Sun and Water
The History of Timekeeping
Course 2 · Chapter 2 · Sun and Water
Before there were gears there were shadows and dripping water. For thousands of years, the two best ways to tell the time of day were to watch where the Sun's shadow fell and to measure how much water had run out of a vessel. Both were in use in the ancient Mediterranean and the Near East, both had clear weaknesses, and both left us something lasting: the habit of dividing the day into hours. This chapter follows the shadow clock, the sundial and the water clock, looks at the lesser candle and incense clocks, and explains why an "hour" did not always mean the same length of time.
Shadow Clocks and Obelisks
The simplest clock is a vertical post. As the Sun climbs, the shadow shortens; as it descends, the shadow lengthens, and its direction swings round. Tall stone obelisks in ancient Egypt are often described as serving this purpose among others (check how far this was their main use). Smaller portable shadow clocks, with a crossbar casting a shadow along a marked scale, survive from Egypt from perhaps the middle of the second millennium BC (check the date).
The Sundial
A sundial improves on the post by tilting the shadow-casting edge, the gnomon, so that it points at the celestial pole, parallel to the Earth's axis. A gnomon set at this angle gives the same shadow direction at the same time every day of the year, so one marked dial can show equal hours. Greek and Roman writers describe a range of dial forms, and the Roman writer Vitruvius, in his work on architecture, lists several types (check). Sundials remained common long after mechanical clocks appeared, and many were used to set the early clocks.
| Type | How it works | Where you may see it |
|---|---|---|
| Horizontal dial | A flat plate with a raised gnomon, angled for the local latitude | Gardens and parks |
| Vertical dial | Fixed to a wall, usually facing south | Churches, houses, castles |
| Equatorial dial | The dial plate is parallel to the equator, so the hour marks are evenly spaced | Modern gardens and sculptures |
| Portable dial | A small dial, sometimes with a compass for orientation | Museum collections |
Water Clocks
A sundial is useless at night and on a cloudy day. The water clock, called a clepsydra in Greek, answers both problems. In its simplest form, water drips from a hole in a vessel, and marks on the inside show how much time has passed as the level falls. Examples are known from ancient Egypt, where a surviving vessel from the reign of Amenhotep III is often cited (check), and the device was used in Greece and Rome, among other things to time speeches in the law courts (check).
Outflow type
Water leaves a vessel through a small hole, and the falling level is read against marks. Simple, but the flow slows as the level drops.
Inflow type
Water flows into a vessel with a float, which rises and points to the hour. Better designs kept the supply tank topped up to give a steadier flow.
Elaborate versions
Hellenistic and later makers added gears, figures and alarms. A famous medieval example is an elaborate water clock built in China in the late eleventh century (check the date and the maker).
The main trouble with the water clock was control of the flow. Warm water is thinner than cold, and the pressure changes as the level changes, so accuracy was always limited. Good makers addressed this with constant-level tanks, but the clock needed regular checking against the Sun.
Candles, Incense and Sand
Other devices measured time by burning or flowing at a steady rate.
- Candle clocks. A candle marked in bands burns down at a roughly even rate. They are described in various sources, with a famous account of King Alfred using candles to measure his day, which may be partly legend (check).
- Incense clocks. Used in East Asia, a trail or stick of incense burns along a marked path, sometimes with threads that drop weights at set points.
- Hourglasses. Sand running between two glass bulbs. The device appears in records from about the fourteenth century (check), and became common at sea.
How the Day Got Its Hours
The division of the day into twenty-four parts is usually traced to the Egyptians, who divided the night by the rising of certain stars and the day by the Sun, and to the Greek astronomers, who later fixed equal hours for their calculations (check the details). The division into sixty minutes and sixty seconds is usually credited to the Babylonian number system, which used base sixty (check). The sixty-based system lives on in our clocks and in angles on a circle.
Exercises
Explain in three sentences why a daytime "hour" of twelve parts between sunrise and sunset would be longer in an Edinburgh June than in an Edinburgh December.
List one strength and one weakness of each of these: a sundial, an outflow water clock and a candle clock. Which would you rather rely on for timing a night watch, and why?
Make a simple water clock: pierce a small hole near the bottom of a plastic cup, hold it over a bowl and fill it with water, and record how much has run out at 30-second intervals. Write two sentences on whether the flow stayed steady.
Quick Reference
- Shadow clock: a post or bar whose shadow length and direction mark the hour
- Seasonal hours: daylight split into twelve parts, so longer in summer
- Sundial: gnomon parallel to the Earth's axis, giving equal hours
- Equation of time: sun time differs from clock time by up to about a quarter of an hour
- Water clock: clepsydra; steady flow was the main problem
- Other timers: candles, incense, hourglasses
- Base sixty: the Babylonian number system lies behind minutes and seconds
- Check: early dates, the obelisk purpose, named makers and the account of King Alfred
Next chapter: Calendars, which covers lunar and solar years, the Roman and Julian calendars and the Gregorian reform.