Transcript: The simplest division of time used by hunter-gatherers was to divide the day into the time when the sun was rising, ahead of the Meridian, and the time the sun was setting, beyond the Meridian.  We still use that terminology.  "AM" means "Anti-Meridian," or before the Meridian. "PM" means "Post-Meridian," or past the Meridian. So the first people divided the day into two, roughly equal parts.  The practice of dividing a day into hours originated 4 or 5000 years ago with the Egyptians.  They kept track of time at night using "decans" or time-keeping stars, and they roughly divided the day and the night into twelve equal portions using these time-keeping stars.  That's the basis of our hours.  In the northern latitude of the Earth, if you divide the day throughout the season into equal amounts, hours will not be of equal length.  The hour will vary if you divide daylight into twelve pieces, from 75 minutes in the summer to about 45 minutes in the winter.  So hours did not have equal length.  The first uniform time-keeping dates from the time of the first clocks.  The first clocks were mechanical devices, enormously expensive for the medieval cultures who created them from the 14th and 13th century.  The Great Clock in Salisbury has been working continuously since 1365.  The word "clock" comes from the word for "bell."  The earliest clocks had no hands and no faces and only bells because almost nobody could read.  Division of time within an hour dates back to the Babylonians, who chose to divide the hour into sixty minutes and the minute into sixty seconds.  But the practice of keeping time this way relied on the invention of the pendulum as a time-keeping device during the time of Galileo.