Transcript Astronomy Journey to the Cosmic Frontier

History of Astronomy
Why did ancient cultures study
astronomy?
• Several cultures kept very exact records
of astronomical events (and could even
predict events) such as the
Mesopotamians (Babylonians) and
especially Chinese.
• WHY??????
• timekeeping
• agriculture (planting crops or hunting
and fishing cycles)
• astrology/religion
• and to keep accurate historical records.
• But, there were no real models of why
astronomical events happened…that’s
what the Greeks did.
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Greek Astronomy
General Information of Historical Models
• Models tried to explain motion planets
and why they move through the fixed
stars.
• Models tried to explain motion of moon
and sun as well.
• Models tried to account for retrograde
motion of planets (this was the biggest
problem in finding a model that worked
accurately).
• Astronomy and mathematics advanced
side-by-side because it gave cultures
problems to solve and calculate
• GEOcentric--Earth centered universe
• HELIOcentric--Sun centered universe
A Timeline of
World Events and
Famous People
(left) and Notable
Greek
Astronomers
(right)
The
Pythagorean
Model of the
Solar
System
Pythagorean Model
• Earth moved around a
“central fire” but we
couldn’t see it because
it was blocked by a
“Counter Earth”
• made music as it spun
• Why it’s important:
• one of first models
• planets are spherical
and orbit in circular
paths
Eudoxus’ Model
• Tried to account for
retrograde motion of planets
• opposite motion of spheres
produced figure eight
motion of planet
• Notice: still circular
• Problems:
• 1. Not exact
• 2. Planets should be same
brightness all the time and
they aren’t
Aristotle’s (Aristotelian) model
• ~350 B.C.
• Geocentric model of
the universe
• Problem: Not very
exact
Eratosthenes (~ 200 B.C.):
Calculation of the Earth’s radius
Angular distance between
Syene and Alexandria:
~ 70
Linear distance between
Syene and Alexandria:
~ 5,000 stadia
 Earth
Radius ~ 40,000
stadia (probably ~ 14 %
too large) – better than
any previous radius
estimate.
Ptolemy’s Model--tried to account for
retrograde motion using epicycles
(still not exact, but better
and was accepted for 1500 yrs !)
The Complete Geocentric Model
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From Ptolemy into the Renaissance
Ptolemy’s model was relatively
accurate and stuck around for
1500 years before another
replaced it. Note...Ptolemy’s was
geocentric.
Aristarchus proposed a
heliocentric model around 300
B.C., but no one accepted it
because they thought the position
of stars should shift if Earth were
moving.
Not until 1500’s (only 500 years
ago !) did we see that the Earth
wasn’t the center of the universe
with Copernicus.
Coming up next… Copernicus,
Tycho, Kepler and Galileo in the
Renaissance.
A Timeline of Notable
People and Events
(left) and the Four
Most Prominent
Renaissance
Astronomers (right)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Renaissance Astronomy
Copernicus
• First accepted fully
developed heliocentric
model (sun at center)
• published in 1543
• Explains diurnal motion of
stars, explains motion of sun
• Important: Retrograde
motion occurs whenever
Earth passes or is passed by
another planet
• Difference between
epicycles (Ptolemy) and
Copernican model
The Position of the Sun Against the
Backdrop of Stars throughout the year
The Retrograde
Motion of Mars
according to the
Heliocentric Model
•The retrograde motion of
mars would occur over a
couple of months.
•The movement is
apparent—mars doesn’t
really move like that in
space. The position of mars
shifts against the backdrop
of distant stars.
Tycho Brahe
• Took very detailed and
accurate measurements of
stars and planets from his
observatory
• Also discovered a
supernova in 1572 (a
brilliant “new star”) that
suggests that Aristotle’s
starry sphere was not
perfect and it changes
Tycho and Kepler
• Kepler took accurate
measurements from Tycho
and devised his own
mathematical laws.
• Kepler’s 1st law: planets
move in ellipses and not in
circles (the sun is at one of
the foci)
• Kepler’s 2nd law: planets
move faster when close to sun
and slower when farther
away.
• Kepler’s 3rd law: P2  a3
(P=sidereal periods and
a=semimajor axis)
Kepler’s Second Law
Galileo
• used telescope (but wasn’t the
inventor)
• found Milky Way is really a lot of stars
• showed sunspots on sun—sun isn’t a
perfect celestial body
• showed moon has mountains and
valleys like Earth (suggests Earth is a
celestial object…part of the “heavens”)
• showed 4 satellites orbiting Jupiter
(proof that Earth wasn’t the center of
everything)
• showed phases of Venus (which
crippled Ptolemaic model)…this can’t
be explained in the geocentric model
Some of Galileo’s
Drawings of the
Moon
Phases of Venus—can’t see this in
the geocentric model
Galileo’s
drawings of
Jupiter and the 4
main moons