Chapter 2

Through investigations you will understand some important features of the Solar System. A systems view of the Solar System shows that planets and other astronomical objects, such as moons, interact with each other and the Sun in many ways. Motion is an endless quality of the parts of the Solar System. A planet's motion influences simple, but important, phenomena. In the Earth-Sun system, day and night are caused by the rotation of our planet about its axis. The length of an Earth year and the seasons we experience depend on the geometry of Earth's orbit around the Sun.

Gravitational forces play a fundamental role in keeping the planets in a nearly circular motion around the Sun. By looking at interactions in the Earth-Moon-Sun system, you learned that the Moon and Sun's gravitational attraction causes the tides, as Earth's water is shaped into bulges. Gravity keeps the Moon in orbit around the Earth, and the illumination of the Moon by the Sun enables us to observe its phases.

When looking at the Solar System as a whole, the gigantic proportions of space became clear by focusing on the distances between the planets. Only a tiny fraction of the Solar System is occupied by matter. Yet, all matter, whatever its size, is governed by the law of gravity and has been since the formation of the Solar System as explained by nebula theory. This theory explains the characteristics and distances from the Sun of the denser inner planets compared to the less dense outer planets.