Summary
▶ Archaeoastronomy (p. 50) is the study of the astronomy of ancient
peoples. Many cultures around the world observed the sky and marked
important alignments. Structures such as Stonehenge, Newgrange, and
the Sun Dagger have astronomical alignments. In most cases, ancient
cultures, having no written language, left no detailed records of their
astronomical beliefs.
▶ Greek astronomy, derived in part from Babylon and Egypt, is better
known because written documents have survived.
▶ Classical philosophers accepted as a fi rst principle that Earth was
the unmoving center of the universe. Another fi rst principle was that
the heavens were perfect, so philosophers such as Plato argued that,
because the sphere was the most perfect geometrical form, the heav-
ens must be made up of spheres in uniform rotation. This led to the
belief in uniform circular motion (p. 53).
▶ Many astronomers argued that Earth could not be moving because they
could see no parallax (p. 56) in the positions of the stars.
▶ Aristotle’s estimate for the size of Earth was only about one-third
of its true size. Eratosthenes used the well at Syene to measure the
diameter of Earth and got an accurate estimate.
▶ The geocentric universe (p. 54) became part of the teachings of the
great philosopher Aristotle, who argued that the sun, moon, and stars
were carried around Earth on rotating crystalline spheres.
▶ Hipparchus, who lived about two centuries after Aristotle, devised
a model in which the sun, moon, and planets revolved in circles
called eccentrics (p. 55) with Earth near but not precisely at their
centers.
▶ Retrograde motion (p. 56), the occasional westward (backward)
motion of the planets, was diffi cult for astronomers to explain.
▶ About AD 140, Aritotle’s model was given mathematical form in
Claudius Ptolemy’s book Almagest. Ptolemy preserved the principles
of geocentrism and uniform circular motion, but he added epicycles
(p. 57), deferents (p. 57), and equants (p. 57). Ptolemy’s epicycles
could approximate retrograde motion, but the Ptolemaic model was
not very accurate, and it had to be revised a number of times as
centuries passed.
▶ Copernicus devised a heliocentric universe (p. 58). He preserved the
principle of uniform circular motion, but he argued that Earth rotates
on its axis and revolves around the sun once a year. His theory was
controversial because it contradicted Church teaching. He published his
theory in his book De Revolutionibus in 1543, the same year he died.
▶ A hypothesis (p. 66) is a specifi c statement about nature that
needs further testing, but a theory (p. 66) is usually a general
description of some aspect of nature that has been tested. Some
theories are very well understood and widely accepted. A natural
law (p. 66) is a fundamental principle in which scientists have
great confi dence.
▶ (^) Because Copernicus kept uniform circular motion as part of his theory,
his model did not predict the motions of the plants well, but it did
offer a simple explanation of retrograde motion without using big
epicycles.
▶ (^) One reason the Copernican model won converts was that it was more
elegant. Venus and Mercury were treated the same as all the other
planets, and the velocity of each planet was related to its distance
from the sun.
▶ (^) The shift from the geocentric paradigm (p. 61) to the heliocentric
paradigm is an example of a scientifi c revolution.
▶ Although Tycho Brahe developed his own model in which the sun and
moon circled Earth and the planets circled the sun, his great contribu-
tion was to compile detailed observations of the positions of the sun,
moon, and planets over a period of 20 years, observations that were
later used by Kepler.
▶ Kepler inherited Tycho’s books of observations in 1601 and used them
to discover three laws of planetary motion. He found that the planets
follow ellipses (p. 64) with the sun at one focus, that they move
faster when near the sun, and that a planet’s orbital period squared is
proportional to the semimajor axis, a (p. 64), of its orbit cubed.
▶ The eccentricity, e (p. 64), of an orbit is a measure of its departure
from a perfect circle. A circle is an ellipse with an eccentricity of zero.
▶ Kepler’s fi nal book, The Rudolphine Tables (1627), combined heliocen-
trism with elliptical orbits and predicted the positions of the planets
well.
▶ Galileo used the newly invented telescope to observe the heavens, and
he recognized the signifi cance of what he saw there. His discoveries
of the phases of Venus, the satellites of Jupiter now known as the
Galilean moons (p. 68), the mountains of Earth’s moon, and other
phenomena helped undermine the Ptolemaic universe.
▶ Galileo based his analysis on observational evidence. In 1633, he was
condemned by the Inquisition for disobeying instructions not to hold,
teach, or defend Copernicanism.
▶ Historians of science view Galileo’s trial as a confl ict between two
ways of knowing about nature, reasoning from fi rst principles and
depending on evidence.
▶ (^) The 99 years from the death of Copernicus to the death of Galileo
marked the birth of modern science. From that time on, science
depended on evidence to test theories and relied on the mathematical
analytic methods fi rst demonstrated by Kepler.
Review Questions
- What evidence is there that early human cultures observed
astronomical phenomena? - Why did Plato propose that all heavenly motion was uniform and
circular? - In Ptolemy’s model, how do the epicycles of Mercury and Venus differ
from those of Mars, Jupiter, and Saturn? - Why did Copernicus have to keep small epicycles in his model?
- When Tycho observed the new star of 1572, he could detect no
parallax. Why did that result undermine belief in the Ptolemaic
system? - Does Tycho’s model of the universe explain the phases of Venus that
Galileo observed? Why or why not? - How do the fi rst two of Kepler’s three laws overthrow one of the basic
beliefs of classical astronomy? - How did The Alfonsine Tables, The Prutenic Tables, and The Rudolphine
Tables differ? - Explain how each of Galileo’s telescopic discoveries contradicted the
Ptolemaic theory. - Galileo was condemned, but Kepler, also a Copernican, was not. Why
not? - How Do We Know? What is a paradigm, and how it is related to a
scientifi c revolution? - How Do We Know? How would you describe the difference between
a hypthesis, a theory, and a law?