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Timeline of Astronomy
Adapted from Prentice Hall
Models of the Universe
Geocentric
 The moon, sun, and known planets orbit earth
o All known as the “7 wonders”--The sun, the moon, Mercury, Venus, Mars, Jupiter, and Saturn.
o Each was thought to have a circular orbit around Earth.
 Beyond the planets was a transparent, hollow sphere (called the celestial sphere) on which the stars
traveled daily around the earth).
 Ptolemy and Aristotle supported this model
Ptolemaic System
 Ptolemy presented a model of the universe that accounted for the movements of the planets (based on
the Geocentric Model).
 Very precise—able to predict the motion of the planets
 Went unchallenged for nearly 13 centuries.
 However, the motion of the planets against the background of stars seemed odd. Each planet, if watched
night after night, moves slightly eastward among the stars. Periodically, each planet appears to stop,
reverse direction for a time, and then resume an eastward motion. The apparent westward drift is called
retrograde motion and is diagrammed in Figure 4 on page 617.
 It is difficult to accurately represent retrograde motion by using the Earth-centered (Geocentric) model.
 Even though Ptolemy used the wrong model, he was able to account for the planets’ motions.
Heliocentric
 The earth, the moon, and the known planets orbit the sun
 Aristarchus (312–230 B.C.) was the first Greek to believe in a sun-centered, or heliocentric, universe
 Copernicus was the first modern scientist to suggest that this was correct and he published the model
Ancient Greeks
Aristotle (384–322 B.C.)
 Concluded that Earth is round because it always casts a curved shadow on the moon when it passes
between the sun and the moon. This theory was abandoned in the Middle Ages.
Eratosthenes (276–194 B.C.)
 The first successful attempt to establish the size of Earth 39,400kilometers—a measurement very close
to the modern circumference of 40,075 kilometers.
Hipparchus (second century B.C.)
 Probably the greatest of the early Greek astronomers was, best known for his star catalog.
 Determined the location of almost 850 stars, which he divided into six groups according to their
brightness.
 Measured the length of the year to within minutes of the modern year and developed a method for
predicting the times of lunar eclipses to within a few hours.
MODERN ASTRONOMY
Copernicus – 1473-1543 (Poland)
 Copernicus concluded that Earth is a planet. He proposed a model of the solar system with the sun
at the center.
o VERY different from the original proposal---a motionless Earth lies at the center of our universe.
o Used perfect circles to represent the orbits of the planets.
 There was a problem though! The planets seemed to stray from their predicted
positions….wonder why? SEE KEPLER!
Brahe - 1546-1601 (Denmark)
 Brahe’s observations, especially of Mars, were far more precise than any made previously.
 The telescope had not yet been invented; he used other instruments to pretty accurately measure
locations of “heavenly bodies” (as they were called)—More accurately than any other astronomer to that
date.
Kepler – 1571-1630 (Germany)
 Kept most of Brahe’s observations and put them to exceptional use.
 Discovered three laws of planetary motion
1. Planets orbit the sun (center) in an elliptical path as opposed to a circular path
2. The radius vector describes equal areas and equal times—TRANSLATION—the planets
must continuously change speed while moving in an ellipse—Closer to the sun = faster.
3. The orbital period of revolution is measured in Earth years. The planet’s distance to the sun
is expressed in astronomical units. The astronomical unit (AU) is the average distance
between Earth and the sun. It is about 150 million kilometers.
Galileo – 1564-1642 (Italy)
 Galileo’s most important contributions were his descriptions of the behavior of moving objects.
 All astronomical discoveries before his time were made without the aid of a telescope. SO Galileo
improved on the basic spyglass (the only other magnifying object) to make his own telescope.
 He made many discoveries that supported Copernicus’s view of the universe, such as the following:
o The discovery of four satellites, or moons, orbiting Jupiter.
o The discovery that the planets are circular disks, not just points of light.
o The discovery that Venus has phases just like the Moon
o The discovery that the moon’s surface was not smooth.
o The discovery that the sun had sunspots, or dark regions
Newton – 1642-1727 (England)
 Formulated and tested the law of universal gravitation
 Many scientists had attempted to explain the forces involved in planetary motion.
o Kepler believed that some force pushed the planets along in their orbits.
o Galileo reasoned that no force is required to keep an object in motion and that a moving object
will continue to move at a constant speed and in a straight line (inertia).
o Newton described a force that extends from Earth into space and holds the moon in orbit around
Earth. Although others had theorized the existence of such a force, Newton was the first to
formulate and test the law of universal gravitation.
 The law of universal gravitation also states that the greater the mass of the object, the
greater is its gravitational force.

Newton used the law of universal gravitation to redefine Kepler’s third law.
Einstein – 1879-1955 (Germany/Switzerland)

Famous for his theories of relativity:
o E = mc2 (energy (E) and mass (m) are equivalent and mass can be converted to energy
and vice versa). The conversion factor is the speed of light (c) squared, an enormous number!
 This means that a small amount of matter can be transformed into a huge amount of
energy. That's the secret of stars! (High temperatures and densities allow lighter atoms to
fuse into heavier ones. Each heavy atom weighs less than the combined weight of the
lighter atoms that formed it, and that difference in mass becomes the energy that keeps
stars shining.
o Also looked at gravity --the equations of relativity predict that gravity, or the curvature of spacetime by matter, not only stretches or shrinks distances but also will appear to slow down
time.—A bit controversial.
Hertzsprung – 1873-1967 (Denmark)
 With the help of Henry Russell, he developed the Hertzsprung- Russell diagram. This diagram plots
Luminosity as a function of Temperature for stars (Brightness vs. Temperature according to size and
age).
 The H-R Diagram is a useful way to see the changes that take place as a star evolves. Most stars are on
the Main Sequence because that is where stars spend most of their lives, burning hydrogen to helium
through nuclear reactions (E = mc2!!). As stars live out their lives, changes in their structure reflected in
changes in their temperatures, sizes and luminosities, which cause them to move in tracks on the H-R
Diagram.
