Measuring time

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

Why was Greenwich chosen as where east meets west at Longitude 0°?

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

Ever wondered whether midday or midnight is 12 a.m. or 12 p.m.?

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.

Next time someone asks you the time, you may enquire if they want to know the atomic, universal, civil, local, solar or sidereal time…

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