|MadSci Network: Astronomy|
The five planets known about in pre-telescopic times were (in order of brightness, from brightest to dimmest):
Venus (Mean visual magnitude -4.4)
Visual magnitude is a logarithmic scale where the lower the number, the brighter the object. As you can see from the planets, increasing brightness extends into negative numbers: the Moon is at -12.7, the Sun is listed at -26.8.
The dimmest stars visible to the naked eye are around magnitude 6.0, and there are only a dozen or so stars brighter (that is, lower numbered) than magnitude 1. (The scale was originally developed to divide stars - by eye - into six classes). When refining the scale, it was decided to make five levels of magnitude exactly equate to a 100 times difference in brightness - so, for example, Saturn at +0.7 is about 100 times brighter than the dimmest stars.
Our prehistoric ancestors, with no light pollution in their night skies, and with hunting and eventually agriculture dependent on knowing where in the year they were, would have been very aware of stellar motion - the slow cycle of the stars around the sky during the course of each year. The five planets listed above were (and still are) clearly different to the stars. The planets are obviously brighter, don't tend to flicker like stars can (as they are not true point-objects like the stars), and they all exhibit regular motion across the seemingly fixed starry background.
So, it is safe to assume that all five of the above planets were well known in prehistoric times. However, there is a interesting question over the status of the next planet to be discovered (the first of telescopic times). Uranus lies about twice as far from the Sun as Saturn, and it is a smaller body. Its average visual magnitude is +5.8, so it is potentially visible to the naked eye, but its motion (with a year of some 84 Earth years) is much less obvious than that of Saturn's. Whether it was ever known about (and then lost again) in prehistory is unknown.
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