The term "planet" originally meant "wanderer": it was observed long ago that certain points of light wandered (changed their position) with respect to the background stars in the sky. In ancient times, before the invention of the telescope and before one understood the present structure of the Solar System, there were thought to be seven such wanderers or planets, as listed in the following table:
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This list differs in several respects from the modern list of planets in the Solar System. These differences are due both to further observations, and to a changing perception of what a planet is.
| 1. Earth is missing, because it was not understood that the points of light wandering on the celestial sphere and the Earth on which we stood had anything in common. |
| 2. Uranus, Neptune, and Pluto are missing because they would only be discovered after the telescope was invented (see the right panel). |
| 3. The Sun and the Moon were classified as planets because they wandered on the celestial sphere, just like Mars and Jupiter and the other planets. |
A central theme of our initial discussion will be how the "seven planets of the ancients" (only five of which are really planets) evolved into our present list of Solar System planets.