Our Planet and Solar System

Ancient and Pre-Modern Theories of the Universe/Solar System

• Aristotle’s theory of four elements• Astronomy and Astrology exist• But Heavens are considered to be a separate

realm from earthly objects

Geocentric vs. Heliocentric Models

• Geocentric Model– Places Earth at the center of the universe– Objects in Heavens (planets, Sun, stars) are

fixed crystalline “spheres,” nested around Earth– Big achievement: it explained the motion of the

Sun, moon, planets, and stars.• Established c. 500 B.C.

– Based on:• Observations• Belief that the heavens must be geometrically

“perfect.”

– Ptolemy c. A.D. 100-170• Combined others’ work to create a quantitative

model• Used circles within circles• Was fairly accurate for the time

Problems with Geocentrism

• Circles within circles inelegant (or imperfect)

• Didn’t explain phases of the moon• These problems not considered big enough

to push alternative theory

Nicholas Copernicus (1473-1543)

• Used a heliocentric model of the solar system• Proposed by a Greek

astronomer Aristarchus in 260 B.C.

• Copernicus applied math to the data (which wasn’t all that good)

• Even though more correct, still assumed perfect circles

• predictions weren’t much better than the geocentric model.

Tycho Brahe (1546-1601)• Took really good data

and tons of it.

Tycho Brahe (1546-1601)

• Was funded by the king of Denmark and then the

German emperor.• Had an island observing station• Died without knowing whether

he had made any breakthroughs.

• Brahe was convinced other planets orbited the Sun, but concluded (b/c he couldn’t detect stellar parallax) that Earth must remain stationary.

Johannes Kepler (1571-1630)

• Worked for Tycho• Approached the data

mathematically, assuming circular orbits.

• After 8 years, found a mathematical model for a circular orbit of Mars that almost matched Tycho’s observations. There was a difference of 8 minutes of arc.

His “aha” moment:

“If I had believed that we could ignore these eight minutes [of arc], I would have patched up my hypothesis accordingly. But, since it was not permissible to ignore, those eight minutes pointed the road to a complete reformation in astronomy.”• This decision to trust the data over his

preconceived beliefs marked an important transition point in the history of science.

• Kepler discovered that the planets did not orbit in circles, but rather ellipses.

• Even through Kepler’s model made much better predictions, many scientists still believed in the geocentric model.– Earth couldn’t be moving, otherwise birds and

thrown rocks would be left behind.– Non-circular orbits meant heavens weren’t

perfect

Galileo Galilei (1564-1642)• Demonstrated Newton’s 1st law

before Newton was even born (in 1642…)

• Firm believer in scientific inquiry– Remember our pendulum experiment?

Galileo Galilei (1564-1642)

• Demonstrated Newton’s 1st law before Newton was even born (in 1642…)

• Saw sunspots on the sun – so now the Sun isn’t perfect

• Saw topography (mountains) on the moon– So the Moon isn’t perfect either

• Discovered that Jupiter has moons that orbit it– Clearly don’t revolve around the Earth

• Aristotle’s doctrines considered part of Catholic Church doctrine.

• Galileo persecuted by church for publishing his results late in life

The imperfections just pile up. . .

Phases of the Moon

Retrograde Motion – Planets switch directions in their paths in the sky

Seasons

Our Solar System

• What was evidence for Geocentrism?

• What was evidence for Heliocentrism?

Comets and Minor Planets

Extra-solar Planets