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

Phases of the Moon• Key ideas– ½ the Moon is always lit.– The Moon reflects the Sun’s light.– We aren’t seeing different sides of the Moon,

what is changing is what is lit.• There is no one “dark side of the Moon.”

– Period of about 27 days.– Phases are due to Moons orbit around the Earth– Moon rises and sets due to the rotation of the

Earth.

Eclipses• Two types– Lunar eclipse• Earth is directly between the

Sun and Moon– Earth’s shadow falls on the

Moon

• Can only be seen at night

– Solar eclipse• Moon is directly between Sun

and Earth– Moon’s shadow falls on the

Earth

• Can only be seen during the day

Seasons

Our Solar System

• What was evidence for Geocentrism?

• What was evidence for Heliocentrism?

Comets and Minor Planets

Extra-solar Planets