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BROCK UNIVERSITY
ASTR 1P01
Introduction to Astronomy I
Summary Notes for Unit 3, The Story of Astronomy
Dr. S. D’Agostino
One of the main goals of this course is not only to describe some of the fascinating facts
about our universe, but to understand how we came to know these things. This chapter of
our course presents an overview of the history of astronomical discoveries up to the time of
Newton.
Some histories of astronomy unfairly treat the ancients as if they were backward compared
to modern thinkers. In fact ancient astronomers were quite ingenious, and made amazing
advances in our understanding of the heavens with careful observations and clever reasoning.
Their advances in understanding the cosmos are truly remarkable.
The ancients were so good at explaining the heavens that progress in our understanding
was subsequently very slow for the next 2000 years! Aristotle, who wrote influential works
on what was then called “natural philosophy” (study of the natural world), was taken as
an authority for many generations of later scholars. However, there were glimmerings of
independent thought, which grew stronger by the 1500s, culminating with the work of the
great post-Renaissance thinkers Copernicus, Brahe, Kepler, Galileo, and Newton. These
great thinkers utterly changed how humans viewed the universe and our place in it, and
knowing a little bit about their story and how their arguments carried the day provides a
valuable perspective on human intellectual history. Galileo and Newton, in particular, were
instrumental in creating modern science, so their amazing work goes well beyond astronomy.
The work of ancient astronomers is remarkable, and the work of the post-Renaissance
thinkers is no less amazing. Together their story is an exciting and awe-inspiring tale of adventurous thought, pioneering technological developments, strident arguments and counterarguments, and courageous searches for the truth in the face of life-and-death attacks by the
orthodoxy.
Textbook Reading: Chapter 2, The Rise of Astronomy, Pages 38–71.
Learning Objectives:
• explain historical evidence from basic observations that the Earth is round
• understand Eratosthenes’s calculation of the radius of the Earth
• understand that the relative distances and sizes of the Moon and Sun can be estimated
from basic observations
• explain why ancient astronomers believed the Earth is at the centre of the universe;
describe the ancient understanding of the universe, including what planets are and the
motions of planets
• explain Copernicus’s arguments that the Earth is a planet orbiting the Sun, and explain
how his reasoning accounts for the retrograde motion of planets
• describe the characteristics of planetary orbits discovered by Kepler as given by his
three laws
• calculate the period of a planet’s orbit given its radius, or calculate its radius given its
period
• describe Galileo’s telescopic observations, and discuss why these were so upsetting to
Renaissance beliefs about the nature of the universe
• describe the general trends in the development of astrophysics in the centuries after
Kepler and Galileo
• read and understand the essay on “Backyard Astronomy”
Questions to Guide Your Reading and Review:
• How did ancient Greek astronomers know that the Earth is round? Describe as many
arguments as you can.
• Describe in detail the reasoning and calculations that Eratosthenes used to determine
the size of the Earth. How accurate was his calculation?
• How did Aristarchus determine the relative sizes of the Moon and Sun? How accurate
were his results?
• How did Aristarchus determine the relative distances of the Moon and the Sun? How
accurate were his results?
• What is the angular size of an astronomical object? How is it measured? What is its
unit of measure?
• How did Aristarchus determine the relative sizes of the Earth and Moon? How accurate
were his results?
• What led Aristarchus to propose that the Sun is at the centre of the universe, not the
Earth?
• What is stellar parallax? When was it first observed?
• What were some counterarguments to Aristarchus’s proposal that the Sun is at the
centre of the universe?
• What is the diameter-distance relation for astronomical objects? How can you use it
to determine the diameter of an astronomical object given its distance, or vice versa?
• What are planets? How did the ancients recognize that planets are different from
stars?
• How do the planets move? How do they appear to move as observed from Earth?
• What is prograde motion? What is retrograde motion?
• What role did retrograde motion have in the acceptance and rejection of various models
of the Solar System? How did retrograde motion conflict with ancient ideas about the
celestial sphere?
• What is an epicycle? How did Ptolemy’s model of the solar system keep the geocentric
hypothesis alive?
• What were some of the contributions of Islamic and Asian astronomers to our understanding of astronomy?
• How did Copernicus’s work support his heliocentric model of the solar system?
• What were the major criticisms of Copernicus’s model of the solar system?
• What were some of Tycho Brahe’s contributions to our understanding of astronomy?
• What were some of Kepler’s contributions to our understanding of astronomy?
• Describe Kepler’s three laws of planetary motion?
• What were some of Galileo’s contributions to our understanding of astronomy?
• Which of Galileo’s observations supported the heliocentric model of the solar system?
• What were some of Newton’s contributions to our understanding of astronomy?
• Compare and contrast astronomy and astrology.