Part 1: The Solar System

Beginnings

In ancient times, people looked at the sky and tried to explain what they saw. They thought that the Sun went around the Earth, that it rose in the East and set in the West, but they were wrong. They thought that the Moon was the same size as the Sun, and that it changed its shape all month long, but they were wrong. They thought the stars were tiny lights set in a huge dome that revolved around the Earth, but they were wrong. Whenever they looked into the distance, such as out across the ocean, they observed that the horizon seemed to be a straight line. They were very smart people, they thought deeply about their observations, and they came up with some interesting explanations for what they saw, but they were still wrong.

The Sun does not rise and set, and it does not go around the Earth. It just looks that way because we do not notice the spinning of the Earth beneath us. The Moon, on the other hand, does go around the Earth, but it is much smaller than the Sun. It just looks the same size because it is so much closer. It looks as if it changes shape because of the way the Sun's light hits it. The stars were not tiny points of light, they are huge distant suns, and some things that look like stars are collections of billions of suns called galaxies, and they are terribly far away from us and getting farther away every day. The Earth is not flat, but as round as a ball.

Many people today still do not understand how the universe works. This is partly because the huge distances of space are hard for the mind to imagine. Also, it is hard for human beings to believe that they are not the most important creatures in the universe. Earth, rather than being the center of the Universe, is not even at the center of the Milky Way galaxy or at the center of the Solar System. Earth is a very small speck of matter in a very large universe.

We are learning more about that universe, and about the Earth on which we live, every day. Yet some of the things we think we know about the universe may turn out to be just as wrong as what the ancient people believed. For instance, when I started teaching at The Haverford School, no one had ever proved that any other star had any planets of its own and we thought our Solar System might be the only planetary system in the universe. Now we know of many planets circling other stars.

Everything we learn about the Solar System, the Milky Way Galaxy, and the universe, we have had to learn from a long way away. Human beings have never been any farther than the Moon, and we have sent unmanned space probes no farther than the edge of the Solar System. Yet using telescopes and mathematics we have learned a great deal about the Universe and we are learning more every day..

Solar System

Our Sun is a medium-sized star in the middle of its life. A star is a body which gives off light and energy. The Sun makes light because of gravity. Gravity, as we learned before, is a field force between objects with mass, and the Sun has so much mass that its gravity is simply enormous. The gravity of the Sun is so strong its atoms are jammed together in the Sun's core, causing a constant nuclear explosion which gives off light, heat, and radiation. The force of the explosion is what keeps the Sun from collapsing into itself. Even though the Sun is 150 million kilometers away, this tremendous explosion is what gives us almost all of our energy here on Earth.

Picture of Sun's gravity forcing atoms together.

Our Sun has a system of nine planets--four inner planets and five outer planets. The planets, large bodies which do not give off their own light, move around the Sun. The paths planets follow around the Sun are called orbits. Orbits are not perfect circles, they are ellipses (el-LIP-sees), rounded shapes which are more like ovals. The orbits of most of the planets are more or less in the same plane, like the lanes on a running track are all on the same level. In other words, the planets don't go around the Sun like moths around a street light but like runners around a track.

The planets move around the Sun at very high speeds. Planets, like all matter, tend to stay in motion in a straight line if they are moving. Why don't they fly off in every direction, then? Because the Sun's gravity is very strong and keeps them from escaping.

Diagram of Solar System

The planets in order from the Sun outwards are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. The inner planets (Mercury, Venus, Earth, and Mars) are made of rock and are small compared to the outer planets. The first four outer planets (Jupiter, Saturn, Uranus, and Neptune) are what we call gas giants. They are very large and are made of very cold gas. Some astronomers do not think Pluto is a planet at all, because it is very small and very solid doesn't fit the pattern of the other outer planets. They think it might be a moon of Neptune which escaped.

Pictures of Planets

It is hard to draw a picture of the Solar System which is correct. This is because if you show the distances between the planets correctly, you have to make the planets too small to see in the picture, and if you show the correct sizes of the planets, you have to draw them too close together.

Earth

The Earth is the third planet from the Sun. It takes the Earth just about 365.25 days to make one revolution (one complete orbit) around the Sun.