Our Planet and Solar System Ancient and PreModern

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 1 st law before Newton was even born (in 1642…) • Firm believer in scientific inquiry – Remember our pendulum experiment?

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