Giz Explains: Why There Are 24 Hours In A Day

We live in a base-10 world. The decimal system governs everything from the binary functions of computers to the amount of change you get when you buy a Slurpee. So why isn't the standard Earth day just 10 hours long? Credit the Egyptians for that one.

As human civilisation moved from transient hunter-gatherer societies to agricultural communities, people realised the need to tally their objects and property. (If you go out to pasture with five goats in the morning, then come back with three in the evening — and can't tell the difference — you're doing it wrong). The concept of written language was just catching on at that point, so people learned to count the same way kids do today — using their 10 fingers.

Egyptian hieroglyphs from as early as 3000BC show the use of a base-10 decimal system. So why did they set their clocks to base-12? Many believe that the base-12 system arose from a counting system the Egyptians inherited from the earlier Sumerian culture, counting not by the whole finger but by each individual knuckle. That is, if you open your left hand and use the tip of your thumb to touch each of the three knuckles in your four fingers, you'll total 12. To measure time using this method, the Egyptians divided the day into 12-hour halves — or, more accurately, a 10-hour day, two hours of morning and evening twilight, and 12 hours of darkness.

The Egyptians based the hours themselves on the movement of the heavens. They tracked a series of 36 small constellations, known as "decans", which rise consecutively over the horizon approximately once every 40 minutes. The rising of each decan marked the start of a new hour. The start of a new decade — the Egyptian 10-day period — began with the appearance of a new decan in the Eastern sky just before dawn.

By the 9th Dynasty (about 2100BC), Egyptians had augmented their solar calendar with the regular appearances of these stars to create a unified annual calendar. Its 36 decades constituted the 360-day Egyptian year. The new system proved precise enough to accurately predict the annual flooding of the Nile with the rising of the star Sirius, even though the actual length of individual hours varied according to the season. "Tables were produced to help people to determine time at night by observing the decans. Amazingly, such tables have been found inside the lids of coffins, presumably so that the dead could also tell the time," Dr Nick Lomb, the Sydney Observatory's consultant curator of astronomy, told ABC News.

While the new calendar made telling time dead simple, it's hourly flexibility made such a system useless to the Greeks — they needed a day with fixed-length increments. Hipparchus, considered by many to be the greatest astronomer in antiquity, is credited with synthesising the Egyptian star clock into the standardised equinoctial clock we use today, wherein each period of light and dark on the Equinoxes is divided into 12 equal-length segments.

Now, if only someone could explain why we still bother with Daylight Savings.

[ABC News, Cornell, Wikipedia, Andrew Tobias]


    Just had to point out that binary is base-2 not base-10 (decimal).

      Beaten to the punch.

      " The decimal system governs everything from the binary functions..". Contradiction right there.

    Daylight savings is fantastic. Unless society as a whole decided they wanted everyone to start work an hour early in summer, I'll happily take my pretend extra hour of leisure daylight in the evenings.

      Daylight Saving makes cows fade.

        I vaguely remember a Qld politician arguing that if they introduced daylight savings, the curtains of his dear voters would fade more quickly and he didn't want them to face that expense. Another suggested that we already have a high enough incidence of skin cancer and it would only be exacerbated by daylight savings. And these are the people that run our country...

          Didn't you know that if a population of people collectively set their clocks back on the same day; the movement of the Sun and planets can be altered?

          Queensland: beautiful one day, backwards every other.

    Daylight savings exists so I can still play golf after work instead of going home in the dark just because cows don't wear watches.

      Play before work? :)
      Daylight savings exists because large populations in higher latitudes in the Northern hemisphere really do feel the loss of the sunshine at that time of year. In the southern hemisphere no population of any decent size lives at equivalent latitudes, so the whole daylight savings thing is more for the cosmetic tradition of the idea than anything else. -You don't gain that much light.

        Dunno about you but in Sydney during daylight savings the sun isn't down until sometime between 7:30-8pm, whereas outside of daylight savings, it's usually dark by 6pm.

        You do actually get a fairly significant chunk of extra daylight during your waking hours.

          Daylight Savings is just lying to yourself as to what time it is. Forcing you to get up an hour earlier. I used to work in Surfers Paradise and go for a swim before work. That would be a little harder if there were Daylight Savings in Queensland!

          What you forgeting here is that the length of day changes with seasons, so comparing Winter's short days (where it gets dark early) with summers long days (lighter longer into the evening) combining summer's long days with a shift in the distribution of the 24 hours, you get a large difference. But in reality the summer sunset time is only 1 hour different on the clock to what it was with out the saving. So you don't get a significant amount of extra time. You get your long summer days offset to the usual time pattern you live at 9 months of the year.

          Why is this so hard for people to understand. Nothing changes but your schedule. Idiots!

          Personally I think the rest of the country needs to harden the fuck up. Daylight savings... what the bloody hell do we need that for ?

          Same in Radelaide.

            Well in Tassie at the height of summer and daylight savings you can sit outside and read a book at 9:30pm, its messed up lol I moved down here 10 years ago from QLD and am still not used to it.

      It also makes it much harder to contact someone over east from Perth when they're three hours ahead. If an issue at work arrives just before knock off, at 5pm, trying to call a boss in Sydney is a bit harder when it's 8 pm over there

    Very interesting read, thanks Giz

    The sumerian and egyptian base 12 system is a consequence of their mathematics. Base 12 is much easier to work with when your maths is based on combinations of common fractions. Halves, thirds and quarters are all whole numbers with 12.

    Daylight Savings - Sun rises earlier and sets later. Without DST - The extra sunlight is wasted when people are still asleep. With DST, you get more daylight to play games, exercise, have walks, after work.

    Also, think about the savings of electricity - we are talking about saving energy, going green and the carbon tax, aren't we ? If you sleep at midnight everynight, without DST, you have to turn on your lights from 7-12 in summer. With DST, you turn them on from 8-12, some days even just 9 to 12.

      Actually it rises later and sets later. Mankind can't affect how long the sun stays in the sky just by moving the hands on their clocks. As for the 'going green' argument, it also means you may need to turn on a light in the morning, which you wouldn't need to do without DST since the sun would already be up.

      I like DST for the extra afternoon/evening light to enjoy, but don't kid yourself about other benefits.

    The 12 hour breakdown is Babylonian, chosen because it can be broken into halves, thirds and quarters easily, where as decimal system would allow halves and fifths and is therefore less practical.

    I always thought it was 12 hours because you can split a circle in to 12 segments easily with its radius on the quarters therefore a clock face

    360 degrees in a circle because its the closest easily divisable number to 365.25 days

      Interesting! I've never really considered things like evenly dividing a circle or counting the knuckles on your hand with your thumb as mathematical methods used by early civilisations. It's little stuff like this that keeps bringing me back to Giz!

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