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Astro 201: Sept. 2, 2010
• Do on-line practice quiz #2 (see d2l)
• Homework #2: posted on web page after
class
• Today:
– Ancient Greeks
– Ptolemy V. Copernicus; retrograde motion
– Galileo
– Astronomical basis for calendars
– Tycho, Kepler, Newton
Despite Aristarchus, for 2000 years, a GEOCENTRIC
model of the Universe was favored
STARS affixed to the celestial sphere
Moon, Planets and the Sun
are between the EARTH and STARS
EARTH at the center
PTOLEMY 90-168 AD
Ptolemy’s ALMAGEST was translated into Latin in 1496
GEOCENTRIC: Earth at center
Copernicus (1473-1543)
Heliocentric Model: Sun at the center
Ptolemy’s Epicycles
• In order to understand the motion of the
planets in the sky Ptolemy’s model had to be
modified
• In terms of the heliocentric model, the planets
orbit the Sun in ellipses, not circles
• Retrograde motion of the planets with respect
to the stars required EPICYCLES
Ecliptic
the fact that the
planets orbit the
Sun in a plane
means that they
always appear to
lie on a great
circle on the sky,
called the ecliptic
Retrograde motion
The planets sometimes appear to be traveling west
to east, unlike the stars and Sun which always go
east to west
Next slide: animation
Astronomy Picture of the Day for Dec. 20, 2001.
Jupiter and Saturn showing retrograde motion.
Combining 23 pictures taken at 2 week intervals
from June 2000 - May 2001.
Planet = “wanderer”
Ptolemy’s explanation for retrograde
motion of the planets
Earth Off Center
Earth Equant
Epicycle
Deferent
The planet moves along its epicycle as the epicycle moves along the deferent around the
Earth. To make the observations as accurate as possible, it was necessary to place the Earth
slightly off center of the orbits, but to preserve symmetry that meant that there was an equal
place (“Equant”) opposite the Earth from the center. The combined motion of the planet and the
resulting retrograde motion are shown.
Copernicus’ explanation for retrograde
motion
Copernicus:
“On the Revolution of the Heavenly Spheres”
Published at the time of his death, in 1543
Major Conclusions:
1. The planets orbit the Sun
2. The apparent daily motion of the
Sun and stars is the result of the
Earth’s rotation
3. The stars are much farther away
than the Sun
Heliocentric model: distance from Sun to stars
must be much greater than distance from Sun
to Earth.
Since Earth orbits Sun, stars should show
parallax (a shift in apparent position) over
the course of half a year.
OBSERVATION: Parallax of stars
is TOO SMALL to be seen
by the naked eye.
Not observed until 1800s.
Implication: distance to stars is
several thousand times Earth –
Sun distance.
Parallax
Unit of distance: the PARSEC
A star which is one parsec from Earth shows a parallax of one arcsecond
1
d
p
Remember: 360 degrees in a circle, 60 arcminutes per degree, 60 arcseconds per arcminute
Reactions to Copernicus:
On March 5, 1616, Copernicus' work was banned from being taught and discussed
by the Congregation of the Index "until corrected."
It stayed on this list of prohibited books and teachings until 1822.
Martin Luther (1483-1546): [Copernicus] “is a fool who wishes to
reverse the entire scheme of astronomy; but sacred scripture tells us
that Joshua commanded the Earth to stand still, not the Sun.”
Giordano Bruno (1548-1600): burned at the stake for advocating that stars
are suns in their own right, and that there is a plurality of worlds like the Earth.
Cosmological Models:
Version 1.0:
“Superdome” model
Version 2.0:
Geocentric model
v. 1.0
v. 2.0
Version 3.0:
Heliocentric model
v. 3.0
Which is “right”?
Occam's Razor:
Pluralitas non est ponenda sine neccesitate
"plurality should not be posited without
necessity."
Given a set of otherwise equivalent models of a
phenomenon, the simplest one is the best.
Keep it simple, stupid.
William of Occam
1285-1349
English philosopher from Ockham
Franciscan Monk
Got into trouble with the Pope for advocating
“apostolic poverty”
Died of the Black Death while in exile
Advocated
Epistomological Parsimony
c.f. Ontological parsimony
Epistomology = theory of knowledge
Ontology = what exists?
Galileo Galilei (1564 – 1642)
Italian
First to use a telescope to look at the sky
1610 “Siderius Nuncius” (The Starry Messenger)
• “Spots” on the Sun; the Sun rotates
• The Moon has mountains, craters, rocky surface with
imperfections
• The “planet” Jupiter is not a pinpoint star – but a disc in
the sky WITH MOONS that orbit it
• Venus has “PHASES” like the MOON
Galileo Galilei (1564 – 1642)
Spots on the Sun
The Sun rotates
Eventually went blind
Galileo Galilei (1564 – 1642)
The Moon Has Mountains and Valleys
Galileo Galilei (1564 – 1642)
Phases of Venus
Galileo observed that Venus
showed phases entirely like those
of the moon from full to crescent,
which it must do if the Copernican
theory was correct.
According to the Ptolemaic theory
Venus would have to be a
perpetual crescent.
Observations consistent with Copernicus,
inconsistent with Ptolemy.
Galileo Galilei (1564 – 1642)
Jupiter Has Moons
Galilean Moons – 4 Largest “moons” of Jupiter
Io, Europa, Callisto, & Ganymede
Galileo Galilei (1564 – 1642)
1632: "Dialogo Dei Massimi Sistemi”
Dialogue on the Two Chief World Systems
In Italian, Not Latin -- For the common people
Two people, one representing the view of Ptolemy and other the view of Copernicus,
present their arguments before an intelligent layman.
The Pope Urban II thought that
Simplico, the character who
upheld the views of Ptolemy in the
book, was a deliberate and
insulting caricature of himself
1633: Trial by Church (threatened
with instruments of torture),
forced to recant views
"Eppur Si Muove“
"(And, yet it moves!")
1633: Galileo is ordered to his house in Arcetri, under house
arrest, where he remains for the rest of his life.
The book remained on the index of forbidden books until 1822
Pope John Paul II finally gave an address that admitted that there
had been errors in the treatment of Galileo by the church, in 1992.
ASIDE: Contrast Galileo with Nicolas Steno (1638-1686), who laid
the groundwork for the interpretation of the fossil record.
See The Seashell on the Mountaintop by Alan Cutler.
Modern Scientific Method
1. Observations and Experiments
Data which describes the physical world
2. Theory, Hypotheses, Models:
-> Organize Facts from experiment & observations
Unifying principles
Make testable predictions
Creationism and Intelligent Design vs. Evolution.
The example of gravity as a scientific theory:
Newton’s Law c. 1700 AD
Einstein’s theory of General Relativity c. 1916
Photon versus wave description of light
The most interesting scientific questions to work on for a professional scientist
are those we don't know the answer to.
Interesting theories to work on are those which fail to explain an observation or fact.
Many modern astronomers are deeply religious people.