Learning Objectives

• Compare the geocentric and heliocentric models of our solar system.

• Describe the key discoveries of astronomers like Copernicus, Galileo, and Newton.

• Explain how gravity and inertia work to keep the planets in orbit.

• Understand how scientific knowledge builds upon earlier discoveries over time.

Key Vocabulary

Geocentric Model

This is a model of the universe in which the Earth is at the center.

Heliocentric Model

This is a model of the solar system in which the Sun is at the center.

Inertia

It is the tendency of a moving object to continue moving in a straight line.

Gravity

It is the attractive force that exists between any two objects that have mass.

Ellipse

It is an oval shape which is the true path of a planet's orbit around the sun.

Ptolemy and the Geocentric Model

• In A.D. 140, Ptolemy said Earth was the unmoving center of the universe.

• This geocentric model seemed correct and was believed for over 1,500 years.

• He argued that gravity pulled all objects toward a stationary Earth.

• However, the model could not explain the observed motions of the planets.

Copernicus and the Heliocentric Revolution

• Nicolaus Copernicus was a Polish astronomer who challenged the geocentric model.

• He proposed the heliocentric model, placing the Sun at the center.

• Earth's rotation causes day and night; its revolution around the Sun causes a year.

• He delayed publishing his work because he feared it would be controversial.

Refining the Model: Brahe, Kepler, and Galileo

• Tycho Brahe collected precise planetary data, suggesting orbits were not perfect circles.

• Johannes Kepler used this data, finding planets move in elliptical (oval) orbits.

• Kepler also found that planets move faster when they are closer to the sun.

• Galileo saw Venus had phases, which supported the Sun-centered model.

Explaining Planetary Orbits

Inertia

• According to Sir Isaac Newton, the two forces balanced to keep planets in orbit are the gravitational force pulling the planet toward the Sun and the planet's inertia.

• Inertia is the tendency of a moving object to keep moving in a straight line.

• A planet's inertia constantly directs it to travel forward and away from the Sun.

• Without any other force, a planet would simply fly off into the vastness of space.

Gravity

• Gravity is the powerful attractive force that exists between any two objects with mass.

• The Sun has immense gravity that constantly pulls all the planets inward toward it.

• This inward pull of gravity balances a planet's inertia, creating a stable, curved orbit.

Common Misconceptions

Summary

• The model of the solar system shifted from an Earth-centered to a Sun-centered view.

• Galileo's observations and Tycho Brahe's data provided evidence for the new model.

• Kepler discovered that planets move in elliptical orbits, not perfect circles.

• Newton explained that gravity and inertia work together to keep planets in orbit.