From the earliest periods man has used some form of time measurement, be it only the seasons of the year or phases of the moon. This was all that was needed in simple nomadic or agricultural communities and precise enough for their daily needs. As people began to congregate in villages, and forms of religious ceremonies began, more refined methods of time measurement were needed. Civilisation in early times was concentrated around the Mediterranean, where there was lots of sunshine and water aplenty for the then relatively small populations. Here time keeping was developed along two main lines - from the shadow stick, probably the earlier, and then the water clock. Although crude by modern day standards, sundials and water clocks were eventually developed to give surprising accuracy.
Figure 1 shows the sundial in its simplest form - the shadow stick.
For pupils, this raises many
questions on the apparent movement of the Sun and thus the rotation of the
Earth. Time was judged from the position and length of the shadow. A deluxe
model would have a pebble to place at the shadow's end. This was, and is, a
method used by many nomadic peoples. But ever more inventive and more accurate
means of using the Sun to track time were soon devised.
Some of these, like the Egyptian T-stick (Figures 2 and 3), were portable.
But probably the most accurate, early, devices were those designed and used
by the Chaldeans. The Chaldeans, the tribe of Moses, are the first people
attributed with dividing the day, and the night, into twelve hours each. Their
sun-clocks took the form of a hole chipped out of a large rock, with the hour
lines converging at the base of the stick or peg. Making your own T-stick is
easy. All you need is two house bricks (no jokes please!) a stick about a metre
long and a strip of paper (Figure 3). 
Figures 4 below and 5 each show simple ways of making other sun-clocks.
In about the same era the Chinese were dividing the day into 100 units. Well into the 19th century the peoples of the East divided the day differently from those of the West
Probably one of the earliest methods of giving a repeatable measurement of
time was a 'sinking bowl ' (Figure 6).
This manner of timing, a crude
water clock, is still in use but nowadays mainly for goods 'on hire'. In rural
communities in developing countries the goods on hire may be an iron for
pressing clothing, hand tools or even farm implements. The method uses a large
basin, or bowl, filled with water. A smaller bowl with a hole in the bottom is
placed on the surface of the water in the basin. The water gradually fills the
smaller bowl and time's up when it sinks.
As material standards of living in civilizations grew, so the purely
functional became more and more ornate. From simply telling the passage of
unspecified periods of the day or night, the peoples of the Middle East moved on
to divide the day into more or less the same periods we use today and clock
designs become ever more sophisticated (Figure 7 )
For example, the Ancient Greeks
introduced alarm clocks using water, novelty clocks, with whistling mechanical
birds, and even an early form of the public town clock.
A messy but enjoyable class activity is the making of DIY water clocks.
Although the sinking bowl is a simple method, the challenge would be to time an
event over an agreed period of time. For example. two or three minutes is a good
period to choose if you are using the components shown in Figure 8.
Does the shape of the sinking
bowl make a difference? Does the position or the diameter of the hole make a
difference? No doubt the children will offer other ideas.
More sophisticated styles are shown in Figure 9
We use pill boxes scrounged from
our local pharmacies. This is a good source of useful containers, but be quick
these boxes are soon to be replaced by other packaging. There are probably many
other designs for simple water clocks and perhaps a brainstorming session is the
best stimulus to further work?
Water clocks also lend themselves to extension into other areas of science, technology and maths. The more obvious of these are floating and sinking, estimating, producing a scale and how about water pressure and energy? The water clocks in the drawing show the water dripping out. In reality it will be forced out in a jet to begin with and slowly diminish to a drip. Water pressure from the model clock could then lead on to a study of simple hydraulics and pneumatics.
Despite early fascination with time-keeping by water or candle, undoubtedly the most accurate early time tellers were sundials. Time was measured from noon one day to noon the next, that is for one revolution of the Earth on its axis. Many medieval churches had sundials long before the more usual steeple clocks. A few old churches have simple marks gouged in the stone of the walls with a hole where a peg would be inserted to cast its shadow to indicate the various times of worship. These are known as scratch dials.
But sundials measure only LOCAL apparent time. A clock in Edinburgh shows exactly the same time as a clock in Glasgow. But sundials in those same cities will always show a different time. A sun-clock in Edinburgh will appear to be running ahead of Glasgow time. Why is this? If the Earth completes one revolution in 24 hrs then 1 hr. will equal 360/24 = 15ø. The Greenwich meridian is taken as 0ø of longitude, a large imaginary line drawn around the Earth and passing through both the north and south pole. So every degree we travel west of this line we have to add about 4 minutes and for every degree we travel east of the line we have to subtract 4 minutes.
Early mechanical clocks were bad timekeepers and were often adjusted by the local sundial. As the timekeeping of these clocks improved so the shortcomings of some of the cruder local sundials became apparent, However country people, ever suspicious of townsfolk, took a long time to accept the more accurate and universal time measurement. A clockmaker from a village near Chester was the butt of the village in asserting that his clocks were right and the Sun was wrong. A memorial to him reads;
"Here's the cottage of Peter that cunning old fox, Who kept the Sun right by the time of his clocks."
That's enough of the boring bits!
Figure 10 shows how we can
make our own sundial. It should be a fairly straightforward exercise. The angle
of the gnomon (gnomon is a Greek word meaning indicator) in our model, will
serve for most places in Britain and give a reasonable approximation of the
time. Remember to ask the children why the sun-clock and any of their watches
are one hour different in the Summer. (And only then make the adjustment for
British Summer Time). You may also wish to make an approximate adjustment for
longitude. Perhaps the class could make up a table of time conversions for the
various cities of Scotland?
SSERC can Supply full size paper masters of the sundial design together with a sheet of instructions. We also have a design for a device for telling time by the stars (a nocturnal). If you wish to have a copy of these, please send an A4 self addressed and stamped envelope to:
SSERC, 24 Bernard Terrace, Edinburgh EH8 9NX
The motto Carpe Diem was to be found carved on many a sundial and engraved on the faces of mechanical clocks well into the 19th century. Roughly translated it means seize the opportunity. But, not surprisingly, this was usually rendered by the old clockmakers as: make good use of the day.
And, finally, a little poem, which in this age of quartz timepieces may well puzzle present day pupils:
There was a man, he had a clock, his name was Mr. Mears,
Every night he wound that clock for five and forty years
And when at last that clock turned out an eight-day clock to be
A madder man than Mr. Mears, I never hope to see!
Further reading: Sundials & Timedials, Jenkins G. and Bear, M. ISBN 0 906212 59 6.