Wandering Stars?

 For thousands of years, people have been fascinated by what they called ‘wandering stars.' These moved slowly but noticeably against the background of the other stars. Today, we call these ‘planets’, from the Greek for ‘wandering’. Together with the sun and many smaller bodies, such as comets, they and their moons are part of the solar system (Latin sol = sun). Studying the movements of the planets helped to develop modern science.

Puzzling motions

Ancient astronomers believed that everything in the heavens revolved around the earth (called geocentrism, from Greek gē = earth). But planets occasionally seemed to go backwards briefly (retrograde motion), before going on their forward path again. How is this explained?

The Greek astronomer Ptolemy, who lived about 100 years after Christ, had a complex answer. In his book called the Almagest, he proposed that planets moved in epicycles—circles upon circles. This was the main astronomy textbook for well over a thousand years.

Eventually, astronomers realized that this explanation wasn’t correct. A few astronomers proposed a novel idea: the earth and planets all revolved around the sun. This is called heliocentrism (from Greek hēlios = sun). In this system, planets occasionally pass each other. This explains the apparent backwrd motion.

Some very high-ranking churchmen in the late Middle Ages (14th and 15th centuries) discussed this idea. Then Nicolaus Copernicus (1473–1543), a Polish church officer, published a famous book promoting the sun-centred solar system. Much later, Italian scientist Galileo Galilei (1564–1642) famously defended this system.

Controversy: Heliocentric vs Geocentric Systems

Because Ptolemy was so highly respected, almost all astronomers at first rejected heliocentrism. But many in the church were very interested in this idea. However, they were cautious about challenging the science of their day. Galileo unfortunately insulted the Pope, so the Church demanded that he stop teaching his view as proven fact. And at the time, it was not.

Astronomers pointed out many problems. For example, why don’t we notice the motion? The answer is: they are all travelling together. You can see this for yourself next time you’re moving in a car: drop something to the floor—it should fall straight down. 

Solution: Kepler and Newton

Also, Copernicus’ model wasn’t much more accurate than Ptolemy’s. Johannes Kepler (1571–1630) proposed that planets orbit the sun in ellipses instead of circles.

Kepler was a very devout Christian. His scientific writings were even full of praises and prayers to God. Kepler said that his scientific work was “thinking God’s thoughts after Him.”

Then the greatest scientist of all time, Sir Isaac Newton (1642–1727), proposed the laws of motion and the law of gravity. Newton showed that these laws explained why planets followed Kepler’s laws. Newton’s laws also work for projectiles (like cannon balls), falling bodies on Earth, and for our moon and the moons around other planets.

Newton was also an extremely devout Christian—he wrote more about the Bible than about science. He taught that “this most beautiful” solar system could have come only from an almighty creator God.

Ptolemy believed that planets moved in a small circle upon a large circle. The large circle had the earth at the center. This hollow circle in turn was thought to be orbiting around the earth. See how this would make the planet move backwards sometimes? If you think this is complicated, this is the simple version! Ptolemy’s real model needed the best mathematics experts to understand it.

Copernicus and Retrograde Motion

The earth orbited faster than Mars and outer planets. When the earth overtakes these, the planets appear to move backwards. Think about what happens when you’re in a car that overtakes a slower one. When you look out the window, the slower car seems to be moving backwards compared to your vehicle.

Mars as viewed from Earth seems to move quite strangely. At time, Mars seems to slow down and appears as if it is about to change direction. At times, Mars seems to have moved backwards. At times, Mars seems to have started moving forward again.

Kepler discovered that the planets move in an elliptical orbit, with the sun at a focus. Most planets have orbits that are nearly circular. Comets, on the other hand, have very eccentric orbits.