Timers that need no sunshine
Sundials stop working at night and on cloudy days, and water clocks freeze or need constant attention. Two humble devices filled some of the gaps: the candle clock, which measures time by burning, and the hourglass, which measures it by trickling sand. Neither tells the time of day the way a modern clock does. Both measure how much time has passed since you started them, and for many everyday purposes that was exactly what people needed.
How a candle clock works
The principle is simple. A candle of uniform thickness and wax burns down at a roughly steady rate, so marks along its length can stand for equal periods. Light it, watch where the flame reaches, and you can estimate how long it has burned.
Early references to candle timekeeping are hard to date with confidence. A Chinese poem attributed to You Jiangu around the year 500 is often cited as an early mention. In Europe the best-known story concerns King Alfred of England, whose biographer Asser described candles used to divide the day, protected from drafts by lantern horn. Historians treat the details with some caution, since the account was written down later and the story serves partly as a portrait of a disciplined king.
Variations existed. Oil lamps with a glass reservoir marked with hour lines were used in several regions. In East Asia, incense sticks and incense trays with channels of burning powder were used to mark time, in some cases laid out with different scents to signal the hour. A thirteenth-century Arabic engineering text by al-Jazari describes ornate candle clocks with mechanisms that pushed the candle up as it burned.
Weaknesses of burning
Candle clocks were useful, but not exact. Drafts make flames burn faster, the wax recipe changes the burn rate, and the wick needs trimming. Nobody would have trusted one to the minute. They suited a monastery, a chamber or a night watch, where it was enough to know that roughly three hours had passed or that dawn was near.
The sand glass appears
The hourglass is a pair of glass bulbs joined by a narrow neck. Sand runs from the upper bulb to the lower one at a roughly constant rate, regardless of how full the upper bulb is, which is one reason sand works better than water here. When the top is empty, turn it over.
The device appears in European records and art from the fourteenth century. A fresco painted in Siena in the 1330s is sometimes cited as an early depiction, although scholars debate how to read such images. Earlier use is possible but poorly documented, and nobody knows who made the first one. The technology depended on skilled glassmaking, which grew in cities like Venice, and on fine, evenly sized, dry grains. Writers mention powdered eggshell and ground marble among the materials, along with ordinary sand.
Where hourglasses were used
Hourglasses spread because they were cheap, small, reusable and could not fall out of step with the weather. Common uses included:
- Churches, where a preacher might keep a glass nearby to pace a sermon.
- Courts and councils, where speeches could be limited.
- Workshops and kitchens, to time a process.
- Physicians and scholars, who needed some way to time an interval before better instruments existed.
Their appearance in art also made them symbols. Painters used them as emblems of mortality, along with skulls and candles.
Sand glasses at sea
The most important home for the hourglass may have been the ship. A pendulum clock fails on a moving deck, and the earliest weight-driven mechanical clocks needed a firm base, but sand keeps running when the ship pitches, although a heavy sea can disturb the flow somewhat. On sailing ships, the crew kept time by turning a half-hour glass and ringing a bell each time. This routine marked the watches that divided the day for the crew, and the custom of striking ship’s bells grew out of it.
Glasses also helped measure speed. A navigator would throw a log, a floating piece of wood on a line with evenly spaced knots, off the stern and let the line run out while a short sand glass emptied, commonly said to be about half a minute. Counting the knots that passed gave the ship’s speed, which is why speed at sea is still measured in knots. These measurements fed into dead reckoning, the method of estimating position from speed, heading and elapsed time. Navigators on a caravel ship in the age of exploration would have worked this way, and later crews on clipper ships did the same, with the glass remaining in use long after better clocks existed.
Errors added up. A glass turned late, or one with a slightly different flow, meant a slightly wrong estimate. The mistakes matter because the later push for reliable sea-going clocks, in the form of the marine chronometer, grew from these limits.
Decline and survival
Mechanical clocks gradually took over the work of marking the time of day, first in towns and later in homes. Sand glasses survived where an interval was all you wanted. In kitchens, board games and some professional settings, a small sand timer is still used, though it now sits next to quartz and digital timers.
What these timers teach
Candle clocks and hourglasses turn a steady physical process into a scale. The idea is the same as in a pendulum or a crystal: find something that repeats or flows evenly, then count it. These humble versions show the earlier stages of that idea, with the strengths of being cheap and portable and the weaknesses of drift, the need for constant tending, and a lack of any link to the position of the Sun.