Measuring time

Keeping track of time helps us make sense of our world. Read all about the history of timekeeping, find out who the pioneers of timekeeping were and learn about the instruments they created. Plus, discover what gems are on display at the Time & Longitude gallery of the Royal Observatory Greenwich.

While days and years are (fairly) neat astronomical events, what explains months, weeks, hours and minutes?

The Royal Observatory Greenwich is famous world-wide as the home of Greenwich Mean Time (GMT). But what is GMT and why is it so important?

Find out what longitude is and what makes it so hard to measure.

The story of Astronomer Royal George Biddell Airy and the remarkable Airy Transit Circle telescope he designed at the Royal Observatory Greenwich.

Why was Greenwich chosen as the home of the Prime Meridian and Greenwich Mean Time, and what do those terms mean?

How do we calculate the precise moment of sunrise or sunset? And exactly when is twilight?

To avoid confusion, the correct way to express 12 o'clock is 12 noon or 12 midnight.

Because the Earth takes a little over 365 days to orbit the Sun, we need to make adjustments to keep the seasons from drifting: leap years and even leap seconds.

For thousands of years, the sundial has told the time and divided the day.

How do you know that your watch, clock or phone is telling exactly the right time? At one time, the only way was to look to the roof of the Observatory.

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