The Solar System
Creation for Kids
Published in Creation 42(4):32–35, 2020
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 come?
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. See Diagram 1 for why they would appear to reverse course briefly. 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 backward motions (see Diagram 2).
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. (Remember to pick it up afterwards!)
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 (see Diagram 3).
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.
Diagram 1. Epicycles
Ptolemy believed that planets moved in a small circle upon a large circle. The large circle had the earth at the centre. In the diagram, the planet is orbiting around the tiny hollow circle, in the path shown by the small dotted circle. This hollow circle in turn is orbiting around the earth, shown by the dotted large circle. 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.
Diagram 2. Copernicus and retrograde motion
The earth is orbiting 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 you.
In the diagram, the numbers mean different times in the orbit of Earth and Mars. The curve on the right means the position of the planets as they appear against the background of the stars.
From time 1 to time 2, Mars as viewed from Earth seems to have moved quite far, as shown by the length of the right curve. At time 3, Mars seems to have slowed down and is about to change direction. At times 4 and 5, Mars seems to have moved backwards. At times 6 and 7, Mars seems to have started moving forward again.
Diagram 3: Ellipses (activity)
Put two pins in a board. Tie string in a circle and loop around the pins. Pull the string tight with a pencil, and trace the curve, keeping the string tight with the pencil. This curve will be an ellipse.
Place the pins further apart and repeat. The ellipse will be longer and thinner, or more eccentric. Then put them closer together and repeat. The ellipse becomes more like a circle. When the pins are in the same position (in effect one pin), it will be a circle. So a circle is a special type of ellipse.
The position of each pin is called the focus (plural foci) of the ellipse. Kepler discovered that the planets move in an elliptical orbit, with the sun at a focus. Most planets have orbits that are nearly circular. The comets have very eccentric orbits.
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