Idle Curiosity - Load Averages

[Geoff]

This may be somewhat nonpractical, but how exactly are load averages
calculated? I know that my load average sits at about 0.5 most of the
time, and that higher numbers mean higher load, but how exactly is it
calculated? CPU usage times ram used divided by pi or something? And
also, when i run top, there are 3 numbers for load average. (Average for
the last minute, last hour, last day?)

Thanks,
Geoff