MadSci Network: Astronomy |
Hi Ashlie!
The five planets known about in pre-telescopic times were (in order of
brightness, from brightest to dimmest):
Venus (Mean visual magnitude -4.4)
Mars (-2.8)
Jupiter (-2.2)
Mercury (-1.8)
Saturn (+0.7)
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.
Andy Goddard
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