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
Our Planet and Solar System: Two Competing Theories
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 – He was a Roman citizen that lived in Egypt and wrote in Greek. – Combined others’ work and created a quantitative model – Model was fairly accurate in making predictions (within ~5-10 degrees of arc)
Did you record how to estimate angular sizes in your notes?
However, some planets exhibited a really strange thing. Sometimes, they turned around a went backwards…
Ptolemy used circles within circles to account for this retrograde motion: when planets appear to go backwards.
Problems with the Geocentric Model Circles within circles inelegant (or imperfect) Didn’t explain phases of the moon Predictions weren’t perfect Problems were not considered big enough to switch to an 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.
Nicholas Copernicus (1473-1543) Copernicus applied math to the data (the data 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 was convinced other planets orbited the Sun, but concluded (b/c he couldn’t detect stellar parallax) that Earth must remain stationary.
Parallax: the apparent movement of an object due to your point of view moving, rather than the object itself moving.
Tycho Brahe (1546-1601) 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; assumed circular orbits based on the idea the heavens are perfect.
Johannes Kepler (1571-1630) 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 arcminutes.
Johannes Kepler (1571-1630) A minute of arc is 1/60 of a degree of arc. This is like the thickness of your fingernail when held at arm’s length…. Remember, Ptolemy’s geocentric was off by ~5-10° of arc
How Science Works Guess → Compare to Experiment/ Experience Compute Consequences →
Johannes Kepler (1571-1630) How do they determine if this is an error in the model or in the data? How do you decide if an error is significant?
Johannes Kepler (1571-1630) How would they “know” if this is correct??? – Remember, no one at this point in time knows the answer.
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.” - Johannes Kepler Kepler’s decision to trust the data over his preconceived beliefs marked an important transition point in the history of science.
Johannes Kepler (1571-1630) Even though model was really close, he wasn’t willing to accept it. Tried ellipses instead of circles and the new model made better predictions.
Many scientists continued with the geocentric model for the following reasons:
– Earth couldn’t be moving, otherwise birds and thrown rocks would be left behind.
Galileo Galilei (1564-1642) Aristotle and Ptolemy’s doctrines considered part of Catholic Church doctrine, so not open to question. Galileo persecuted by church for publishing his results late in life