The Solar System
Our solar system is made up of our star, the Sun, and all the objects that orbit around it. This includes planets (and the moons that orbit them), asteroids, comets, dust, and gas. There are 8 planets in our solar system. The inner four, Mercury, Venus, Earth, and Mars are called the terrestrial planets. They are small, dense, and rocky. The outer four planets, Jupiter, Saturn, Uranus, and Neptune, are called the jovian planets. They are massive, low density, and gaseous. The terrestrial and jovian planets are separated by the asteroid belt. Beyond the Jovian planets, there is a second asteroid belt called the Kuiper belt. Our solar system is also home to many comets, balls of ice and dust which orbit the Sun along highly elliptical orbits with their tails always extending away from the Sun. Information about the planets in our solar system can be found in the Earth Science Reference Tables.
Early astronomers believed in a geocentric model of the Universe, in which all celestial objects orbited around the Earth, which was stationary in the model. As we collected more and more observations and data, it became clear that our solar system is actually heliocentric, with all objects orbiting around the Sun. The heliocentric model was initially accepted because it better predicted the positions of the planets in the sky.
In the early 1600s, astronomer Johannes Kepler published his three laws of planetary motion. The laws state that...
1 The orbit of a planet is an ellipse with the Sun at one of the two foci.
2 A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time.
3 The square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit.
An ellipse is a rounded shape that can be any shape between a perfect circle and a straight line. All planets orbit the Sun along elliptical orbits, some of which are very circular, and some are very elliptical (oval). The describe the shape of an orbit, we use the calculation of eccentricity. The close the eccentricity is to 1, the more stretched out the ellipse. The close to 0, the more circular it is. When a planet is closest to the Sun, it experiences stronger gravity and therefore travels at a higher velocity. This point is called perihelion. When a planet is farthest from the Sun, it experiences weaker gravity and therefore travels at a lover velocity. This point is called aphelion. The Earth experiences perihelion in the winter and aphelion in the summer.