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: Ptolemy c. A.D. 100-170 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 Surprisingly, not new. 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 Was funded by the king of Denmark and then the German emperor. Died without knowing whether he had made any breakthroughs.

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…) 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