The earliest clocks relied on the movements of the sun and stars. A sundial consists of a stick or gnomon that casts a shadow on a flat surface with numbers to represent hours or other measures of time.

The “merkhet,” developed by Egyptians around 600 B.C., was a nighttime clock as well as an astronomical instrument. It marked the meridian (the north-south line) by aligning with the North Star, then determining nighttime hours by noting when other stars crossed the meridian.

Water clocks, also called “clepsydras,” let water drip out of or into a vessel at a nearly constant rate. Marks on the inner surfaces measured the passage of time. Babylonia first used them in 1600 B.C., followed by Egypt, then Greece and China, then throughout Europe until the 14th century.

In China, incense clocks burned different colors or smells to mark the passage of time. Other fire-based clocks burned threads that held weights, which would drop onto a gong when the thread was burned through.

Sand glass clocks appeared around A.D. 300. By the ninth century, candles with time markings had appeared in medieval Europe.

In the 14th century, the first mechanical clocks based on a verge and foliot mechanism appeared in Europe after the invention of the escapement in about 1300. These clocks used weights to convert rotational motion into oscillating motion. They were not very accurate, losing as much as several hours per day.

In 1510, Peter Henlein invented the spring-driven precursor to the modern pocket watch, but it slowed down as the mainspring unwound and measured only the passage of hours.

In 1656, Christian Huygens, a Dutch scientist, used Gali- leo’s discovery that a pendulum swing takes the same amount of time regardless of the size of the swing to make the first clock that was accurate to about 10 seconds per day. Shortly thereafter, Huygens and the Abbe d’Hautefeuille simultaneously developed a spiral spring, which we now know as the hairspring that was the primary mechanism in watches until the 1950s.

In 1737, clockmaker John Harrison won the 23,000-pound Longitude prize (equivalent to $2.5 million in 2015) after spending 45 years building the first accurate marine chronometer. This allowed navigators to determine longitude at sea accurately by comparing the ship’s clock with the local time in port.

In 1840, Edinburgh, Scotland, clockmaker Alexander Bain created the first electric clock, and by the early 1900s clocks and personal watches had entered into mass production.

Quartz clocks using the properties of quartz crystals to vibrate at a specific frequency, first used in 1920, improved accuracy to a few seconds per month, although they did not become a consumer item until the 1970s. Quartz crystals are the basis for watches and computer clocks today.

In 1949, scientists developed the atomic clock that measured vibrations of atoms in ammonia molecules, and later cesium atoms. The latter, used by the National Institute for Standards and Technology, would lose just 1 second in 20 million years.

The most precise atomic clock to date is the optical lattice clock, which uses strontium atoms to keep time accurately to within 1 second per 15 billion years.

Richard Brill is a professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.