Transcript Lecture 3: History of Western Astronomy

Early Views of
the Solar
System
(With thanks to Mark Ritter & John Bloom)
The Roots of Astronomy
• Already in the stone and bronze ages,
human cultures realized the cyclic
nature of motions in the sky.
• Monuments dating back to ~ 3000 B.C.
show alignments with astronomical
significance.
• Those monuments were probably used
as calendars or even to predict eclipses.
Stonehenge
Summer solstice
Heelstone
• Alignments with locations of
sunset, sunrise, moonset and
moonrise at summer and
winter solstices
• Probably used as calendar.
Constructed: 3000 – 1800 B.C.
Examples All Over the World
Big Horn Medicine Wheel (Wyoming)
Examples All Over the World
Caracol (Maya culture, approx. A.D. 1000)
Ancient Astronomers
• No written documents about the significance
of stone and bronze age monuments.
• First preserved written documents about
ancient astronomy are from ancient Greek
philosophy.
• Greeks tried to understand the motions of the
sky and describe them in terms of
mathematical models.
Ancient Greek Astronomers
Models were generally wrong because they
were based on wrong “first principles”, believed
to be “obvious” and not questioned:
1. Geocentric Universe:
Earth at the Center of the Universe.
2. “Perfect Heavens”:
Motions of all celestial, ethereal bodies described by
motions involving objects of “perfect” shape, i.e.,
spheres or circles.
General Greek
Principles of Science
• Nature can be understood
(Not just randomly occurring
phenomena)
• The diverse behavior observed in
nature is held together in patterns
• Tendency to
“Save the Phenomenon”
Euclid ~ 600 BC
• What he got right:
– Predicted solar eclipses
– Moon shines due to reflected sunlight
– Universe explainable by ordinary knowledge
and reason
• What he got wrong:
– Thought Earth was a flat, rotating disk
– Thought water was a fundamental element
from which all other matter derives
Pythagoras ~ 530 BC
• First to suggest that Earth is
a sphere
• One of the earliest advocates
of a geocentric solar system:
– Earth at center, surrounded
by system of concentric,
rotating, transparent
spheres
– Bodies attached to spheres
in this order:
Moon, Mercury, Venus, Sun,
Jupiter, Saturn, stars
Philolaus ~ 410 BC
(student of Pythagoras)
• Suggested Earth
moves around a
central fire
• (NOT the Sun)
• What were his
new ideas???
Ancient Greek Astronomers
• Eudoxus (409 – 356 B.C.):
- Model of 27 nested spheres
• Aristotle (384 – 322 B.C.), major
authority of philosophy until the late
middle ages:
- Universe can be divided in 2 parts:
1. Imperfect, changeable Earth
2. Perfect Heavens
(described by spheres)
• Aristotle expanded Eudoxus’ Model to use 55 spheres.
the Aristotelian
universe ~ 340 BC
• In ancient times
philosophers argued from
first principles, things
that were accepted as
“obviously” true
• Several important ideas
ruled the ancient world…
Aristotle ~ 340 BC
• first: the earth was round
• gave 4 reasons for earth’s roundness:
– Symmetry: the sphere is a perfect shape
– Elements have their natural places: Earth’s pieces fall naturally
to Earth’s center, pressing it into a spherical shape (circular
reasoning!)
– Shadow: Lunar eclipse shadow (Earth’s shadow on Moon) is
always circular (what if Earth were a circular disk?)
– North Star: Polaris gets higher in the sky the further north one
goes
• second: the earth was
located in the center of
the cosmos!
• third: the heavens
were perfect, the earth
imperfect
• all the cosmos traveled
in perfect circles
around us
• Plato pushed this, his
student, Aristotle
adopted it
• he gave us the
Aristotelian way of
looking at the cosmos…
• geocentric universe with all the heavenly objects moving in
uniform circular motion
• Aristotelian concepts dominated ancient ideas about the
universe for thousands of years…
• Two major themes here:
1) our place in the cosmos
2) the character of planetary motion
Aristarchus ~ 240 BC
• Rotation of Earth on its own axis
accounts for daily motion of stars
• Earth revolves around Sun in a
yearly orbit
He had it right before 200 BC!
But his ideas failed to catch on…
Eratosthenes ~ 235 BC
• Calculated Earth’s radius to within about 5% accuracy!
• Basic method:
– Measured shadow lengths at two different cities directly North-South of each
other (Syene and Alexandria)
– Calculated Earth’s circumference and radius using geometry
– Central angle = 7.5 degrees, 500 miles from A to S, times 48 = 24,000 miles;
– Actual angle = 7.2 degrees, 500 mi x 50 = 25,000 miles.
Ptolemy ~ 120 AD
• great ancient astronomer of the 2nd
century
• wanted to put it all this orbital motion
into a mathematical model
• final effort to “Save the Phenomenon”
• no significant changes for 1300 years!
• Ptolemaic view = geocentric view
•
•
•
•
•
why did they
think we were
in center
anyway?
popular
worldview
the Sun
appears to rise,
transit the sky,
and set
it doesn’t feel
like we are
moving!
and there was
the problem
of…
• parallax, the
apparent motion of an
object b/c of the motion
of the observer
• if we were going around
the sun, the background
stars should change
position!
• (they do, but not a lot
since they are so far
away)
• planetary motion was difficult for
the ancients to explain
• planets (wanderers) would suddenly
backtrack!
• called
retrograde motion
epicycles
(little circles - blue) on deferents (big circles -
• Ptolemy tried to explain it with
white)
• all in an attempt to keep the Aristotelian
(geocentric) view of the cosmos
• all Ptolemy’s attempts to mathematically explain
Aristotle's universe were put together in a book
preserved by the Arabs, Al Magisti, then translated
into Latin,
The Almagest
• but unknown to them it was all wrong! enter…
Copernicus
~ 1500 AD
• a Polish monk,
Mikolai Kopernik (Lt.
Nicolaus Copernicus)
rediscovered the
heliocentric
model of the
universe
• but being for
heliocentrism was
being against
Aristotle, and 
against the Church!
De Revolutionibus
• finished in 1530, Copernicus wrote De
Revolutionibus Orbium Coelestrium
(“on the
revolution of
the celestial
orbs”)
• he didn’t give
permission for
its publication
until he was
dying
• most important idea?
• he put the sun (Sol) in the center!
• with the sun at
the center the
whole universe
was simplified and
elegant, and could
explain things like
retrograde
motion
• we, moving faster
on the inside, see
other planets
“seem” to move
backward
• but!…
• Copernicus insisted on circular orbits!
• so he had to come up with his own
epicycles so it worked better…
• his model was incorrect
• but his hypothesis with
the sun in center was
right
• but without a telescope it
couldn’t be substantiated
with evidence
• AND his circle obsession
meant it couldn’t predict
things very well
• AND it upset people
because their whole
universe changed!
• so it wasn’t accepted for
a long time
Tycho
Brahe
~ 1500s
• the greatest pretelescopic
observational
astronomer
• in 1572, he saw a supernova which he called a new
star
• it showed no parallax, so he deduced it to be a part
of the starry sphere
• but the star sphere was “perfect and unchanging”!!!
• so the new star
challenged
Ptolemy and
Aristotle
• Brahe wrote a
book on it called
De Stella Nova
• which got the
notice of lots of
people and made
him famous…
• Danish king Frederick II gave him island of
Hveen to build an observatory (Uraniburg)
• here Tycho
shows off the
latest mural
quadrant
• with tools like
these (notice no
scopes) he
observed the
heavens and
took over 20
years of data!
• Tycho had his
own version of
what was going
on
• neither
Ptolemaic nor
Copernican
• Earth was at
center, but all
the other
planets went
around the sun
• Tychonic view
• Tycho ended up in
Prague and hired
some new
assistants to help
prove his system
was correct
• one of the
assistants was
Johannes Kepler
Johannes Kepler
(1571 – 1630)
• Kepler had a notso-fun life
• born in 1571 in
Germany, he was
poor, had an absent
father, was sickly,
had a nasty mother,
etc., etc.
• a Copernican, Kepler even wrote a book
Mysterium Cosmographicum which tried to
prove that all the planets were built with
regular solids as spacers!
joining
Tycho
• eventually, after
being persecuted
for being a
Protestant, he
headed for Prague
to work for Tycho
• then Tycho died…
• so, Kepler took all
of Tycho’s work
to finish the
Rudolphine Tables
himself
• with all of Tycho’s
data, Kepler tried to
find out how the
planets really moved
• he discovered that
Mars moves in an
ellipse, not a circle!
• and it traveled at
different speeds!
• he wrote a lot after
this and in 1619
published The
Harmony of the
Worlds in which he
said that the radii of
the planets’ orbits are
related to their periods
• all this led to his
three fundamental
laws of planetary
motion…
Kepler’s three laws of
planetary motion
• first law relies on the ellipse
– an oval drawn around 2 points
• points called foci (sing: focus)
• semimajor axis (a) =
half the longest diameter
• eccentricity (e) is half
the distance between foci
divided by the semimajor axis
• a circle is an ellipse with e = 0
• greater e  more elliptical
• Kepler’s First Law (Law of Ellipses)
states that all planets travel in
ellipses with the sun at one focus
• Second Law (Equal Areas-Equal Times)
planets sweep out equal areas in equal
times
• translation: they go faster when closer to
the Sun
•
•
•
•
• Third Law
T2 = R3
• states that the
closer a planet
is to the Sun
the shorter its
“year”
T = period in Earth years
R = distance in AUs
given one, you should be able to find the other
Kepler never knew why his laws worked, just
that they did
the Rudolphine
Tables
• finally, in 1627,
Kepler finished
the R Tables, a
precise model of
planetary motion
• (notice Tycho as
part of the pillars
of astronomy)
• a contemporary
of Kepler was…
Galileo Galilei
(1594 – 1642)
• helped push
science to a
new level…
telescopic
observations
• GG had an academic
upbringing and was
quietly Copernican
• it was the telescope
that pushed him over
the top
• he used this recent
invention to examine
the sky and gather
evidence for the
heliocentric view
• he immediately
published his
findings in
Siderius
Nuncius (The
Sidereal
Messenger)
• here he reported
several big
discoveries…
• 1: The
Moon was
not
perfect!!!
• remember
Aristotle
said
everything
in the
heavens
was
perfect?
• but the
Moon
looked sort
of
like Earth!!!
Major Discoveries of Galileo
• Surface structures on the moon; first estimates
of the height of mountains on the moon
• 2: There
were lots
of stars up
there
(many more
than were
previously
supposed)
• the Milky
Way (and
the rest of
the
heavens) is
substantial,
like the
earth, not
ethereal
• 3: He saw four
little “stars”
near Jupiter
• these points of
light were going
around Jupiter!
• but they weren’t
supposed to!!!!
they should have
been left
behind!!!
• So, not
everything orbits
the Earth…
• and, if Jupiter
can move and
keep its “moons”
so could Earth
Major Discoveries of Galileo
• Moons of Jupiter
(4 Galilean moons)
(What he really saw)
• Rings of Saturn
(What he really saw)
Major Discoveries of Galileo
4: He saw that the sun had spots
(proving that the sun is not perfect!),
and that it rotated
5: He observed that Venus went
through phases (including “full Venus”)
So what? it can only have all phases if
it goes around the sun, not the Earth!
• but even with all
this evidence people
were still upset
• some didn’t even
look through the
telescope!
• The Catholic Church,
still in love with
Aristotle, told
Galileo to be quiet
• it was in 1629 that
he wrote his great
defense of the
Copernican way…
Galileo’s Dialogue
• it was an imaginary debate between three
friends:
Salviati (pro-Copernican)
Sagredo (indifferent)
Simplicio (pro-Ptolemy)
• problem: Simplicio presented the Pope’s
view of things … bad move!
• Galileo was brought
before the Inquisition
• charged with not
shutting up
• condemned to life
imprisonment
• note: what Copernicus
and Kepler and Galileo
taught and discovered
didn’t contradict the
Bible…
• it contradicted
people’s
interpretations,
traditions, and
worldviews
The Birth of Modern Science
• in trying to understand the skies in the 16/17
century, modern science was born
• nothing more than a logical way of observing,
studying, and thinking about nature
• scientific method (Roger Bacon, 13th century)
• cause and effect (Francis Bacon, 17th century)
• et al…
Isaac Newton
and orbital motion
• by now we have a
new outlook on the
universe and new
ways to study it all
• perfect time for this
guy to show up…
Isaac
Newton
• Isaac was very
smart, raised in
England,
educated at
Trinity, but…
• during the Black
Plague of 1665
went home to
Woolsthorpe
• while there he
thought through
his laws of
motion…
• which he wrote
about in his
Principia
• from the work of all
the great guys
before him he gave
us three laws of
motion which we still
use
• he realized the same
force that brings an
apple to the Earth
keeps the Moon in
orbit
• and those other
things must be
pulling on us, too
• he figured if
something has
mass it has
gravity
• mass is how much
stuff/matter
something has
• weight is the force
that something
exerts because
gravity pulls on it
• He also
realized that
the distance
plays a huge
role
• and is an
inverse
square
relation
• double the
distance and
gravity
decreases by
4x.
• know the relationships…
• the force of gravity attracting two
objects is proportional to their
inversely
proportional to the squares of their
masses, but
distances
orbital
motion
• here’s why satellites orbit celestial
bodies
• throw something hard enough and the
Earth will curve out of the way
• it will “fall” forever
• that is being in orbit
• this animation
shows how
• circular
velocity is the
speed you have to
achieve to orbit in a
circle
• less, you crash;
more, you might
even leave!
• the Shuttle needs
to get to about 8
km/s to do it.
• less over smaller
bodies