Candle Clock: Telling Time with Fire and Wax
Before clocks ticked, some people simply watched a marked candle burn down. Done carefully, it worked better than you might expect.
The simplest clock you can make
A candle clock is exactly what it sounds like: a candle marked along its length so that you can read how much time has passed from how far it has burned. If the candles are made the same way every time, each mark stands for roughly the same span.
It needs no gears, no water and no sunshine. That made it attractive indoors and at night, the two situations where a sundial is useless. Guinness World Records lists the candle clock as the first indoor clock.
The weakness is just as obvious. A flame is easily disturbed. A draft, a thicker wick or a softer batch of wax can make a candle burn faster or slower, and every bit of error adds up over the night.
See also: how the hour became a standard unit on OnlineClock.app.
King Alfred's six candles
The best-known description of a candle clock comes from Anglo-Saxon England. Asser, a Welsh churchman at the court of King Alfred the Great, wrote a life of the king that explains why Alfred needed one.
Alfred, Asser says, had vowed to give half his time, by day and by night, to God. Keeping that promise meant measuring the hours, but darkness hid the night and clouds and rain often hid the sun.
His answer was wax. According to Asser, Alfred's clerks weighed out wax against coins until it balanced 72 pennies, and made it into six candles of equal weight, each twelve inches long. Burned one after another before the holy relics that traveled with the king, the six together lasted a full day and night.
Guinness World Records, describing the same clock, says each candle lasted four hours and was marked at 20-minute intervals, which works out to one mark per inch. In the Guinness version, Alfred used it to share his day between study, prayer, royal business and rest.
Weighing the wax rather than eyeballing it was clever quality control for a ninth-century court. Candles of equal weight and length should burn at a similar rate, so anyone in the household could read the time by counting the inches left.
The lantern that fixed the drafts
Asser is frank that the system did not work at first. Wind blew in through doors and windows, through gaps in walls and wooden panels, and through the thin cloth of the king's tents. In a draft, a candle sometimes burned out early, and the count of hours went wrong.
The fix was a lantern made of wood and ox horn. Horn shaved thin enough lets light through, so the candle could still be seen, but the wind could not reach the flame. A small door, also made of horn, let a fresh candle be put in when the old one had burned away.
With the lantern in place, Asser says, the six candles burned for exactly 24 hours. Whether the count was really that precise is another matter, but the problem he describes is real for anyone who has watched a candle gutter in a breeze.
How much should we trust the story?
Asser's biography is our main source, and historians have argued about it for a long time. The preface to Albert S. Cook's 1906 English translation notes that its authenticity was challenged in 1841, 1876–77 and 1898.
Two leading scholars of the day, Plummer and Stevenson, defended it. Plummer concluded that the work could not be later than 974, and Stevenson found no anachronism proving it a later forgery. Both still urged caution: Stevenson pointed to remaining difficulties and possible exaggeration, and Plummer warned about later additions and Asser's imaginative style.
So the Alfred candle clock is best treated as a well-attested tradition rather than a museum object. No candles or lanterns from it survive.
Al-Jazari's mechanical candle clock
Candles could also power something more theatrical. The engineer Badi' al-Zaman ibn al-Razzaz al-Jazari, who worked in Mesopotamia, wrote a treatise on ingenious mechanical devices in 1204 that includes a candle clock.
The Isabella Stewart Gardner Museum in Boston gives al-Jazari's dates as 1136 to 1206, so he wrote the treatise near the end of his life. The museum owns a page from an illustrated Egyptian copy made in 1354, a sign that his book was still being copied 150 years after he died.
As the museum describes it, a yellow candle burns down and releases metal balls one at a time. Each ball drops into a red sling on the arm of a small swordsman, and the extra weight brings his sword down to trim the candle's wick.
The ball then rolls on into a falcon at the base, marking that an hour has passed. The hourly wick trimming is a neat touch: the clock tended its own flame, a chore a human timekeeper would otherwise have to remember.
The page is painted in gold and colors on paper and measures 32 by 25 centimeters. The museum suggests that Isabella Stewart Gardner, who loved Venice and its links with the Middle East, may have included Islamic paintings in her collection to show how much Islamic art and learning fed into the Renaissance.
Why candle clocks faded
Candle clocks ask a lot of the user. Every candle must be made to the same standard, kept out of drafts, lit on time and replaced the moment it burns out. Miss a changeover and the count is lost.
They also consume themselves. Alfred's system used up six large, carefully weighed candles every single day, so the clock had to be rebuilt from fresh wax daily.
Mechanical clocks removed most of that labor. Yet the candle clock's basic idea, a slow, steady process with marks along the way, never quite disappeared. It is the same thinking behind the incense clocks of East Asia, which measured time by a slow-burning trail rather than a melting candle.
See also: the wider history of timekeeping on OnlineClock.app.
Questions people ask
How did a candle clock work?
A candle of known size and weight was marked along its length. Because it burned at a fairly steady rate, the marks showed how much time had passed.
Did King Alfred really invent the candle clock?
He is the most famous early user, through Asser's biography, but the text does not claim he invented the idea. Some historians have also questioned how reliable Asser's account is.
How long did Alfred's candles burn?
Asser says six candles burned for 24 hours in total. Guinness World Records puts each candle at four hours, with marks every 20 minutes.
Why did Alfred use a lantern?
Drafts made the candles burn too fast. A lantern of wood and thin ox horn kept the wind off the flame while letting the light show.