Planets

The planets are spherical bodies which rotate around the hub of the Earth. The planets follow a similar daily route across the sky as the Sun, along a path called the ecliptic. Five planets — Mercury, Venus, Mars, Jupiter, and Saturn are visible to the naked eye and were known to the ancients as "wandering stars;" entities which appear to move differently from the fixed path of the stars. The word "planet" comes from the Greek word planetes, meaning "wanderer."

Topics

• Discovery of Neptune
• Precession of Mercury's Orbit
• Kings Dethroned
• Astronomical Prediction Based on Patterns
• Three Body Problem

Dance of the Planets

The Sun moves in Northward and Southwards paths across the sky over the course of the year when changing seasons and the planets follow along that path, with an apparent relationship to the Sun. Mercury and Venus appear to be rotating around the Sun, while the other planets have a less direct effect. The below animation shows the Sun's progress across the sky over the course of a year as a panorama, its Northward and Southward motions, and the 'dance of the planets' as they follow the sun.

(Source)

The Copernican Revolution of the 16th century held that this relationship with the Sun was evidence that all of the planets of the Solar System moved around the Sun. At the time the idea of a Round Earth was already widely prevalent, based on the teachings of the Ancient Greeks. It was deduced that since the Earth is a round body in a Sun-centered celestial system, that the earth must also be a body in motion similar to the planets seen in the sky.

Quotes

  “ Accordingly, since nothing prevents the earth from moving, I suggest that we should now consider also whether several motions suit it, so that it can be regarded as one of the planets. For, it is not the center of all the revolutions. ”
                  —Nicolaus Copernicus

  “ I demonstrate by means of philosophy that the earth is round, and is inhabited on all sides; that it is insignificantly small, and is borne through the stars. ”
                  —Johannes Kepler

  “ In the middle of all sits Sun enthroned. In this most beautiful temple could we place this luminary in any better position from which he can illuminate the whole at once? He is rightly called the Lamp, the Mind, the Ruler of the Universe: Hermes Trismegistus names him the Visible God, Sophocles’ Electra calls him the All-seeing. So the Sun sits as upon a royal throne ruling his children the planets which circle round him. The Earth has the Moon at her service. As Aristotle says, in his On Animals, the Moon has the closest relationship with the Earth. Meanwhile the Earth conceives by the Sun, and becomes pregnant with an annual rebirth. ”
                  —Nicolaus Copernicus , De Revolutionibus, Of the Order of the Heavenly Bodies 10.

Prediction in Astronomy

Prediction in astronomy is performed through patterns. By analysis of historic tables it is possible to construct functions which can predict where a planet will be in the future. This is how prediction in astronomy has been performed since times of antiquity, and how it is performed today. See: Astronomical Prediction Based on Patterns