Astronomy 294
Life in the Universe
Prof. Scott Gaudi

Lecture 2: Modern Science and Astronomy

Key Ideas


: Simple observations suggest a geocentric model.; Planets pose the problem of retrograde motion.; -- Ptolemaic model solves this, but is complicated.; Copernicus advocates a heliocentric model; -- Beauty and simplicity over philosophy.; Tycho and Kepler use data to refine model.; Galileo conducts observations that confirm model.; Newton proposes a physical, rather than empirical model.; Relevance to Astrobiology:; -- Development of modern science.; -- Laws of physics are the same elsewhere as here on Earth.; -- Earth does not occupy a unique place in the Universe.

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 Sun 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.

Other observations

Earth appears stationary.

Stars don't move.

Philosophy

Aristotelian world view.

--Earth stuff different than sky stuff.

--Heavenly perfection.

These observations suggest Earth is the center of the Universe!

The problem of the planets

Mostly move Eastward relative to the fixed stars.

Sometimes, however:

Slow down, stop, reverse direction, slow down again, stop, and finally, resume moving Eastward.

Very hard to understand in the simple geocentric uniform motion picture.

Ptolemaic Model

Elaborate system of epicycles.

Very complicated, although predicted the motion of the planets.

Copernicus

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!)

Kepler

Strongly influenced by data (Tycho's)

Questioned his assumptions.

Suggested elliptical orbits.

Modern science in action

Galileo

Built telescope to make observations.

Sunspots, moon craters --> Heavens not perfect

Moons of Jupiter --> Not everything orbits the Earth

Phases of Venus --> Must orbit the Sun

Newton

Formulated physical laws in place of empirical laws.

Hallmarks: quantifiable, physical, unified, and simple

Relevance to Astrobiology

Development of Modern Science

- Attempt to understand underlying reasons for observed phenomenon.

- Hypothesis construction, comparison with data, rejection of hypotheses that don't match data or prejudices.

- Emphasizes the predictive power of science.

Copernican Principles

- The laws of physics are the same elsewhere as the are here on Earth.

- The Earth does not occupy a unique place in the Universe.

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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