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Chapter 3: Motion of Astronomical Bodies. Observed Motion of the Planets. At its core, astronomy is an observational science. Thus, we must start by simply observing. After that, we can try to explain why things move as they do. - PowerPoint PPT Presentation
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Chapter 3:Motion of
Astronomical Bodies
Observed Motion of the Planets
At its core, astronomy is an observational science. Thus, we must start by simply observing. After that, we can try to explain why things move as they do.
The ancient Babylonians were the
first to make
records of celestial motions
The Ancient Greeks were the first to try to Explain
Celestial MotionsSun, Moon and stars are no problem. Each rotates around Earth on a giant crystal sphere. This is known as the Geocentric model.
The Physics of Aristotle All celestial
motions must be circular
The motions of the planets were difficult to explain
The Ptolemaic (ancient Greek) ModelEarth is at the center of
everything. All celestial motions are circular (as per Aristotle). Circles on top of circles gives a reasonably close description of planetary motions.
Watch ClassAction Renaissance Astronomy module Animation Ptolemaic Orbit of Mars
Nicolaus Copernicus1453 – 1543
Moved Earth out of the center and
placed the Sun there instead. The Earth was just another
planet moving around the
Sun.
The Copernican Solar System
De Revolutionibus Orbium Coelestium was published in 1543, the year of his death.
The orbits were still circles so he still needed epicycles but they were very small. Still, there were problems, it wasn’t perfect.
Watch Museo Galileo Copernican System video
The Copernican system explained retrograde
motion in a much simpler way
Watch ClassAction Renaissance Astronomy module Animation Retrograde Motion
Tycho Brahe: The Father of Observational Astronomy
1546 – 1601
The Observatory at Uraniborg
In the late 1500’s, if you wanted to study astronomy with the best, you went to Uraniborg on the Danish island of Hven
Johannes Kepler 1571 – 1630
Tycho hired Kepler to come work for him as an assistant. Kepler couldn’t see very well so his job was to make calculations. When Tycho died Kepler took the data from Tycho’s heirs and used them to develop his Laws of Planetary Motion.
Kepler had pre-existing ideas about how the solar
system worked Kepler’s planetary spheres were nested inside the five perfect geometrical solids
Once he started his work on the data, he let the data lead him to the answer rather than trying to force it to his pre-existing idea
Kepler’s Laws of Planetary Motion are
empirical lawsThe laws are based on the observational data of Tycho Brahe. They were derived to fit the data. They do not try to explain why the planets move as they do.
Kepler’s 1st Law
The planets move in elliptical orbits with the Sun located at one focusCheck out Eccentricity Demonstrator In ClassAction Renaissance Astronomy module Animations
The Law of Ellipses
This was a major change since everyone that came before had used circles, including Copernicus
Kepler’s 2nd Law
A line drawn from a planet
to the Sun will sweep out equal areas in
equal time periods
Check out Planetary Orbit Simulator in ClassAction Renaissance Astronomy module Animations
The Law of Areas
Kepler’s 3rd Law:The Law of harmonies
The ratio of the square of the orbital period to the cube of the orbital semimajor axis (the radius) is the same for all the planets
Galileo Galilei was a contemporary of Kepler’s
Galileo preformed physics experiments and developed new Physics to replace the old physics of Aristotle
Galileo discovered the Law of Falling Bodies
Watch Apollo 15 Feather and the Hammer video
Astronomical Discoveries of Galileo
Sunspots Lunar Observations
Saturn Drawing
Phases of Venus
Watch ClassAction Renaissance Astronomy Phases of Venus animations
Galileo sent his
observations of Jupiter to
Kepler to verify Kepler’s
LawsThe ratio of the square of the orbital period to the cube of the orbital radius was the same for all four bodies orbiting Jupiter but it wasn’t the same constant as the planets orbiting the Sun
It took 50 years after Galileo and
Kepler before Isaac Newton explained why things moved as they did on Earth and in the heavens
1642 – 1727
Newton’s 1st Law of MotionThe Law of Inertia
An object in straight line uniform motion will continue that motion unchanged unless some external force acts on it
This law was based entirely on the work of Galileo
Newton’s Second Law The Force Law: F = maThe acceleration a body experiences is directly proportional to the net force acting on it and inversely proportional to its mass
changetotakesitlongHowchangesvelocitymuchHow
onAccelerati
There can be an acceleration even when there isn’t a
change in speed
Mass plays an
important part in
Newton’s 2nd Law
Newton’s Third LawThe Action-Reaction Law
For every force there is an equal and oppositereaction force
The Action-Reaction forces always apply to different
objects
While the force on each object may be the same, the acceleration (and thus damage) each experiences depends inversely on their mass according to Newton’s 2nd Law
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