Models of the Solar System

About Scientific Theories

​

• Science is based on direct observation.

​

• Scientific Theories are attempts to explain what we observe.

​

• The test of a scientific theory is whether or not it can predict what will happen under new circumstances.

Theories about Celestial Motion

​

• People have been observing and measuring the motion of objects in the sky for thousands of years.

​

• Theories attempt to explain how the universe must be structured for things to move as they do.

​

• New unexplainable observations are often related to improvements in technology.

As soon as we make observations that cannot be explained by the currently accepted theory, the theory has to change

​

Early Theories – Flat Earth

​

• Many early cultures believed the earth was flat, and that the sky was a dome over the earth.

​

• This belief was based on observation.
• The earth looks flat, and the sky looks like a dome.

Evidence that the earth is not flat.

• Greek Astronomers observed that…

​

• The shadow of the earth on the moon during an eclipse was curved.

​

• Ships sailing into the distance seemed to disappear below the horizon.

Erastothenes measurements

​

• 250 B.C. - Erastothenes made measurements of sunlight angles

​

• He used these to calculate the circumference of the earth.

​

• He was accurate to within a few thousand kilometres.

​

• The earth’s circumference is about 25,000 km – he calculated it to be about 28,000 km.

The Geocentric Universe

​

• Early civilizations believed the earth was the centre of the universe.

​

• Everything else, sun, moon, planets, stars, etc. revolved around the earth.

An Accurate Geocentric Model

​

• Greek astronomer Ptolemy

​

• Made an accurate mathematical model of the solar system that had the earth at the centre.

​

• Accurately predicted the movement of the stars and planets, given the measuring abilities of the times.

Improved Measurement

​

• 1500’s
• Measurements of the heavens became more accurate

​

• Ptolemy’s theory did not predict exactly the location of celestial bodies.

​

• A more accurate theory was needed.

The Heliocentric Model

• Nicolas Copernicus
• Suggested the Sun might be the centre of the solar system, not the earth, and everything traveled in circles around the sun

​

• This model did a better job of predicting the positions of the stars.

Improvements to the theory

• Further improvements were made to the sun-centered model.

​

• Johannes Kepler
• Determined that the orbits of the planets were not circular, but elliptical.

​

• Isaac Newton
• Explained the forces of gravity that held the planets in place.

Modern Ideas

• As our ability to observe, i.e. measure, has improved, our picture of the universe has developed.

​

• We now know our solar system to be just one of many in an extremely complex universe.

​

• Our theories are accurate enough to allow us to send space craft to other planets.