Celestial MotionIs the Earth the Center of the Solar System?

Modelling Celestial Motion

From your research into ancient cultures that used astronomy, you know that people have long searched for explanations for the motions of the stars and planets.

First people began to notice patterns to the motions, for example, they saw that the Sun came up in the east and set in the west.

Next people (usually scientists) had to come up with basic ideas or theories about why this might be.

Once they has some type of theory they would build a model to see if their theory could be right.

Early scientists developed two models to test and explain the motions of the Sun, Moon, planets and stars.

The first model was an Earth-centered or geocentric model. ( Geo means earth). It was based on the work of a Greek philoso pher named Aristotle and dominated thinking about celestial motions for nearly 2000 years!

The Earth Centered Model

As you have observed, all celestial bodies seem to move across the sky from east to west, revolving around the Earth.

Early astronomers imagined that this was because the Earth was at the center of a giant sphere on which the Sun, Moon and planets were attached.

Aristotle used math and geometry to create a model of perfect spheres. He placed the stars ( whose position didn’t appear to change) on the surface of an outer sphere which he called the celestial sphere.

Inside of this sphere he arranged more concentric spheres on which were attached the Sun, the moon and the five planets that were known.

It looked like this . . .

Well really like this . . .

The model worked fairly well for predicting dates and times when celestial bodies rose and set but it was very complicated.

In order to explain why some planets appeared to reverse direction ( we now know this is because of retrograde motion), astronomers had to keep adding more spheres and cycles of motion. The model got very, very complicated but it did seem to make a certain amount of sense.

A better way of explaining . . .

During the Renaissance ( the period between the 14th and 17th centuries), the Western world saw rapid expansion and exploration. This was possible because of improved navigation of unknown waters and lands.

Advances in navigation were developed from more refined astronomical observations.

The Sun Centered Model

In the early 1500’s a polish astronomer named Copernicus,

proposed a different model to explain the celestial motions observed from Earth.

You might not have guessed but he was a totally radical

thinker for his time!!

Copernicus was trouble by the complicated model that Aristotle had created. It was hard to understand and didn’t really explain retrograde motion satisfactorily.

Copernicus began to think and he created a much simpler model.

In 1543, He proposed that, rather than the Earth being fixed at the center of a giant sphere, the Sun was fixed and a rotating Earth travelled around the sun.

It looked like this

Copernicus placed the Sun at the center of the solar system and arranged the planets in orbits all on the same solar plane. This is called a Heliocentric model. (Helio means sun)

Controversy

Despite it’s simplicity, the model proposed by Copernicus was slow to catch on.

Jump ahead 60 years and we find Galileo ( an Italian astronomer) and Johannes Kepler ( a German astronomer) doing work that supports the Sun centered model.

Using an early telescope Galileo was able to see the phases of Venus, the rings of Saturn and 4 moons orbiting Jupiter.

Kepler, who was also a mathematician, calculated that the predictions of celestial movements would be more accurate if the orbits of planets were ellipses rather than circles.

Both the Protestant and Catholic churches reacted strongly to this new model. Many felt that ideas presented by Copernicus, Galileo and Kepler were heresy and a direct opposition to Scripture.

In 1633 Galileo was convicted of grave suspicion of heresy for "following the position of Copernicus, which is contrary to the true sense and authority of Holy Scripture.”

Hewas placed under house arrest for the rest of his life.

Our View of the Solar System Today

The geocentric model was developed by early scientists , based solely on observations with the un-aided eye. As technology advanced, scientists looked for new explanations and the heliocentric model emerged.

Today, technology continues to evolve and advance. We are constantly learning new things about our solar system and the universe.