Why 12 hours in a day

The Eqyptians were fond of counting in base twelve instead of base 10 which is commonly used today. This is thought to be because they counted finger joints instead of fingers. Each of your fingers has three joints, so if you count by pointing to finger joints with your thumb you can count to twelve on each hand. This might seem arbitrary, but is actually just a strange as counting in base ten simply because we have ten digits. Feb Update : Thanks to a "Curious" reader for pointing out that another reason the Egyptians and Indians liked counting in base 12 is that 12 has a larger number of integer factors than The Egyptians divided the clock into 12 hours of daytime and 12 hours of night-time or alternatively 10 hours between sunrise and sunset, an hour for each twilight period and 12 hours of darkness.

This is known because of various sundials from the period which have been found to be marked with hours. Interestingly this means that hours started out changing in length with the seasons as the amount of daylight vs.

There is a more in-depth explanation for the division of night-time into 12 hours which is based on the number of "decan" stars which were seen to rise during summer nights in Ancient Egypt.

A "decan" star was a star which rose just before sunrise at the beginning of a day "decade" in Ancient Egypt. During summer nights, 12 decan stars rose - one for each "hour". However, hours did not have a fixed length until the Greeks decided they needed such a system for theoretical calculations. Hipparchus proposed dividing the day equally into 24 hours which came to be known as equinoctial hours because they are based on 12 hours of daylight and 12 hours of darkness on the days of the Equinoxes.

Ordinary people continued to use the seasonally varying hours for a long time. Only with the advent of mechanical clocks in Europe in the 14th Century, did the system we use today become common place. Follow-up question Apr : How come there are 36 decan stars but only 12 in a night. Why aren't there 18 each night? Are some of the decan stars below the southern horizon part of the year.

What am I missing here? There was not a decan star every modern hour. Remember that the length of darkness in the summer is actually less than 12 "modern" hours. The Egyptian "hours" marked by the rising of each of the 12 decan stars were shorter than what we call an hour now.

As I said, hours did not have a fixed length until much later when people decided that would be useful! The "hours" in this era were only equal to our current hours on the equinoxes. I could not find a list of decan stars or star groups in some cases , in modern terminology. A list of them in Egyptian terms is here.

Karen was a graduate student at Cornell from This changed in , when the second was redefined as the duration of 9,,, energy transitions of the cesium atom.

Interestingly, in order to keep atomic time in agreement with astronomical time, leap seconds occasionally must be added to UTC. Thus, not all minutes contain 60 seconds. A few rare minutes, occurring at a rate of about eight per decade, actually contain Already a subscriber? Sign in. Thanks for reading Scientific American.

Create your free account or Sign in to continue. See Subscription Options. Go Paperless with Digital. References Time's Pendulum. Jo Ellen Barnett. Plenum Press, A History of Mathematics. Florian Cajori. MacMillan and Co. History of the Hour. Gerhard Dohrn-van Rossum. University of Chicago Press, Get smart. Sign up for our email newsletter. Sign Up. Support science journalism. Decimalisation has started creeping in by the back door. And of course it is easier to denote a part of an hour with decimal notation if you are using a word processing package.

But it seems as though we are stuck with our antiquated system of denoting time. Blame the Sumerians, I say. The rest of Europe drives on the right, so why do the British drive on the left? Martin Fone, author. Nothing says 'Spring is here' better than that wonderful aroma of newly-cut lawn. Martin Fone, author of 'Fifty Curious Questions',.

We've all been given the familiar advice by parents anxious to get us eating our vegetables, but is there any. Can you calibrate the intensity of different insect stings? Martin Fone, author of 'Fifty Curious Questions', investigates. Credit: alamy. Curious Questions. Credit: Alamy. Curious Questions: Who compiled the first crossword?