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