http://www.haroldcarr.net Harold Carr en http://www.haroldcarr.net/2004/07/18#2004-07-18-outOfEgypt24hours <p>24 hours in a day seems arbitrary (unlike 365 days in a year). There is some evidence that it is based on the Egyptian belief that the Sun-god, Re, spent the ours of the night voyaging through th underworld in a boat. The underworld was said to be divided into twelve regions. The god must pass through each, passing one hour in each - the word for hour was written as a star.</p><p>The astronomer-priest selected twelve bright starts which rose in succession through the night, and, calling the interval between each an hour, charted the progress of the Sun-god through the underworld.</p><p>In practice it was not a single star but a 36 constellations (not ours) known as decans, since each one rose heliacally (i.e., the first pre-dawn rising) for ten days.</p><p>This whole system is made explicit in the painted star clocks which were prevalent from around 2200 BCE, in which twelve star-groups are charted for each ten-day period of the year.</p><p><div class="mediaobject"><img src="http://www.haroldcarr.net/blog/environment/egyptianDecanStarClock.jpg"></div></p><p>The division of the day into twelve hours was made by analogy with the night - the daylight hours being measured with shadow-clocks.</p> http://www.haroldcarr.net/2004/07/18#2004-07-18-observeMemorizeBuild <p>Humans observe patterns of nature and then use that knowledge to regulate our lives. Before writing, we built structures such as Stonehenge with precise astronomical alignments. The data necessary for such alignments would need to be collected across multiple generations, but writing, to our knowledge, did not exist at that time. This implies an oral tradition of astronomical knowledge.</p> <p><a href="http://www.haroldcarr.net/blog/environment/stonehenge.jpg" target="_top"><div class="mediaobject"><img src="http://www.haroldcarr.net/blog/environment/stonehenge_s.jpg"></div></a></p>