Astronomy in the Middle Ages to the Modern Era

From Copernicus to Galileo to man on the moon

Copernicus• Began the process of

disproving the geocentric theory

• A heliocentric model of the universe offers a much simpler explanation of the universe (remember Occam’s Razor)

Heliocentric Model• Explains retrograde motion• Planets will appear to change direction

every time Earth overtakes their orbit• Could now determine the distance a

planet is from the sun using geometry• There were still problems with his model

because he said the orbits of the planets were circular

• His views were slow to be accepted

Tycho and Kepler

• Copernicus opened the door to new ideas• A cultural renaissance was taking place in

Europe• The Protestant Reformation had begun• The new world was being settled• It was an environment ripe for intellectual

stimulation

Tycho Brahe• Believed that God

placed the planets in the heavens as a sign to humankind of events on Earth

• Made precise measurements with instruments he constructed

Tycho Brahe

• First to show the existence of heavenly bodies outside of our solar system

• Suggested that the heavens were more complex than we first believed

• He had Earth still as the center of the universe with the Sun orbiting around it and the other planets orbiting the Sun

Johannes Kepler• Explained Brahe’s

precise observations in mathematical terms

• Developed three laws that explained most aspects of planetary motion

Kepler’s Laws

• Law of Ellipses• Law of Equal Areas• Law of Periods

Law of Ellipses

• Each planet orbits the sun in an elliptical path

• Each ellipse has two foci – one of which is the sun

• This means the planet is not always the same distance from the sun

Law of Ellipses

• Perihelion – the point where the planet’s orbit is closest to the sun

• Aphelion – the point where the planet’s orbit is furthest from the sun

• The distance given is usually the average of the perihelion and the aphelion

Law of Equal Areas

• Describes the speed at which planets travel at different points in their orbits

• Earth’s orbit is nearly a perfect circle with the sun off center

• Earth moved faster when it was closer to the sun

Law of Periods

• Describes the relationship between the average distance of a planet from the sun and the orbital period of the planet

• Orbital period – the time it takes to make one revolution around the sun

• The cube of the average distance of a planet from the sun (a) is always proportional to the square of the period (p)

The Mathematics

K X a3 = p2 K is a mathematic constantIf we measure distance in AU and use earth-

years, K = 1 and therefore a3 = p2

a = distance in astronomical units (AU)

p = orbital period

Kepler’s Three Laws

• Describe the essential features of planetary motion around our Sun

• Were the first laws to describe the heavens correctly

• Kepler also gave us the word satellite

Galileo Galilei• Used the newly

invented telescope to study the motions of the planets (Note: Galileo did not invent the telescope)

• Proved that the heliocentric model was the correct one

Galileo Galilei• Was the first to use the telescope to study the

heavens• Observed that the moon was a ball of rock• Observed sunspots and that they changed from

day to day and deduced that the sun rotated• Discovered that Jupiter had moons• Examined the Milky Way and saw that it was

made up of millions of stars• Observed that Venus went through phases just

like the Moon

Galileo Galilei• That fact that Venus had phases left no

doubt that it orbited the Sun and put an end to the Geocentric model of the universe

• The originator of the scientific method still in use today

• Was brought up on charges of heresy against the church (1992 the church admitted it made a mistake)

Newton’s Application of Kepler’s Laws• Newton wanted to know why the planets

moved liked this• Newton said that a moving body will only

change its motion if an outside force causes it to do so

• An outside force must cause it to curve• This force he determined was gravity• The gravitational pull of the sun is what

keeps the planets in orbit around it.

Isaac Newton and the Birth of Astrophysics• Some consider him to be the greatest

scientist of all time• Studying the moon he came up with the

laws of gravity• Invented Calculus• His laws of gravity and motion were totally

accepted for 200 years

The Growth of Astrophysics

• New Planets were discovered• Stars had companions that orbit with them• Irregularities in the orbit of Uranus led to

the discovery of Neptune• Bigger telescopes – Telescopes in space• Applying photography• Space Exploration