It is 5 hours EST until 2011, which is reason enough to mention that 5 is one of my favorite numbers. Yes, I have favorite numbers. Don’t you? I’m not alone on 5 either. The ancient Pythagoreans thought numbers were what the universe was all about, and 5 was a big deal for them. First of all, the number and the associated pentagram were symbols of health. Why?
AB/BC is the Golden Ratio, [(the square root of 5) + 1] divided by 2, or 1.61803… This ratio is also derivable from the Fibonacci sequence, which Fibonacci described in 1202 AD to explain the growth of rabbit populations – no kidding – and which includes the number 5 (1,1,2,3,5,8,13…). The ratio and sequence occur frequently in nature for reasons no one yet has figured out. They turn up in everything from leaf growth to sea shells, and the Greeks believed the ratio produced the most aesthetically pleasing physical objects. The long sides of the Parthenon are 1.618 times the short sides.
The Pythagoreans also associated 5 with marriage, but, hey, no number is perfect. (The mathematicians out there are shouting, “You're wrong! Any number that is the sum of its positive divisors excluding itself is ‘perfect.’” That is true, but you know what I mean.) Marriage comes into the picture because 5 is the sum of the first “female” number, 2, and the first ‘male’ number, 3 – they didn’t consider 1 to be a number since it is a unity.
There are 5 Platonic solids consisting of regular polygons around a central point: tetrahedron, cube, octahedron, dodecahedron, and icosahedron.
5 is the largest number most people instinctively recognize without pausing actually to count. So, if you simply glance at a shelf with no more than 5 books on it, you will be able to say instantly how many are there. Unless you are unusual, if there are more than 5 books, you (however briefly) will have to count. Most higher animals lose track at 3, but somehow it is hard to be proud about doing only 2 better. This is one reason counting by 5s is easy; the 5 digits on each hand help too, of course. 5 has a prominent place in many number systems, such as the Roman (V=5, L=50, D=500). Incidentally, the classical Romans almost never did subtractive notation. They did not write IV for 4 or XC for 90 as we do today; nearly always they wrote IIII or LXXXX; the subtractive notation became popular in Medieval times, perhaps to befuddle the peasantry further.
Finally, on a personal note, 5 is the number of truly serious inamoratas who have entered and left my life to date, and all were a handful.