Book: Revolution in Time

A history of clocks by David Landes.  Fascinating subject.

So far though -- surprisingly -- not a word about the history of the measurement of time -- why 24 hours, why sixty minutes, why sixty seconds -- when the days are different lengths at different times of the year (unless you're on the equator). That's another story I want to read.

http://en.m.wikipedia.org/wiki/Day_length#/image/File:Hours_of_daylight_vs_latitude_vs_day_of_year_cmglee.svg

Quote :

Some places divided the day into twenty-four continuous hours (so-called Italian time); this was very hard on bell ringing clocks. Others marked it off into two rounds of twelve hours each ("German hours" in Bohemia, "French hours" in Italy). In the long run of course the needs of commerce and communication were such as to encourage standardization, and usage shifted gradually toward the diurnal pattern we know today: midnight to midnight, twelve hours A.M. And twelve hours P.M.

Nowadays the 24-hour clock survives as "military time."