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Lecture 4

emphasis of lecture 4 and 5

Course Code
Rick Halpern

of 6
Lecture 4: The Copernican Revolution
Geocentric Universe
oAncient astronomers were aware about sun, moon, mercury,
venus, mars, Jupiter and saturn
oPlanet: planets change speed, brightness and have retrograde
oInferior planets: Mercury, Venus
oSuperior planets: Mars, Jupiter and Saturn
oInferior planets have inferior conjunction (closest to earth)
and superior conjunction (farthest from the earth)
oSuperior planets go through opposition(close to earth) and
conjunction (farthest away)
oInferior planet never move too far from the sun
oSuperior planets are not tied to sun and experiences
retrograde motion
oSuperior planets are brightest at opposition
oInferior planets are brightest near inferior conjunction
Ptolemaic Model of Solar systems and Epicycles
oCenter of the epicycle orbits the Earth called a deferent. The
planet orbits the epicycle.
oThis model can correctly predict retrograde motion
oOnly the moons orbit is correct in this model
Copernican Revolution: Heliocentric Model
oSun is the center of the solar system
oRetrograde motion occurs as Earth passes Mars
Birth of Modern Astronomy
oGalileo discovered mountains on moon, sunspots, rotation of
sun, jupiters moons, venus phases
oJupiters moon was nail in the coffin to confirm not all object
revolve around earth
oPhases of venus cannot be explained by geocentric model
Keplers Laws
oDerived using observation made by Tycho Brahe
(observational astronomer)
oKeplers Laws of Planetary Motions
1.Planetary orbits are ellipses, Sun at one focus
Ellipse has two axes
oLonger one called major axes = 2a where a
is semi major axes
oShorter one called minor axes
Foci moved closer together decreases eccentricity
and when brought to the center creates a circle.
Eccentricity = c/a where 2c is the focus
Perihelion: Nearest to the star or perigee for
moon nearest to earth
Aphelion: furthest from the star or apogee for
moon farthest from earth