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Introduction to Classical
Astronomy
Mr. Ross Brown
Brooklyn School for Law and
Technology
Aim: How did early astronomers study
our universe?
• 21 March 2016
• Do now: What evidence do you have that our
Earth is round? Use “naked-eye” observations.
We knew the Earth was round
• Pythagoras, 500 BC (Before Christ, or BCE,
Before Common Era) suspected it
• Aristotle (384-322 BC) observed it
• Naked-eye observations
Aim: How did early astronomers
study our universe?
22 March 2016
Do now: Why does the Moon appear so much
larger at the horizon? Why do the Sun and
Moon appear similar in size in our sky?
Angular size
Ponzo Illusion
• The human mind judges an object’s size based
on its background.
Ebbinghaus Illusion
Angular size of Sun and Moon
Calculating the Earth’s Circumference
• Eratosthenes (276-195 BC)
• Used measurement and geometry
• Parallel lines and their transversal (a line that
passes through 2 parallel lines)
Eratosthenes’ calculations
Relative sizes of Sun and Moon and
Distances to Each
• Aristarchus
Homework
• Provide examples of observational evidence
that the Earth is round.
How did classical astronomers
explain planetary motion?
• Do now: How does the Earth’s motion through
space compare to Mercury? Or to Mars? Is the
Earth moving faster or slower?
The Planets
• Ancient Greeks noticed five bright “stars” in
the night sky that didn’t stay fixed like stars
• We named them planets from the Greek word
for “wanderers”
• Planets seemed to move independently, so
they were named after gods
Where is the center of the Solar
System?
• Geocentric: Earth is the center
• Heliocentric: Sun is the center
Retrograde Motion Worksheet
• You can work with a partner
• Please take your time and use care with this
work.
A Flaw in the Geocentric Model
• Retrograde Motion: planets beyond Earth
relative to Sun sometimes appear to move
backwards. This is due to slower paths around
Sun and being “lapped” by Earth.
Homework #5
• What was Nicolaus Copernicus’ theory about
the Solar System?
• What did Tycho Brahe design and build?
• What did Johannes Kepler learn about
planetary orbits?
Aim: How did our study of the
universe advance?
• Do now: How did retrograde motion illustrate
the likelihood of a heliocentric universe?
Nicolaus Copernicus (1473-1543)
• Used retrograde motion to show that planets
closer to the Sun overtake planets further
away, indicating heliocentric model
• Calculated distances to other planets based on
an astronomical unit (AU)
• Showed that the universe was much, much
larger than thought. People began thinking
other stars might be suns with other planets
orbiting them.
Distance from Sun to its planets
(the universe is HUGE!!!)
Tycho Brahe (1546-1601)
• Danish astronomer, and wealthy. Used his
power to design better telescopes and made
great discoveries.
Johannes Kepler (1571-1630)
• Brahe’s assistant. Used Brahe’s data to create
an accurate picture of the path of Mars.
• Not a circle, but an ellipse (a flattened circle.)
• An ellipse has long and short dimensions, its
major and minor axes.
• ½ the major axis is referred to as the
semimajor axis.
Kepler’s Three Laws
1. Planets move in elliptical orbits with the Sun
at one focus of the ellipse
2. The planet’s orbital speed varies so a line
joining the Sun and the planet will sweep
equal areas in equal time intervals
3. There is a relationship between the time of a
planet’s solar orbit and its orbit’s size, i.e.,
the square of the time to orbit (p2) is equal to
the cube of the semimajor axis (a3)
Aim: How did our understanding of
the universe continue to develop?
• Do now: What keeps the planets of our solar
system from colliding?
Galileo Galilei (1564-1642)
• First person recorded to use telescope to
study heavens
• Saw mountains and features on Moon
• Saw sunspots on the Sun
• Saw objects orbiting Jupiter (which Kepler
named ‘satellites’)
• Got in trouble with the Church during the
Inquisition. Forced to recant and held under
house arrest for rest of his life.
Galileo Galilei (1564-1642)
https://commons.wikimedia.org/w/index.php?title=File%3AApollo_15_feather_and_hammer_drop.ogg
Isaac Newton (1642-1727)
• Contributions to math, physics, astronomy
• First to truly understand importance of gravity
as a force
Homework
• If an asteroid has an average distance from
the Sun of 4 AU, what is its orbital period?