Part 2: Understanding the heavenly bodies – Our solar system decoded

The study of the movement of the heavenly bodies, i.e. the Sun, the Moon, the planets and the stars, is as old as history itself. In every major civilization of the past, we find evidence that people looked to the sky and used the movement of the heavenly bodies to measure time, to navigate the land and sea, to prepare and harvest crop, to inspire thought, and to bring the human race to new frontiers of technology and knowledge. The same study however has a dark side of suffering, cruelty and ignorance fueled by religious beliefs, arrogance and the thirst for power. In this part of our series I will give you a brief history of how clever people decoded the movement of the sun, the stars, the Moon and the planets.

Figure 1A: Calligraphically depiction of the geocentric model.

  

The wrong picture – The Earth as the center of the universe

As early as the time of ancient Greece, philosophers and mathematicians began to ponder about rules and laws which command the heavenly bodies. They quickly convinced themselves that the Earth was at the center of our solar system and combined with the other stars, was even the center of our universe. The term solar system refers to the community of Earth, Sun, Moon and planets. In the geocentric model all bodies move around the Earth in perfect circles. Such a path is called an orbit (from Latin orbita meaning “the track made by a wheel” and orbis meaning circle). The Earth was thought to be still, i.e. not moving. The Sun, the planets and the stars all moved around the Earth in orbits. This idea is referred to as the geocentric  model (the word geo comes from the Greek word ge, pronounced gaeh, and means Earth, the word centric  comes from Greek kentros, which describes a sharp point or fixed point). The universe as it was known up to the Rennaissance, comprised our solar system and the ten thousand odd visible stars. All stars were believed to be the same distance away from Earth, just as if they were glued to a glass sphere with the Earth in the center( see Figure 1).

Figure 1B: Simplified picture of Aristotle’s geocentric model. Notice the position the Sun and the Moon have in this model.

Astronomers begin to predict the movement of the heavenly bodies

The Greek philosopher Thales, pronounced thay-lees, (ca. 624 B/C. – 564 B.C.) was the first person known to predict a solar eclipse, that is, the event when the Sun is darkened by the Moon’s shadow for a short period of time. He is known as the father of Greek science and the first astronomer. He based his prediction on the geocentric model of the universe. This model had its roots in the early ancient Greek period around 700 B.C. Big names of Greek philosophy are associated with further developing ideas such as Pythagoras (560-480 B.C.), Plato (428-348 B.C.) and Aristotle (384-322 B.C.) (see Figure 1). The Aristotle model added the concept of the “Prime Mover” (see Figures 1A & 1B)., representing the outermost cosmic sphere beyond that of the stars. It was thought of as "first cause" of the eternal movements of the Universe with no need for a beginning or end. We may think of it as God’s presence. The reminiscence of the Prime Mover can still be found in modern cosmology in the form of the “big Bang”, impetus of the creation of the Universe as we know it today.

The model of Ptolemy – The longest excepted theory of history

Figure 2: Claudius Ptolemaeus

Nevertheless it was one particular man who strung all the knowledge together and created a practical model of the solar system and the known universe. It was the Greek/Egyptian philosopher, mathematician and astronomer Claudius Ptolemaeus (ca. 83 A.D - 161 A.D.). (see Figure 2) or better known in English as Ptolemy, whose model was the first to give scholars an accurate means of calculating the movement of the Sun, Moon and planets. Ptolemy used “tricks” to get his model to predict the positions of the planets as accurately as nature demanded.

Imagine the planets (well the Sun and the Moon too) orbiting the Earth in circles. These orbits are called deferent (comes from the Latin ferro, ferre, meaning "to carry."). The real trick came in placing each planet in a much smaller orbit who’s center is on the orbit or path of the deferent, i.e. the big circle around the Earth). This small orbit is called epicycle (from the Greek epi meaning upon and cycle from Greek kyklos) The epicycle literally means a cycle on a cycle. (Please see Figure 3).

Figure 3: The geocentric model of Ptolemy

Figure 4: In order to understand the basic movements in the Ptolemy model, one can compare the basic elements with the ‘real’ movement of Earth and Moon around the Sun. The words in brackets refer to terms used in the Ptolemy model which resembles the corresponding objects in the real world. Please note that this is only a conceptual comparison, not a factual one.

One good parallel is to imagine the movement of our Moon around Earth (see Figure 4). The Earth moves around the Sun in a big circle, the deferent. The Moon however, representing now a planet in the Ptolmy model, circles around the Earth. The Moon orbit represents in this parallel the epicycle.

The Ptolemy model needed another trick to get better predictions. The deferent, i.e. the big circles, have their center not exactly with Earth. Instead, the center of the deferent is slightly off-center and causes the planets to wobble around the Earth.

The center is called equant (from Latin aequus for equal) since the planets move through equal angles in equal times around the common center of the model. Please see Figure 3. Imagine all planets moving in such a complicated way around the Earth makes the model more likely to be a blueprint for a sophisticated Swiss mechanical watchmaker’s masterpiece. Despite the wrong assumption that the earth is at the center of the universe, the Ptolemy model was surprisingly able to predict the movement of the Sun, Moon and planets very well. It lasted 1400 years until it was seriously challenged during the Renaissance.

The movement of the planets – A not so easy prediction

You may wonder why a model describing the movement of the Earth, Sun and the planets gets so complicated. Its reason is the strange movement of the planets. At the time of the ancient world and even until the Renaissance, astronomers only knew about 6 planets, i.e. Mercury, Venus, Earth, Mars, Jupiter and Saturn. Planets seemed to have very irregular movements.

Sometimes the planets like Mars, Jupiter and Venus (those are the only planets one can observe without using a telescope) can be seen close together in the night sky, sometimes the planets can only be observed at totally different times.

Figure 5: Retrograde motion of Mars as seen from Earth

However the most mysterious property of the outer planets like Mars, Jupiter and Saturn is that they appear to slow down and stop during their path through the sky, halt and trace a backward loop in the sky only to continue their eastward movement. This is not the movement of a planet during the hours of the day caused by Earth’s rotation, which is from east to west. The path or orbit of a planet is observed over a long period. The position of a planet observed at the same time during the night appears to drift eastward, i.e. from west to east, and is called the direct motion of a planet (see Figure 5). The name ‘planet’ comes from the Greek word planao and means “to wander” or “to go astray”. The Greeks were the first to describe the movement of such “wanderers”, i.e. planets. All planets actually wander on a slim strip across the sky called Zodiac (Derived from the Greek word zoon meaning “animal”), located around the apparent path that the Sun traces out in the sky, called the ecliptic (see Figure 5). The Zodiac contains the twelve known constellations Capricorn, Aquarius, Pisces, Aries Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius.

And again, everything changed

At this part of our journey to unlock the secrets of the heavenly bodies, we take a break. So far we have followed the origin of the first models explaining our solar system and the planets from the times of the Greek philosophers up to the time of the Renaissance. We have learned about the extremely successful geocentric model of Ptolemy, which could explain quite nicely, all known facts of the solar system and stood strong and undisputable for 1400 hundred years. In the next part of the series we will learn who and how everything changed in the Renaissance.

About the Author: Juergen Riedel was born in a small town in north Germany. He was educated in the humanist tradition and completed his Masters in Physics at the Christian Albrechts University Kiel, Germany. In 1997 he went to Nassau. He is married to the Bahamian jewelry artist Kim Riedel. Together they have two boys Judah 7 and Jasper 6. Juergen Riedel is currently a part-time science teacher at Genesis Academy in Nassau and founder and director of The Science Institute ( www.thescienceinstitute.com ) which offers an in-depth science education for children in form of workshops and special activities. He believes that bringing science to children in early age will give them an edge in the international competition for future jobs. He can be reached at jriedel@coralwave.com