Models of the Solar System

Ancient observers noticed that most objects in the sky seem to be in a state of slow but steady motion. Every day the sun appears to rise in the east and set in the west. The moon, too, rises and sets every day. Similarly, the stars move across the sky in a fixed pattern.

Camp out for a few nights and you'll notice, as the ancients did, that the stars appear to circle around a fixed point in the sky. In the Northern Hemisphere, the stars circle around Polaris, the North Star. Most ancient people concluded that Earth was stationary and the sun, moon, and stars moved around Earth.

Ancient observers noticed that a few bright starlike objects seemed to wander slowly among the fixed patterns of stars. These objects were called planets, from the Greek word for “wanderers.” Besides Earth itself, the ancients knew of the five planets that can be seen with the unaided eye: Mercury, Venus, Mars, Jupiter, and Saturn.

Figure 1 The Mayan ruin of El Caracol, also known as “The Observatory,” is located on the Yucatan peninsula. It is thought that the Mayans used El Caracol for astronomical observations.