The Motions of the Planets

Ancient people observed that: The sun appeared to move from east to west across the sky during the day. The stars did the same thing at night, staying in the same position relative to each other. The Greeks noticed that five objects seemed to wander among the stars. They called these objects planets.

The Romans gave these planets names: Mercury Venus Mars Jupiter Saturn

A Greek astronomer Ptolemy (A.D. 100-165 ) theorized that: Objects such as the sun, planets and moon, travel in orbits around the Earth. These orbits are perfect circles.

An orbit is the path that one object takes when moving around another object in space. Ptolemy’s theory is known as the geocentric or Earth-at-the-center of the universe theory. Ptolemy felt that the orbits must be circular because the circle is the most perfect of all forms.

About 1400 years later, Polish astronomer Copernicus (1543) challenged Ptolemy’s theory. He thought that objects orbit around the sun.

Copernicus also said Each orbit is a perfect circle. All planets revolve in the same direction. Each planet takes a different amount of time to complete one orbit.

His model is also known as the heliocentric or sun-at-the-center model of the solar system.

Italian astronomer Galileo Galilei, who lived about 100 years after Copernicus, thought that the heliocentric model was correct.

Galileo constructed a telescope to look at objects in the sky Galileo constructed a telescope to look at objects in the sky. He discovered that Four moons revolve around Jupiter. So not everything revolves around Earth. Venus goes through phases similar to our moon. This would not be what we would see if Earth were at the center.

Figure 1.7 Venus Phases (a) The phases of Venus, at different points in the planet’s orbit. If Venus orbits the Sun and is closer to the Sun than is Earth, as Copernicus maintained, then Venus should display phases, much as our Moon does. When directly between Earth and the Sun, Venus’s unlit side faces us, and the planet is invisible to us. As Venus moves in its orbit, progressively more of its illuminated face is visible from Earth. Note the connection between orbital phase and the apparent size of the planet: Venus seems much larger in its crescent phase than when it is full because it is much closer to us during its crescent phase. (The insets at bottom left and right are actual photographs of Venus at two of its crescent phases.) (b) The Ptolemaic model (see also Figure 1.3) is unable to account for these observations. In particular, the full phase of the planet cannot be explained. Seen from Earth, Venus reaches only a “fat crescent” phase, then begins to wane as it nears the Sun.

In the late 1500’s Danish astronomer Tycho Brahe made very careful observations of the positions of the planets

Johannes Kepler (1609) also supported the heliocentric theory of Copernicus. But…

He determined that the orbit of the planet is an ellipse not a circle

Today, modern astronomers believe that: Each planet travels in a counterclockwise elliptical orbit around the sun. The greater the distance from the sun, the longer a planet takes to complete one orbit.