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Models of the Universe
Throughout history we have looked at the
stars and wondered about the universe
Scientific theories change when scientists
discover new information.
Observational evidence caused the model
of the solar system to be changed from one
in which the sun and planets orbit the Earth
to one in which the Earth and planets orbit
the sun.
Prior to the 1500's it was believed that the
earth was the center of the planets. This
was known as the geocentric model.
Aristotle’s and Ptolemy’s
Universe
 The Geocentric Theory is credited to the Greek astronomers
Aristotle and Ptolemy.
Geocentric ideas
 The Earth is still, motionless, and at the center of the Universe
 Celestial bodies move in perfect circles at uniform speeds
 Stars were set in a rotating sphere that turned E to W once a day
 Planets, moon, sun also set in separate spheres that moved
slower
The Copernican Model:
A Sun-Centered Solar System
Sun Centered
 The distinction between the Solar System
and the Universe was not clear until modern
times
 Polish priest –
astronomer Nicolaus
Copernicus (1473 –
1543) eventually
decided
to reject the
geocentric model
Heliocentric
 Heliocentric – Sun centered universe
Copernicus
Galileo Galilei
Proves Copernicus’ Hypothesis
 Galileo Galilei (1564-1642) was a pivotal figure in the development
of modern astronomy. He proved the Copernican hypothesis.
 He also invented the telescope.
Galileo
 The sun is the center (heliocentric) of our
solar system
 Our solar system is a part of a larger
universe
 Using a new invention, the
telescope, Galileo was able
to view parts of our Solar
System in motion
What could Galileo see?
 New stars (Milky Way made up of stars)
 Mountains and valleys on the moon
 Four moons orbiting Jupiter (now called Galilean
moons)
 Phases of Venus
 Sunspots (rotating around the sun about once a
month)
 The rings of Saturn
 Planets are disks, not pinpoints of light like the
stars
Johannes Kepler (1571-1630)
 Johannes Kepler, a German astronomer who
lived at about the same time as Galileo.
 He showed mathematically that Copernicus'
idea of a sun-centered system worked well if
uniform circular motion was replaced with
uneven (but predictable) motion along offcenter ellipses.
Modern View
 Like any experiment, as we learn more, we
change our hypothesis about the universe
 Geocentric – Ptolemy's model Earth-Centered
Universe
 Heliocentric – Copernicus & Galileo SunCentered Solar System and larger Universe
 Big Bang Theory – the Universe started from
one big explosion
Modern thought begins to take hold
 Other scientists began to look and explain how
the universe worked
 Isaac Newton (1642-1727) proposed that
gravity is the main source that holds our
solar system together.
 The sun's gravitational pull holds the Earth and
other planets in their orbits.
 Things on or near the Earth are pulled toward it by
the Earth's gravity.
So what have we learned?
Before modern thought, the universe was seen in 2
ways
1. Geocentric: Ptolemy's Earth-centered Universe
2. Heliocentric: Copernicus’ and Galileo’s Sun Centered Universe
3. Kepler: explained the orbits of planets
4. Newton: explained gravity is the glue that hold the solar system
together
5. 20th century: newer ideas would change the way we thought about
gravity, space, and time