Astronomy 100


Lectures Table of Contents Astro 100

The Origin of Modern Astronomy

Planetary Motions and the Historical Foundations of Astronomy


  1. Classical Greek Astronomy
  2. The Copernican Revolution

Terms to Know

Retrograde Motion

1. Classical Greek Astronomy: An Earth-Centered Universe

The Data:

  1. All the planets (like the Sun and Moon) stay close the ecliptic
  2. Venus and Mercury never seem to stray far from the Sun
  3. The other naked-eye planets (Jupiter, Mars, Saturn) move mostly west-to-east among the stars, but sometimes do loop backwards (retrograde motion), and then continue in the original direction.
  4. There is no parallax (apparent regular back-and-forth motion, like the difference in view between your right eye and left eye) of celestial bodies visible to the naked eye.
Aristotle (Greek, 384-322 BC), one of the world's greatest philosophers (though not a true scientist by today's definition), proposed a model Universe with the Earth at the center (geocentric), and the Sun, Moon, planets, and stars carried around the Earth on transparent, crystalline spheres.

Ptolemy refined Aristotle's theories by adding mathematical equations and epicycles, or circle-on-a-circle, to the geocentric model. Each epicycle moved around a deferent (larger circle), and the Earth was placed not at the center of the deferent but at an equant (offset point). Even though it didn't predict the positions of the planets very well, this picture of the Universe lasted almost 2000 years!

The Ptolemeic model was favored because the sky was thought to be made of the "most perfect" shape -- the circle. What else could it possibly be made of?

2. Copernicus and the Sun-Centered Universe

Nicolaus Copernicus (1473-1543), a Polish church official, launched modern astronomy by proposing a heliocentric (sun-centered) model of the solar system. His model still assumed circular orbits, so the ability to predict planet motions was little better than in the Ptolemeic system. But epicycles were no longer necessary -- and the Earth was no longer at the center of the Universe!

Copernicus' model put Mercury and Venus' orbits inside ("inferior") the Earth's orbit, and Mars, Jupiter, and Saturn's orbits outside ("superior") the Earth's orbit, thus explaining why Mercury and Venus were never observed far from the Sun.

This is the basic picture of the solar system with which we are all familiar today.

Copernicus' ideas were revolutionary and controversial. Why?


Gravity, Orbits, and the Birth of Modern Astronomy


  1. Brahe, Kepler, Galileo, and Newton: The Birth of Modern Science
  2. Kepler's Laws of planetary orbits
  3. Newton's Laws of motion

Terms to Know

Kepler's Laws
Newton's Laws
Newton's Law of Gravity

1. Brahe, Kepler, Galileo, and Newton: The Birth of Modern Science

Tycho Brahe (Danish, 1546-1601)

  • made very careful measurements of star and planet positions
  • rejected the Copernican model because he could detect no parallax among the fixed stars (why couldn't he?), and rejected the Ptolemeic model because of its poor ability to predict planet locations.
  • proposed a new, complex model with the Sun and Moon revolving around the Earth, and the other planets revolving around the Sun

Johannes Kepler (German, 1571-1630 -- Tycho's successor)

  • Studied the data Tycho had carefully collected on planetary motions
  • Realized that the planets move on ellipses, not perfect circles
  • Supported a heliocentric (Copernican), not geocentric, view.
  • Showed that the planets change speed during their orbit (faster when close to the Sun, slower when far from the Sun)

Galileo Galilei (Italian, 1564-1642) (99 years after Copernicus' death!)

  • was first person to turn a telescope on the sky
  • was first to see moons of Jupiter; sunspots; mountains on the Moon; phases of Venus; stars in the Milky Way; rings ("ears", "handles") around Saturn.
  • proved that Earth could not possibly be the center of the Universe, thus supporting Copernican system

Isaac Newton (English, 1642-1727) (born the same year Galileo died!)

  • studied the work of Galileo, Kepler, and apple trees
  • figured out the laws of optics, gravity, and calculus
  • came up with the physics to explain Kepler's Laws
  • realized that mass and distance were both important in the Law of Gravity, and derived that fact mathematically


2. Kepler's Laws of planetary orbits

  1. Orbits of planets are ellipses with the Sun at one focus
  2. The area swept out by a planet in its orbit around the Sun is always the same in a given time interval (i.e., planets move faster when closer to the Sun)
  3. P2 = a3, where P=Orbital Period (years) and a = Distance from Sun (A.U.)

3. Newton's Laws of Motion

  1. Law of Inertia: A body remains at rest or moving in a straight line unless acted upon by some external force
  2. F = ma : A force F applied to a body with mass m will cause it to accelerate (change speed and/or direction) at rate a.
  3. Action and Reaction: Every force (or action) applied by one body on a second body results in an equal but opposite force (or action) by that second body back on the first body.
Newton's Law of Gravity:



  • Since the Moon travels in an orbit around the Earth (and thus not in a straight line), Newton's First Law requires that some force must act upon the Moon.
  • Newton's insight: This force is the same one that pulls objects to the ground on the Earth.
  • Newton determined a quantitative relationship for the gravitational force between two masses (m1 and m2) separated by a distance r12.


    • G is just a number -- the gravitational constant. If you measure mass in kilograms, separation in meters, and force in newtons(!), then G=6.67 x 10-11.
  • Any two masses attract one another gravitationally, each one -- according to the Third Law -- exerting an equal attractive force on the other.
    • The greater the masses, the greater the force.
    • The smaller the separation, the greater the force.
  • This law provides the physical explanation for Kepler's observations.

Lectures Table of Contents Astro 100

Houjun Mo Astronomy 100