THE COPERNICAN REVOLUTION

THE MIDDLE AGES, AND ASTRONOMY IN NON-WESTERN CULTURES

Learning in Europe went into a massive decline from the fall of the Roman Empire (476 AD) until about 1000 AD. The only Greek astronomy available in this period was that of Plato.

ST. AUGUSTINE (354 - 430) says philosophy (including science) must be a handmaiden to religion.

The ARABS saved much of the knowledge of the Greeks and translated it into Arabic. The Arabic astronomers where strongly influenced by Aristotle and Ptolemy. From universities in Moorish Spain, this knowledge was eventually retransmitted to the West, starting with GERBERT (Pope Sylvester II; 940 - 1003).

• built observatories
• made detailed observations
• continued to refine methods of predicting the positions of the planets
• refined the astronomical instrument called the astrolabe

Arabic science declined following 1200 with the conquest of Spain and Mongol invasions from the east.

CHINA, INDIA and JAPAN had astronomical traditions of their own. Like the Arabs, they concentrated on recording observations and refining methods of prediction. There may have been some influence from Babylonian and Greek astronomy. Hindu numerals were introduced to the West by the Arabs.

The MAYANS of Central America used similar techniques and were particularly good at following the motions of Venus, which apparently had great religious significance for them.

THE RENAISSANCE

The Renaissance was another period of great ferment in all the arts and sciences.

NICOLAUS COPERNICUS (Polish; 1473 - 1543) rediscovers heliocentric model, but publishes posthumously. Copernicus's model still retains circular motion and epicycles. Preface (not written by Copernicus) suggests that model is not physical, but only convenient tool for calculation

GIORDANO BRUNO (Italian; 1548 - 1600) proposes that the Sun is just one star out of an infinite number. Burnt at the stake in 1600

HANS LIPPERSHEY (Dutch; 1587 - 1619) inventes telescope 1608

GALILEO GALILEI (Italian; 1564 - 1642)

Starting in 1609 uses telescope to make astronomical observations that contradict Aristotle

• observed mountains on the moon: suggests Earth not unique
• sunspots: shows celestial bodies are imperfect and not unchanging
• 4 large moons of Jupiter (Galilean satellites): shows that not all bodies orbit Earth
• phases of Venus (and correlation of apparent size and phase) show that Venus orbits the Sun

Also observes:

• rings of Saturn
• Milky Way made of stars

1616 Galileo ordered to treat Copernicanism as hypothetical
1633 Galileo condemned to permanent house arrest (despite papers giving permission to publish hypothetically)
1992 Vatican acknowledges error after 359 years

Galileo argued that the Church should not make scientific positions as articles of faith nor should biblical arguments be applied to science

TYCHO BRAHE (Danish; 1546 - 1601)

• collects detailed and accurate (2' accuracy) observations of planetary positions
• shows comets and novas are extralunar contra Aristotle; comets pass through planetary spheres

JOHANNES KEPLER (German; 1571 - 1630) reduces Tycho's data using triangulation

1. orbits of planets are ellipses with the Sun is one focus  
2. sweep out equal areas in equal times
3. period² a³

animated illustration of Kepler's 1st and 2nd laws
animated illustration of Kepler's 3rd law--use your browser's BACK button to return

NEWTON EXPLAINS IT ALL FOR YOU.

ISAAC NEWTON (English; 1642 - 1727) invents calculus and physics while on vacation from college!

The Three Laws of Motion

1. inertia: an object in motion will tend to remain in motion unless a force acts on it (contra Aristotle)
2. F=ma
3. For every action, there is an equal and opposite reaction

The Universal Law of Gravitation. F=GMm/r²

THE MOON IS FALLING

The motion of the Moon is governed by the same laws as gravity at the Earth's surface. This is the first time in history that it is shown that the laws of Nature are the same on Earth as they are in the heavens.

APPLICATIONS OF NEWTON'S LAW OF GRAVITATION

1. If we know the distance between two bodies and the gravitational acceleration of one of the bodies, we can compute the mass of the other.
2. mass of the Earth can be deduced from the weight of objects near the surface
3. the mass of the Sun can be deduced from the orbital velocity of the planets
4. Orbital mechanics. A body in an elliptical orbit cannot escape the mass it's orbiting unless something increases its velocity to a certain value called the escape velocity. Escape velocity from Earth's surface is 25000 MPH (7 mi/sec)

OBJECTIONS TO THE HELIOCENTRIC MODEL ANSWERED

Advantage of the heliocentric theory:

simpler (Occam's razor)

Objections to the heliocentric theory and the motion of the Earth

why don't we get thrown off? (Earth's rotation isn't fast enough)
why don't we feel the wind of our motion? (the air moves along with the Earth)
why don't we observe parallax? (Aristarchus: it's there, but very small, because the stars are so far away)
why do dropped objects appear to fall straight down if the Earth is moving? (Galileo: dropped objects start with the velocity of the Earth)

Problems of both heliocentric and geocentric models:

lack of explanation (provided by Newton)
lack of direct evidence--see below

PROOF OF THE HELIOCENTRIC MODEL

Actual proof that earth rotates

• Coriolis force (hurricanes are ccw in N hemisphere)
• Foucault pendulum

Actual proof that earth revolves around the sun

• aberration of starlight observed 1729
• parallax observed 1838

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