Time Answers

by Deb on November 11, 2011


Now if only I had more of it. What do you know about time?

1.     The official unit of time is the second. It was originally defined as 1/86,400 of a mean solar day, before we realised that we don’t actually know what a mean solar day is – it keeps changing. So now it is defined as “the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom at rest at 0 Kelvin.” That’s much easier.

It is certainly easier in that it won’t change, but I think I’ll just stick with my watch.

2.     One of the major reasons for developing accurate clocks or chronometers was to be able to navigate at sea. Most early voyages either hugged coasts or island hopped to islands that could be seen. No-one’s quite sure how the Polynesians got out there, and we’ll never know how many didn’t make it.

The next method was to take off towards something you were pretty sure was there and keep going until you hit it. This is the method Christopher Columbus used (when he was wrong and lucky) and the reason there were so many shipwrecks on the Western Australian coast – people who were aiming for the Indies but ended up too far south.

With knowledge of the stars it was possible to work out latitude, but longitude is much more difficult because it keeps changing as the earth spins around. To work it out needed very accurate records of how far the ship had sailed, which meant accurate speeds and times. This is what pushed the early development of chronometers, both because of the accuracy needed and because many early clocks just wouldn’t work on a moving ship.

Ship's chronometer

Photo courtesy of Bjoertvedt

3.     The Long Count is a Meso-American calendar. There are two different ways of counting time that are both useful in different situations – cyclical and linear. Cyclical counts are what calendars are generally about, with months and years tracking the lunar, solar or even astral cycles. They are important to be able to prepare crops for planting or store food for winter. Linear counts tell us about the time since or until a unique event.

The Mayan cyclical calendar repeated roughly every 52 years. To date things uniquely they used the Long Count, which basically calls everything ‘xxxx days after creation.’ The Mayan date of creation appears to be the equivalent of August 11th 3114 Before the Christian Era.

Within the Long Count there were divisions, the same as we have centuries and millennia.  One of these divisions will end on 21st December 2012, which is what the completely modern New Age hysteria about the world ending is based on. The Mayans considered it cause for celebration.

4.     What is it with all the 60s? Today we base most of our maths on decimals – base 10. In some earlier systems 12s were important, and we still use dozens and inches. I don’t know or understand what the Americans use 😉 But we come from a world of decimals, calculators and importantly, zeroes.

The early Babylonians and probably even back to the Sumerians didn’t have that. They were the first to set up the counting and measuring systems we have inherited (interesting note – the earliest writing we have found seems to be records and inventories, so measuring was important). They didn’t use decimals, but a sexagesimal or 60-based system. It wasn’t a pure system and used 10 as a sub-base, it’s very interesting if you want to look at it further.

We will never know the original reason for 60, but we can guess. There are some interesting counting systems in Asia that use finger counting based on 5 x 12, but given that the Sumerians used the 10 sub-base that probably isn’t it. What is special about 60 is that it is easy to use for fractions.

Using 60 you can get simple whole number fractions for 1/2, 1/3, 1/4, 1/5, 1/6, 1/10, 1/12, 1/15, 1/20 and 1/30. Or in other words, it can be divided by 1, 2, 3, 4, 5, 6, 10, 12, 15, 20 and 30. It’s even relatively simple to divide by things like 8 because that would just be half of a quarter.

In a lot of ways fractions are easier to understand than decimals, because you can concretely see something cut in half. This is why some of the base-12 measures still hang on today, because they make it is easy to get a half, a third and a quarter. This is why a circle has 360 degrees. And even though we don’t talk about half a minute, it’s why we’ve inherited 60 seconds and 60 minutes.

5.     The picture is an ancient sundial. A stick or gnomen at the top would cast a shadow down the cone. The lines would tell the time, depending on the time of year and how accurate it needs to be there are different corrections. Sundials are the most ancient known clocks for telling the time of day.

This rather more modern sundial is in Sydney’s Botanical gardens. I had to use a correction chart to take 20 minutes off, then add an hour for daylight savings. Given that I took it at about 2:40 that’s fairly accurate.


This link will help you construct your own sundial that is the correct time for your latitude, and the video below shows you how to place it.

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{ 2 comments… read them below or add one }

Dave McArthy November 15, 2011 at 11:25 am

What a great project. How accurate will this be? My 8 year old daughter is here asking me questions.
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Deb November 15, 2011 at 12:07 pm

There is a correction factor depending on the time of year given by what is called ‘the equation of time.’ It ranges from around 16 minutes fast to 14 minutes slow and needs to be allowed for with any sundial. So you can have fun and know it’s right to within about 15 minutes, or you can make up a correction table or read it off a graph like the one on the link.


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