Historical Models of the Universe and Ancient Scientists Lesson

Astronomy is considered one of the oldest known sciences. It is natural for human beings to notice the things happening around them each day and to ask questions. It is all part of the process of learning and growing our knowledge base. In science we make observations and we apply our prior knowledge to our observations and then we can make inferences. Let's explore how ancient astronomers stepped through the scientific method to develop models to help explain things that were too large to fully see. We will also see how technology and new tools helped scientists over time change models based on new learning.

Aristotle

Aristotle (384-322 B.C.) was a philosopher who lived over 2000 years ago. Aristotle was proud of being a Macedonian, but he admired the Greeks and the Greek way of life. He spent much of his time in ancient Greece. Aristotle made fantastic contributions towards science and education. He was one of the teachers of Alexander the Great and he established the initial beliefs that would later develop into a recognized model of the Solar System. Aristotle looked to the sky and saw celestial bodies. He believed there were circular layers or spheres around Earth and that these celestial bodies followed a path around these spheres around Earth.

There was a problem. Sometimes celestial objects would appear to move backwards or retrograde. This motion could not be explained by Aristotles model.

Aristotle raises a series of common sense questions about the nature of the universe that anyone might ask.

Does the earth move?

Where is the earth in the universe? Why?

What is the shape of the earth? How do we know?

What is the motion of objects in the heavens?

Why do things move? Do all things move the same way?

Is there empty space? What is space?

Ptolemy

Ptolemy (90 - 168 AD) Almost everything that the earliest astronomers knew about the universe came from what they could discover with their eyes and minds. The Greek philosopher, Ptolemy, thought that Earth was at the center of the universe and that the other planets and the sun revolved around Earth.

Ptolemy embraced the generally accepted theory that the Earth did not move and was located at the center of the universe. This is known as a geocentric model. Geocentric meaning the Earth at the center. Ptolemy developed explanations for the motion of the planets assuming that they were all going around the earth. Thinking that the planets are going around the earth, some of them sometimes would seem to shift direction and go backwards for a while - Ptolemy called this "retrograde motion" and developed complicated mathematical formulas to predict when each planet would go into retrograde motion. But really, the planets only appear to be going backward as seen from the Earth - they are just going in plain circles around the Sun.

Because the Greek culture believed in the perfection of nature, Ptolemy wrote that the planets and stars moved continuously in perfectly circular orbits. In order to make his system fit with most of the observations that astronomers had recorded, he departed from traditional mathematics, and explained that these bodies had varied speed and the epicycles had different diameters.

Geocentric Model

Geocentric System

Nicolaus Copernicus

Nicolaus Copernicus

Copernicus (1473 - 1543) was a Polish astronomer who had been educated at the University in Krakow. Copernicus is credited for developing the heliocentric model or sun-centered model of the solar system. Copernicus's theory included the rotation of the earth on its axis every 24 hours and revolving around the sun every year. This theory further included all the other planets revolving around the sun too.

Heliocentric Model

Heliocentric Model- Fixed Background Stars, Saturn, Jupiter, Mars, Moon, Earth, Venus, Mercury, Sun

Remember! Accepted scientific models can change when new information is discovered or something different is observed. Aristotle and Ptolemy observed natural phenomenon with their eyes. But, new innovations, new inventions are yet to be discovered!

Enter the TELESCOPE!

Galileo Galilei

Galileo Galilei (1564–1642), was an Italian astronomer, mathematician, and physicist. Although Galileo did not invent the telescope, he was the first person to use it to systematically study the sky, and what he found was amazing. Galileo's careful observations provided recorded evidence that would eventually change long-held beliefs about the universe. Galileo

By using a telescope Galileo was able to make new observations that could not have been made by just looking without a tool.

Galileo observed that Venus appeared in phases like the Moon, these phases could not been seen without the telescope.

Hold Everything The phases of Venus raised questions that the geocentric model could not explain. Galileo will later discover Jupiter has four of the largest moons that revolve around it thus proving not everything revolves around the Earth.

When Galileo published a book that introduced his evidence supporting Copernicus, who had written he believed the Sun was the center of the Universe, the Church found Galileo guilty of heresy and sentenced him to house arrest for the rest of his life. Today, Galileo is recognized for his revolutionary work in both astronomy and physics, and he has been referred to as "the father of modern science." In addition to his findings in support of the Copernican system, his theories regarding falling objects and objects in motion laid the groundwork for classical mechanics. He was also a pioneer in the approach we call the scientific method, which utilizes observation and experiment. In 1992, three hundred and fifty years after his death, the Church officially apologized for condemning Galileo and his scientific research.

The word "helios" in Greek means, "sun." Heliocentric means that the sun is at the center. A heliocentric system is one in which the planets revolve around a fixed sun. Thus Mercury, Venus, the Earth, Mars, Jupiter and Saturn all revolve around the sun. The moon is the only celestial sphere in this system, which revolves around the earth, and, together with it, around the sun.

Tycho Brahe

Tycho Brahe

Tycho Brahe (1546- 1601) was a Danish astronomer who was known for his accurate and comprehensive astronomical and planetary observations. As an astronomer, Tycho worked to combine what he saw as the geometrical benefits of the Copernican system with the philosophical benefits of the Ptolemaic system into his own model of the universe, the Tychonic system. Furthermore, he was the last of the major naked eye astronomers, working without telescopes for his observations.

Johannes Kepler

Johannes Kepler

Johannes Kepler (1571 - 1630) was a German astronomer and mathematician. He became an assistant to Tycho Brahe. Kepler did work with telescopes and refined the process to make a refracting telescope. Kepler used the observations Brahe had made to discover orbits of the planets were elliptical not circular.

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