Transcript History of Astronomy

What is astronomy?
Astronomy is the science that studies matter in
outer space, especially the positions, motion, and
composition.
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 catalogs
 First to divide circle into
360 degrees
 Made calculations of
daylength changes,
planet motions and
lunar eclipses
Chinese Astronomers
 600 BC onward
 Compiled star
catalogs
 Observed and
predicted comets
and eclipses
 First to record a
supernova
Greek Astronomy
 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 Jupiter.
 Some Greek astronomers believed that the sun,
moon, and the known planets followed a geocentric
(earth center) model, others a heliocentric (sun
center) 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?
Aristotle
 Greek mathematician and
philosopher
 384 BC – 322 BC
 He said that the universe was
a system of 55 spheres with
Earth at the center
Aristarchus of Stamos
 Greek astronomer and
mathematician
 310 BC - 230 BC
 First heliocentric model
of the solar system
Eratosthenes
 Greek mathematician
 276 BC – 195 BC
 Calculated the
circumference of Earth
 Also measured the tilt of
Earth’s axis
Eratosthenes (cont.)
Measuring the sun’s
angle at noon on the
solstice at Alexandria
and knowing that the
sun’s angle at Syene
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
circumference!
Ptolemy
 Greek mathematician,
astronomer
 c. AD 90 to c. AD 168
 Using Aristotle’s model, he
developed an epicycle
model of a geocentric
solar system.
Ptolemy’s Model
 Geocentric (earth
center)
 Each planet moved in a
small circle (epicycle)
around the Earth
Retrograde Motion
retrograde motion of a planet, when it appears to
move backward. This apparent motion is caused by
the different orbital speeds of Earth and the other
planet.
Let’s take a break: Turn to your neighbor on
your left or your right and answer these
questions. Be prepared to share your
answers.
 What is the difference between astrology and
astronomy?
 Why does Mars appear to have retrograde motion?
Western Europe in the
4th through 12th centuries
 These were the Dark Ages when scientific studies
declined.
 By the 10th Century) the Arabic world had knowledge
about astronomy. Western European scholars traveled
to Spain and Sicily and translated the writings.
Aristotle’s Model
Aristotle’s and Ptolemy’s model of the universe was
reintroduced to scholars.
1. The stationary Earth is at the center of the universe.
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 the 16th Century, scientists began to develop more
modern ideas about the universe.
 They investigated and discovered the natural laws
that govern the universe.
 Scientists who expanded astronomy were Copernicus,
Brahe, Kepler, Galileo, and Isaac Newton.
Nicolaus Copernicus
 Polish astronomer
 1473 - 1543
 Earth was a planet
 Circular orbits of
planets about the sun
 Understood that the
stars are very, very far
away
Tycho Brahe
 Danish astronomer
 1546 - 1601
 Made extremely accurate
observations about the solar
system, 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
observations,
Brahe designed
many sighting
instruments like this
wall quadrant
 He kept detailed
records
Johannes
Kepler
 German astronomer
 1571-1630
 Father of modern
astrophysics)
 Originally an assistant to
Brahe
 Developed 3 laws of
planetary motion using
observations of Brahe
 Supported the heliocentric
model but with elliptical
orbits instead of circles!
First Law: The orbit of every planet is an ellipse with
the Sun at one focus.
Kepler’s Laws of Planetary Motion
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.
Planets
revolve around
the sun at a
speed that
varies
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
sun.
Galileo Galilei
 Italian physicist,
astronomer,
mathematician
 1564 -1642
 Father of modern
observational astronomy
 Refined the refracting
telescope
Galileo Galilei’s Observations
1. Saw that planets were
2.
3.
4.
5.
disks rather than points
of light
Saw craters on the
moon
Discovered 4 moons of
Jupiter
Observed sunspots
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
supported Copernicus’
heliocentric model.
Geocentric model
Heliocentric model
At long last, data to disprove one of the two
conflicting models, geocentric and heliocentric!
Sir Isaac Newton
 English physicist
 1643 -1727
 First to understand that
planetary motion is due to
gravity
 Proposed the Universal Law
of Gravitation
The Gravity between two bodies depends on both
masses and the distance between them.
This means that the more massive an object is and
the closer it is to another object, there is more
gravity between them.
OR
The smaller an object is and the further away they are
from each other, there is less Gravity between them