Download The Astronomical Revolution

The Astronomical Revolution
Astronomy in Antiquity
Established facts in ancient times:
1. spherical earth - estimate of it’s size
2. lunar phases and eclipses
3. the apparent motion of the sun on the sky
4. sophisticated calendars and time-keeping
Outstanding puzzle: the motion of the 5 planets, Mercury,
Venus, Mars, Jupiter, and Saturn against the stars
We know that the planets move on orbits centred on the sun
Heliocentric model - Sun-centred model
Early models put the earth at the centre
Geocentric Model – Earth-centred model
sun, moon and planets move on circular orbits about the
earth
Recommended reading: The Sleepwalkers, by Arthur Koestler
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Retrograde Motion
outer planets normally proceed from W to E in sky
near opposition they reverse direction and start going E to W
why? - easy to explain in a heliocentric system
apparent path on sky
Mars
Earth
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In the geocentric model retrograde motion is hard to explain:
Many models were developed over nearly a millenium:
1. Pythagoras - circular motion
2. Eudoxos and Aristotle (rotating spheres)
3. Hipparchus - the epicycle
4. Aristarchus - a heliocentric model!
Ptolemy’s Almagest - culmination of these efforts
planets move on circles about the earth
secondary motions about epicycles
explains the retrograde motion
more than 80 epicycles were built into the whole model!
very good agreement with naked eye observations
used for 1500 years - passed down to Europeans
through Arabic translations
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Mars
epicycle
Earth
deferent
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Copernicus
Established
the
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planets still on circular orbits centred on Sun
advantages over the geocentric model
more natural explanation of retrograde motion
fewer epicycles (although still needs them!)
explains changing brightness of planets over their orbits
more accurate in predictions than the Ptolemaic system
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Galileo
Pointed the newly invented telescope at the sun, moon, and
planets
discovered:
moon has a rough terrain - mountains, valleys, craters
sun has spots - discovered that the sun rotates
phases of Venus
: implies Venus is in orbit about the Sun
moons of Jupiter
: 4 bright stars which orbit Jupiter
: mini solar system
Endorsed the Copernican system
condemned by the Church and forced to recant
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Tycho
Tycho’s accurate data on Mars motion - accurate to 1
arcminute=1/60 of a degree
both Ptolemaic and Copernican models were a relatively
poor fit to the data
Tycho wanted to know why and believed more accurate
observations were needed
compiled 20 years of accurate observations at his
observatory Uraniborg
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Kepler
An astronomer-mathematician - proponent of Copernican
system
Analyzed Tycho’s data - hoped to make more accurate table
for predicting planetary motions
His great discovery was that planetary orbits are ellipses
He also discovered empirical laws which describe planetary
motions along these trajectories
Kepler’s Three Laws
1. The planets move on elliptical orbits with the sun at one
focus.
2. The motion along the ellipse is such that the line joining the
sun to the planet sweeps out equal areas in equal times.
3. The square of orbital period of a planet is proportional to the
semi-major axis of its ellipse cubed.
We can derive the dimensions of the solar system from the 3rd
law, ;=<>@?BACED"FHGJI CLK>NM OQPROQG
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The Planetary Orbits
The Inner Planets
Mercury
Venus
Earth
Mars
The Outer Planets
Jupiter
Saturn
Uranus
Neptune
Pluto
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