Chapter 1:
Origins of Modern Astronomy
22.1: Early Astronomy
Ancient Greeks and Astronomy
• Astronomy is the
science that studies
the universe.
• The first accurate
European
astronomers were
the ancient Greeks.
Aristotle (384 - 322
B.C.E.) concluded
that the earth was
round because it
always cast a
curved shadow
when it passes
between the sun
and the moon.
Aristotle’s belief that the Earth is round was
abandoned by the Middle Ages.
Eratosthenes (276
– 194 B.C.E.), an
ancient Greek
mathematician,
calculated that the
earth’s circumference is 39,000
kilometers - very
close to our own
modern measurement of 40,075 km.
The Greeks thought
that the Earth was a
sphere that stayed
motionless at the
center of the
universe. The other
planets and stars
revolved around the
Earth on their own
hollow spheres.
The Geocentric Model
• In this model, the moon, sun, and the
known planets – Mercury, Venus, Mars,
Jupiter and Saturn – orbit earth.
• Every other body in space circled this
system on their own transparent,
hollow sphere.
• This was called the celestial sphere.
A modern
version
of an
earthcentered
system.
The Heliocentric Model
• Aristarchus (312-230
B.C.E.) was the first
Greek to believe in a
sun-centered, or
heliocentric, universe.
• In the heliocentric
model, Earth and other
planets orbit the sun.
Aristarchus was condemned by his
own religious leaders for his theory.
Modern Greece, however, honors him, and
even puts his theories on stamps…
The Ptolemaic System
• Claudius Ptolemy (c. 90 – 168 C.E.) is
our main source for what the ancient
Greeks knew about astronomy.
• He also attempted to explain retrograde
motion, or how each planet appears
sometimes to stop in the night sky,
reverse direction, and then resume
eastward motion.
Ptolemy was wrong –
the planets do not
orbit Earth.
Yet although he used
the geocentric model,
he did try explain the
planets’ apparent
motions.
The Birth of Modern Astronomy
• The first great astronomer after the
Greeks was Nicolaus Copernicus
(1473-1543) of Poland.
• Copernicus proposed the heliocentric
model of the solar system: Earth is a
planet and all planets of the solar
system revolved around the sun at its
center.
Heliocentric Model of the Solar System
Tycho Brahe (15461601) had the King
of Denmark build
him an observatory.
Although the
telescope had not
been invented,
Brahe’s findings
were extremely
precise for the
time.
Johannes Kepler
(1571-1630) was
Brahe’s assistant,
and later the first
important modern
astronomer.
Kepler applied
mathematics to
Brahe’s findings
and discovered
Three Laws of
Planetary Motion.
• First Law: The path of each planet
around the sun is an ellipse, with the
sun at one focus.
How is an ellipse different from a
circle?
• A circle is a
closed curved
shape that is flat.
In a circle, all
points on the
circle are equally
distant from the
center of the
circle.
•An ellipse is also a closed curved shape
that is flat.
•Instead of having all points the same
distance from the center (like a circle), an
ellipse has two focus points.
• Second Law: Kepler determined that a
planet travels most rapidly when it comes
closest to the Sun and moves slowest
when farthest away.
Third Law: Keppler’s third law gives the
precise relation between the distance of a
planet from the Sun and how fast it
completes an orbit, using Astronomical
Units (AU).
One AU equals
150 million km,
the average
distance of the
Earth from the
Sun.
Galileo Galilei (1564-1642)
The greatest
Italian
scientist of
the
Renaissance
One of the
most brilliant
humans who
ever lived.
• Galileo not only
provided
evidence that
proved the
heliocentric
model accurate,
but he also
invented
pendulum clocks
and the modern
thermometer.
• He also created
one of the first
telescopes,
without anything
but a written
description to
guide him. He
even ground
glass for its
lenses himself.
Galileo used his telescope to view
the universe in a new way. He
made important discoveries that
supported Copernicus’ heliocentric
model of the universe.
1. The discovery of
the largest four
moons of Jupiter.
(He also discovered
Saturn’s rings.)
2.Planets are NOT pinpoints of light.
They are actually spheres, like Earth.
3. Venus has phases,
just like the moon.
Therefore, Venus circles the Sun.
4, The moon’s
surface is not
smooth.
(Galileo
thought the
moon’s dark
areas might
be seas.)
5. The sun has sunspots,
or dark regions.
Galileo tracked the movement of these sunspots
and estimated the rotational period of the sun.
Sir Isaac Newton (1642-1727)
•The first one
to explain
that planets
are held in
their orbit by
a force:
Universal
Gravitation
Mathematician, Inventor, General Genius
Universal Gravitation
1. Every body in the universe attracts
every other body with a force
directionally proportional to their
masses.
2. Gravitational forces decrease with
distance.
3. The greater the mass of the object,
the greater its gravitational force.
Newton also
improved on
Galileo’s original
telescope by adding
mirrors, allowing it
to be much shorter.