Time and Its Measurement

The measurement of time grew out of the regular motions of the sky. The rotation of Earth on its axis produces the cycle of day and night and defines the day, while Earth's orbit around the Sun gives the year. The phases of the Moon provided an early basis for the month. Dividing the day into hours, minutes and seconds gave a finer framework, allowing events to be ordered and durations compared with precision.

Early devices tracked time by natural means. A sundial uses the shadow cast by a fixed pointer, which moves across a marked surface as the Sun crosses the sky, indicating the hour from the shadow's position. Water clocks, hourglasses and later mechanical clocks improved on this by measuring time independently of the weather and the Sun. Modern timekeeping relies on extremely stable oscillations, with atomic clocks providing the most accurate standard.

Because the Sun is overhead at different moments in different places, local time varies with longitude. To coordinate timekeeping, the world is divided into time zones, and Greenwich Mean Time (GMT), the mean solar time at the Greenwich meridian in London, long served as the reference from which other zones were measured. Coordinated Universal Time later became the precise international standard, but the Greenwich meridian remains the line from which longitude and the world's time zones are reckoned.

Frequently asked questions

What is Greenwich Mean Time?

The mean solar time at the Greenwich meridian in London. It long served as the worldwide reference from which other time zones were measured.

How does a sundial tell the time?

A fixed pointer casts a shadow that moves across a marked surface as the Sun crosses the sky; the shadow's position indicates the hour.

Why are there different time zones?

The Sun reaches its highest point at different moments depending on longitude, so the world is divided into zones to keep local clocks roughly aligned with the Sun.