Clocks can be split into two broad categories based on how they display information.

Analog, aka mechanical, clocks use moving hands to indicate the current time. Digital clocks, on the other hand, display time as a set of numbers, typically via an LCD or other electronic screen.

(It’s technically possible to have an electronic clock with an analog display, but it’s very rare – we’ll treat analog and mechanical as synonyms.)

Every clock needs three fundamental parts:

1. Timekeeping mechanism: a way to keep accurate track of the passage of time.
2. Energy source: a way to provide energy for the motion of the other various components.
3. Display: shows the user what the current time is.

In the most basic terms, a clock is a device that uses energy to display time, regulated by a timekeeping mechanism.

Consider a sand-filled hourglass – a very simple analog clock. Its energy source is the pull of gravity, its display is the amount of sand held in each half, and its timekeeping mechanism is the relatively constant rate at which sand flows through the narrow opening between the two halves.

In more sophisticated analog clocks, the three fundamental parts are connected via gears, pulleys, and other mechanical systems.

In modern clocks, the mechanical components may be replaced by wires and electrical currents. There are more possible configurations than we could ever cover, so let’s take a closer look at one particular type of clock.

Pendulum clocks are arguably the first modern clocks.

A pendulum, you’ll remember, is a weight hung from a fixed point and allowed to swing back and forth – you can make a simple one by dangling a pair of earbuds.

At the turn of the 17th Century, Italian scientist Galileo Galilei’s experiments in physics led him to discover this unique feature of pendulums: one will always take the same amount of time to complete a full swing.

This is true even as air resistance and other factors slowly reduce how far a pendulum moves with each swing, right up until the moment it stops.

He immediately recognized the potential of pendulums for timekeeping inside a clock mechanism, but it wasn’t until 1656 that Dutch scientist Christiaan Huygens, inspired by Galileo’s work, designed a working pendulum clock.

Huygens didn’t have the skill to implement his design, so he hired professional clockmaker Salomon Coster to build it.

Let’s look at how pendulum clocks work according to the three-part breakdown (timekeeping mechanism, energy source, and display) we used above.

Energy Source: Like an hourglass, the first pendulum clocks used gravity to generate energy through a system of weights hanging from pulleys. Turning a key would “wind” the clock, lifting the weights and storing potential energy by holding the weights up against gravity.

Timekeeping Mechanism: A pendulum and a component called an escapement regulate the rate at which energy from the weights is released. The escapement includes a notched wheel that ensures it can only move in discrete steps, or “ticks.”

Each completed swing of the pendulum releases one tick on the escapement, which in turn allows the weights to drop a tiny bit.

Display: The hands of the clock are connected via gear train to the rest of the mechanism.

When the escapement releases one tick of energy, the gears turn and the hands move the correct amount.

If you assume a one-second pendulum swing, which was common in later designs, every tick ends up moving the seconds hand exactly 1/60th of the way around the clock face.

In the simplest terms: energy is stored using raised weights, then released at a precise rate by timekeeping pendulum mechanism, which turns the hands of the display to show the current time.

It may have occurred to you that a pendulum would not work in a watch, which is constantly moving around.

Instead, mechanical watches use mainsprings and balance wheels. Spring-driven clocks actually predate pendulum clocks by about 200 years, but were considerably less accurate.

The mainspring is wound tight to store energy. The balance wheel is a specially weighted disk; once set in motion it rotates back and forth at a regular rate to act as a timekeeping mechanism.

Today, the most common clocks are quartz clocks, named for their timekeeping mechanism.

Quartz crystals are piezoelectric: if you run an electric current through them, they vibrate at a specific rate. Notice a trend? Almost any process with a specific rate can act as a timekeeping mechanism.

A typical modern battery-powered clock sends a miniscule electric current through a quartz crystal, which is set in a circuit that acts like an escapement: it releases small amounts of electricity from the battery at regular intervals dictated by the vibration of the quartz.

Each regular “ticks” of electricity either powers a motor to move analog hands or controls the output to a digital screen.

You may have seen or heard of an atomic clock.

They’re almost entirely digital, so we won’t get into details, but the basic principles of how they work are the same as the clocks above. The big difference is their timekeeping: they are built around a mechanism that measures the precise rate at which cesium atoms release energy after being “excited” by radio waves.

The International System of Units standardized its definition of one second on the properties of cesium in 1967, and it has remained the standard since.