Download Lesson 4d Models of the Solar System

Do now!
We will have Wednesday
in the computer suite to
finish the presentations
and you will do the
presentations on FRIDAY
Development of models of the Solar
system/universe
You are here
Astronomical observations
 The pattern of the
stars remains the
same from night to
night (over the time of
a life-time)
 These patterns have
been labelled
constellations
Orion’s
belt
Astronomical observations
 The constellations do
not appear in the same
place
 Over the period of one
night they appear to
rotate around the pole
star
 Some stars rise above
the horizon during the
night, and some set
behind it.
Astronomical Observations
 This same movement is continued during the day.
The sun rises in the east and sets in the west.
Maximum height at midday (in the south in the
Northern hemisphere.
Place to place
 The constellation you can see also depends on
where you are
I’m on top of the world!
I can’t see the pole
star!
Constellation position
 The location of the pole star varies from
night to night as we go through the year, so
different constellations may be visible at
different times of the year.
View toward horizon from 59°54'N 11°E, azimuth 0° (N)
Planets
 Some objects that we
can see a night
behave differently!
 Their position DOES
change relative to the
background of stars.
 These objects are
called planets (derived
from the Greek word
for “wanderer”).
Planets
The planets can sometimes exhibit
retrograde motion over the course of several
nights.
The moon
 The moon’s observed motion across the night
sky is different again.
 It takes part in the nightly rotation around the
pole star, but it also wanders.
Phases
 The moon’s appearance changes over the
course of a month. We say it has phases.
The Moon's Phases
Models of the Universe
 Aristotle (384-322 BC)
Wisdom is the reward
you get for a lifetime of
listening when you'd
rather have been
talking.
Aristotle’s Universe
 Earth at centre (geocentric)
 Other objects attached to concentric perfect
crystal spheres turning at slightly different
speeds
 All “fixed” stars were attached to the final
sphere
Aristotle’s Universe
Ptolemy’s (AD 85 – 165) adaption of
Aristotle’s ideas
 To explain the retrograde motion of some
planets, he allowed each planet to move in a
small circle (an epicycle)
 The centre of the epicycle moved on a
bigger circle called a deferent.
 Earth is still at the centre (geocentric)
Ptolemy’s adaption of Aristotle’s
ideas
The Universe of Aristotle and Ptolemy
Heliocentric models
 Sun-centred (heliocentric)
 First proposed by Aristarchus (2nd century
BC), but it has come to be associated with
Copernicus (1473-1543)
 Orbits are still taken to be circular
 Explains retrograde motion of planets
Heliocentric models
The Copernican Model: A Sun-Centered Solar System
This model also
explains why
Mercury and Venus
always appear
close to the sun in
the sky
To account for
slight differences
between the
model and actual
observations,
Copernicus was
forced to add
epicycles too!
Galileo
"All truths are easy to
understand once they are
discovered; the point is to
discover them."
Galileo
 His observations with one of the first
telescopes added further support to the
Copernican view
Galileo’s observations
 There are mountains on the moon
 Venus has phases (like the moon) and
appears to change size
 Jupiter has objects orbiting it (moons)
 There are dark spots on the sun
 The sun rotates and the spots on the
surface move
Galileo in trouble
 In 1633 he was tried by the catholic Church
and forced to recant his ideas. He remained
under house arrest for the rest of his life
Kepler
 At the same time as Galileo, Johannes
Kepler (1571-1642) was adapting
Copernicus’s model
 His new model gave very good agreement
with observations
Kepler’s model
 Each planet orbits the
sun in an ellipse, with
the sun at one focus
Universal Gravitation
If I have ever made any
valuable discoveries, it
has been owing more to
patient attention, than to
any other talent.
Isaac Newton (1642 – 1727)
Universal Gravitation
 Newton came up with a physical explanation
for Kepler’s model
 Newton proposed that every object in the
universe attracts every other object with a
force that is proportional to the masses of
the objects and inversely proportional to the
distance between the objects squared
Universal Gravitation
 This law applies everywhere in the
universe
 It can explain why an apple falls to
earth, but also why the moon stays in
orbit around the earth
That’s it!