientists who expanded astronomy
were Copernicus, Brahe, Kepler,
Galileo, and Sir Isaac Newton.
Nicolaus Copernicus
• Polish astronomer
• 1473 - 1543
• Near death, finally
published a book
explaining heliocentric
view of universe
• Earth was a planet
• Circular orbits of
planets about the sun
• Understood that the
stars are very, very far
Tycho Brahe
• Danish astronomer
• 1546 - 1601
• Made extremely accurate
stellar and solar system
observations, esp. Mars
• Proposed a model where sun
and moon orbited Earth but the
other planets orbited the sun
• His observation of a supernova
in 1572 gave evidence that the
heavens could change
Convinced that
progress in
astronomy meant
more accurate
Brahe designed
many sighting
instruments like this
wall quadrant
• He made nightly
observations for many
years and kept detailed
• Brahe ran his own
printing press
• His observatory was visited
by many scholars
Giordano Bruno (DON’T WRITE)
• Italian friar 1548-1600
• Believed the sun was a star
• Believed that there were
Earth-like, inhabited planets
orbiting other stars
• For these and other
beliefs, the Church
convicted him of
heresy and he was
burned at the stake
Johannes Kepler
• German astronomer and
• 1571-1630
• Father of modern
astrophysics, was the
first to unite astronomy
(math) and physics
(natural world)
• Originally an assistant to
• Developed 3 laws of
planetary motion using
observations of Brahe
• Supported the heliocentric
model but with elliptical
orbits instead of circles!
Kepler’s Laws of Planetary Motion
First Law: The orbit of every planet is an
ellipse with the Sun at one focus.
An ellipse is an OVAL.
Kepler’s Laws of Planetary Motion
Second Law: A line joining a planet and the
Sun sweeps out equal areas during equal
intervals of time.
revolve around
the sun at a
speed that
depending on
where it is in
its orbit.
Kepler’s Laws of Planetary Motion
Third Law: There is a proportional relationship
between a planet’s orbital period and its
distance to the sun.
Orbital period is
the time it takes to
make one full
orbit around the
Astronomical Unit
• A convenient way of measuring distances
in the solar system is relative to the Earthsun distance.
• An astronomical unit (AU) is the average
distance between Earth and the sun; it is
about 150 million kilometers.
Galileo Galilei
• Italian physicist,
• 1564 -1642
• Father of modern
• Refined the refracting
telescope into an
instrument to observe the
Galileo Galilei’s Observations
1. Saw that planets were
disks rather than points
of light
2. Saw craters on the
3. Discovered 4 moons
of Jupiter
4. Observed sunspots
5. Saw the Milky Way is
made of stars not just
nebulous (made of gas)
Galileo observed that the
phases of Venus as
observed from Earth
support the Copernican
heliocentric model.
Geocentric model
Heliocentric model
At long last, data to disprove one of the two
conflicting models, geocentric and heliocentric!
• Galileo defended his views in
"Dialogue Concerning the Two
Chief World Systems“ in which
he said the Copernican model
was ‘hypothetically’ true.
• The Church felt he was
attacking the pope, so Galileo
was tried by the Inquisition,
found “vehemently suspect of
heresy,” and forced to recant.
• Galileo spent the rest of his life
under house arrest.
Sir Isaac Newton
• English physicist
• 1643 -1727
• First to understand that
planetary motion is due to
gravity—same laws in
heaven as on Earth
• Explained why Kepler’s Laws
worked by formulating the
Universal Law of
• Explained that gravity
between two bodies
depends on both masses
and the square of the
distance between them.
Let’s fire a cannonball from
a cannon that is parallel to
the ground (and neglect air
What happens if we
increase the muzzle speed?
Is there a speed that would
have the cannonball NOT
fall to the ground but stay in
Yes, that speed is the minimum orbital speed at that
particular height for the cannonball to achieve orbit
around the Earth and stay there.
As it orbits, a planet
feels a pull directly
toward the sun (blue
arrow) while it is
travelling along its
Planetary Motion
If somehow the sun
could be turned off in
an instant, the planet
would fly off in the
direction it had been
This means the orbiting planet
going (red arrow) at continually falls towards the Sun
that instant. but also continually misses!

History of Astronomy