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Astronomy 141
Life in the Universe
Prof. Scott Gaudi

Lecture 2: Origins of Modern Science, Astronomy, and Astrobiology

Key Ideas

Ancient Greek (i.e. Aristotelian) philosophy asserts that:
The Earth is fixed and unmoving at the center of the Universe
The laws on Earth are different than those in the Heavens
Copernicus advocates heliocentric model, and begins the "Copernican Revolution"
Kepler uses Tycho's data to refine model
Galileo shows the Earth is not the center of the solar system
Newton demonstrates that the very same laws of physics govern the Earth and Heavens.
Physical estimates of the age of Earth indicate that it is much older than our records of human civilization
Estimates of the scale of the Universe indicate it is very large
By the mid-20th century, the key pieces of knowledge are in place for the development of astrobiology
Technological advances turn philosophy into science.

Observing the Sky

Stars are pinpoints of light that appear to move from East to West.
The Sun is a bright disk ~1/2 degree across that moves East to West, but in one year moves relative 360 degrees relative to the stars (from West to East).
The Moon is a pale disk ~1/2 degree across that moves East to West, but in one month moves relative 360 degrees relative to the stars (from West to East), and has phases.

The Geocentric System Geocentric = Earth-Centered

Anaximander of Miletus (611-546 BC)
The first Greek philosopher to suggest a geocentric system:
Earth was a flat disk (cylinder) fixed and unmoving at the center.
Sun, Moon & Stars were affixed to rotating crystalline spheres centered on the Earth.
Sun, Moon & Stars were physical objects.

Aristotle (384-322 BC)

Pupil of Plato, tutor of Alexander.
His On the Heavens refined previous systems
55 crystalline spheres within spheres
Incorporated physical reasoning:
Earth fixed and unmoving at the center as it was too big to move, including rotation.
All spheres were in uniform circular motion.

The Aristotelian System makes certain basic assumptions:
The Earth is a sphere, fixed & unmoving at the center of the Universe.
Natural state of motion on Earth is rest.
The natural state of the heavens is unceasing uniform circular motion.
A rotating or revolving Earth is "unnatural".

The Aristotelian system makes a distinction between the heavens versus the Earth
Uniform circular motion
Constant motion
The Earth:
Center of the Universe
Sphere, fixed & unmoving
Natural state of motion on Earth is rest.

The "rules" of the Earth do no apply to the heavens.
A basic assumption that affects all subsequent ideas.

The problem of the planets

Planets: (Greek: planetai = wanderers)
Objects that move relative to the "fixed" stars.
Stay within a few degrees of the Ecliptic.
In general, the planets move eastward relative to the "fixed" stars.
Sometimes, however, the planets appear to
Slow down, stop, start moving westward, or RETROGRADE, stop again, and then resume moving eastward.
Very hard to understand in the simple geocentric uniform motion picture.

Ptolemaic Model

Elaborated on a system of 'epicycles,' creating a geocentric model that explained retrograde motion
Epicyclic models have a number of successes:
Reproduces the retrograde motion of planets.

The Ultimate Geocentric System
Ptolemy's final system was quite complex:
40 epicycles & deferents required.
It provided accurate predictions of the motions of the planets, Sun, and Moon.
It was to prevail virtually unchallenged for nearly 1500 years.
Was rooted and associated with fundamentally Aristotelian ideas
Essentially precludes even the notion of life elsewhere.


A revolution.

Sun at the center.
Earth rotates about its axis
Earth revolves about the Sun.
+ Explains retrograde motion naturally.
- Didn't work much better, although it was more elegant.
- No observational evidence (no parallaxes!)

Scientific Objections to Copernican Model
No observational evidence of orbital motion:
As Earth orbits around the Sun, it moves 2 AU from one side to another in 6 months.
A nearby star would appear to shift position with respect to more distant stars.
The apparent shift is the "stellar parallax"
Parallax was not observed in Copernicus' time, suggest that the Earth does not move.


Brilliant German Mathematician
Staunch Copernican
Convinced the Universe was governed by physical laws.
Obsessed with finding harmony in the heavens.
Had a genius for data analysis
Inherited Tycho's data
Mars was the key to unlocking the secrets of planetary motion.
Kepler began analyzing Tycho's data on the orbit of Mars.
Last data point did not fit by 8 arcminutes
Kepler listened to the data:
Knew Tycho's data were accurate to 1-2 arcminutes.
Kepler questioned his assumptions:
Forced to abandon uniform circular motion.
Concluded Mars' orbit was not a circle, but instead an ellipse with the Sun at one focus.


Italian contemporary of Kepler:
Gifted mathematician
Brilliant observer & experimenter
Preferred experimentation and measurement to philosophical rhetoric.
Staunch anti-Aristotelian
Often at odds with the scholarly establishment
Built a telescope and observed the sky
Sunspots,mountains on the moon, -> showed that the heavens were not perfect
phases of Venus -> showed that Venus orbits the Sun
moons of Jupiter -> showed that there were other centers of motion other than the Earth


Unified all motions into three simple laws.
Replaced older, empirical or philosophical descriptions with quantifiable, physical explanations of the nature of the World.
Explained the motion of all objects with the same set of self-consistent rules.
Developed the law of Universal Gravitation which governs all things
Demonstrated that the physical laws which govern motion are the same everywhere

The Copernican Revolution Completed

We do not occupy a special or privileged place in the Universe.
The Universe and everything in it can be understood and predicted using a set of laws ("rules").
The entire Universe obeys the same rules.

How Old is the Earth?

James Ussher (1581-1656)
Protestant Archbishop of Armagh
Classical & biblical scholar
Sought a critical chronology of human history, including the date of the Creation.
Annals of the World (1658):
Sunday, October 23, 4004 BC
First Sunday after the Autumnal Equinox in 4004 BC (Julian Calendar).

All of the estimates from Ussher and before are based upon the same central assumption:
Human history can be equated with the physical history of the Earth.
Not surprising given the vestigial Aristotelian philosophy.

After the Copernican revolution, physical estimates of the Earth's age were sought.
Example: Charles Darwin
Theory of Natural Selection
Slow changes in species over time
Takes a long time for profound changes
Concluded that the Earth probably had to be more than 500 million years old

Radioactive Dating of the Earth
Oldest surface rocks known are 4.3 Gyr old
The best estimate of the age of the Earth:
4.5 billion years
Age of the Universe
14 billion years
Civilization: less than 10,000 years

How big is the Universe?

The Parallax View
Stars are more distant than people thought
All stellar parallaxes are less than 1 arcsecond
Cannot measure parallaxes with naked eye.
First observed in 1837 by Friedrich Wilhelm Bessel for the star 61 Cygni.
Used a telescope
Measured a parallax of 0.3-arcsec
Means its distance is ~10 light years
~630,000 times the distance to the Sun!

By the early 1900s, there were two lines of thought about the "Scale of the Universe"
How big is the Milky Way?
How distant are the Spiral Nebulae?
Island Universe Hypothesis: Spiral Nebulae are much more distant than the "edge" of our Galaxy, and so very large (as big as our Galaxy).
Nebular Hypothesis: The Spiral Nebulae are nearby, thus inside our Galaxy and and thus smaller than it.

Debate was ended in 1923 by Edwin Hubble
Used the new 100 inch telescope on Mt. Wilson
Found variable stars that he used to estimate the distance to Andromeda Nebula
Found it was much further away than the size of the Milky Way, and thus was not in the Galaxy
Also found that it was the same size as the Milky Way

The Birth of Astrobiology

By the middle of the 20th Century, the key pieces of knowledge were in place
We do not occupy a special or privileged place in the Universe.
The Universe and everything in it can be understood and predicted using a set of laws ("rules").
The entire Universe obeys the same rules.
The Universe is big!
The Universe it old (but we are young)!

All that was required to turn astrobiology from philosophy to science was the development of technology....

See A Note about Graphics to learn why the graphics shown in the lectures are generally not reproduced with these notes.

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