Download Renaissance Astronomy - Faculty Web Sites at the University of

1
Renaissance Astronomy
2
Prior to the Renaissance
The lack of observable stellar parallax combined with the
sensation that the Earth was stationary led to an Earth-centered
view of the Universe.
3
Prior to the Renaissance
The Greek astronomer Ptolemy (85-165 A.D.) successfully
created a model that explained the complex observed planetary
motion in the context of the dogmatic restrictions of the time.

All motions were perfect circles.

The rate of motion was constant and unchanging.
Explain this????
4
Prior to the Renaissance
Ptolemy solved the problem of retrograde motion by postulating
spheres embedded within spheres (more than 40) all turning at
uniform speed.
5
Prior to the Renaissance
Ptolemy solved the problem of retrograde motion by postulating
spheres embedded within spheres (more than 40) all turning at
uniform speed.
6
Prior to the Renaissance
Alfonso X, King of Castille in the 13th century noted, “If the Lord
Almighty had consulted me before embarking on the creation, I
should have recommended something simpler.”
7
Prior to the Renaissance
This statement captures the essence of “Occam's Razor” - the
simplest explanation for a phenomenon is usually right.
8
A Simpler Suggestion: The Heliocentric Model
Interestingly, 300 years before Ptolemy the Greek astronomer
Aristarchus was suggesting a heliocentric model.


Using the fact that the quarter Moon doesn't happen when the Moon
and Sun are at a right angle – he estimated the relative distances
and thus sizes of the Sun and Moon.
Knowing the Sun was much much bigger than the Moon and Earth
he reasoned, correctly, that it was the center of the Solar System.
9
A Simpler Suggestion: The Heliocentric Model
Interestingly, 300 years before Ptolemy the Greek astronomer
Aristarchus was suggesting a heliocentric model.

The Greeks of this era even had a good sense of the distances and
true sizes of the various bodies via the measurement of the size of
the Earth by Eratosthenes.
10
Copernicus
It wasn't until more than a thousand years later that the
Heliocentric model was revived.

Copernicus asserted that things would be much simpler with the Sun
in the center.
11
Copernicus
Specifically, the Copernican model made the explanation of
retrograde motion simple and obvious.
http://www.astro.illinois.edu/projects/data/Retrograde/
http://www.astro.ubc.ca/~scharein/a310/SolSysEx/retro/Retrograde.html
12
Validating the Copernican Model
Copernicus was no better than Ptolemy in asserting that his view
was right. Observational evidence was required.


Tycho Brahe was THE observer of the pre-telescopic era.
Using simple tools he recorded precise planetary and stellar
positions over more than 20 years.
13
Validating the Copernican Model
Copernicus was no better than Ptolemy in asserting that his view
was right. Observational evidence was required.


Tycho Brahe was THE observer of the pre-telescopic era.
Using simple tools he recorded precise planetary and stellar
positions over more than 20 years.
14
Tycho and Kepler
Ironically, Tycho Brahe was an adherent to the Earth-centered
universe. His assistant, mathematical genius, and mystic
Johannes Kepler, eagerly waited for the opportunity to analyze
Tycho's data (which Tycho kept close and secret).
Kepler discovered remarkable properties of
planetary orbits:
1. All planetary orbits were ellipses with the
Sun at one focus.
2. Planets moved faster when closer to the Sun
in a way that a line between the Sun and planet
swept out equal area in equal time.
3. The orbital period of a planet was related to
its average distance from the Sun. P2=a3
15
Kepler's First Law – Elliptical Orbits
An astounding discovery – Planets
followed detailed mathematical
relationships.



All planetary orbits were ellipses with
the Sun at one focus.
A circle is a special case of an ellipse
where the foci are on top of each
other.
The more separated the foci the
more eccentric the ellipse.
16
Kepler's Second Law – Equal Areas
Another quantitative relationship....


Planets move faster when they are closer to the Sun in such a way
that a line between the planet and the Sun sweeps out an equal
area in the same time interval.
The Law of Equal Areas
17
Kepler's Third “Harmonic” Law
Relates


A planet's average distance from the sun - “a”

specifically ½ of the long axis of the elliptical orbit

for a circular orbit this “semi-major axis” is just the radius of the circle
The time it takes the planet to orbit the Sun – the orbital period, “P”
P2=a3

This equation works as written if P, the orbital period, is expressed in
years, and a, the semi-major axis, is expressed in astronomical
units.

The Earth's orbital period is 1 year and it is 1 A.U. from the Sun.

12=13 .... it works.
18
An Example Using Kepler's Third Law
Consider an asteroid discovered in a circular orbit 4 A.U. from the
Sun. How long does it take this asteroid to orbit the Sun?
– – a = 4 A.U.
P2 = a3
?2 = 43 = 4 x 4 x 4 = 64
82 = 64

This asteroid takes 8 years to complete an orbit around the Sun.
Kepler's Laws in Motion
19
Galileo
Began a longstanding tradition of quick application of new
technology to enable astounding astronomical discoveries.

He put the telescope to astronomical use within a couple of years of
its invention – just over 400 years ago
20
Galileo
Began a longstanding tradition of quick application of new
technology to enable astounding astronomical discoveries.

These observations were the first to provide strong direct
observational support for the Copernican heliocentric mode.
21
Galileo: Craters on the Moon, Spots on the Sun
The Heavens weren't perfect, in fact they could be downright
terrestrial.
Sunspot movie
22
Galileo: Phases of Venus
Venus showed phases that changed over time and that were
consistent with the Copernican model.
23
Galileo: Jupiter's Moons
Galileo discovered four satellites in orbit around Jupiter, and
obeying Kepler's Harmonic Law.
I should disclose and publish to the world the
occasion of discovering and observing four
Planets, never seen from the beginning of the
world.... I noticed three little stars...near the
planet...arranged exactly in a straight line... When
I turned again to look [a few nights later]... I found
a very different state of things.
I therefore concluded... that there are three stars
in the heavens moving about Jupiter, as Venus
and Mercury around the Sun.
24
Galileo: Jupiter's Moons
Any object could serve as the center of motion – so why not the
Sun. Jupiter was an example of the Solar System in miniature.
25
Galileo and Neptune
Another notebook page shows an object that moved with respect
to the background stars over a few nights.

Galileo made a note of it but did not follow up.

It was Neptune...
26
Galileo and Supernovae
Galileo, Tycho, and Kepler all observed supernovae and
recognized that, because they didn't show parallax, these objects
were part of a changing celestial sphere.
27
Galileo and the Church
Galileo died a prisoner, having been under house arrest imposed
by the Inquisition for a decade.
Although characterized by many as a conflict between science
and religion, the details are much more complex.



Many in the Church accepted Galileo's conclusions and even
confirmed the observations for themselves.
Many refused to look through a telescope and were compelled to
enforce the longstanding dogma of a stationary Earth.
Galileo had both friends and enemies. He was a stubborn, irascible,
and outspoken character who was successful and popular. His
enemies were thus well-motivated to plot his demise.
28
I have been told by a friend of mine, a priest who is very fond of
you, that a gang of ill-disposed men, who are envious of your
virtue and merits, met at the residence of the Archbishop of
Florence, and put their heads together in a mad quest for some
means by which they could damage you, either with regard to
the motion of the Earth or otherwise. One of them asked a
preacher to state from the pulpit that you were asserting
outlandish things. The priest, seeing the animosity against you,
replied as a good Christian and a member of a religious order
ought to do. I write this that your eyes may be open to the envy
and malice of these evildoers.
In 1992, Pope John Paul II expressed formal regret for the
handling of the Galileo affair and acknowledged that the
church had made errors.
29
Galileo's Trial and Confinement
Galileo's tomb at Santa Croce