When you travel by plane, you switch your smartphone to flight mode. When you on at your destination, it automatically shows you the local time. It seems normal to us that our phones ‘know’ the exact time and place where we are, but things were not always that easy.


Think back to the Age of Discovery. When explorers began navigating across the globe, sea travel was incredibly dangerous. Many ships were lost because the sailors did not know their exact location. The ancient Greek astronomer and mathematician Hipparchus first suggested putting a grid of lines on a map to establish the location of specific places on earth in about 150 B.C.

However, the idea was not developed until the 15th century, when understanding latitude and longitude became a question of life or death.



Knowing time and place was not only important for sailors. Before the second half of the 19th century, almost every town in the world had its own local time. With the expansion of the railway, postal systems and communications networks during the 1850s, the worldwide need for an international time standard became imperative.


To collect accurate astronomical information for navigation, cartography and time-keeping, King Charles II commissioned Britain’s first state-funded scientific institution – the Royal Observatory – in 1675. It was built in Greenwich, where you can visit it today.


If you divide the world into a grid of lines running north to south (meridians), and lines running east to west (parallels), you can make astronomical observations from those lines and create a very accurate map of the sky. Obviously you need a starting point for those lines. The equator is the obvious choice for latitude 0°, but zero degrees longitude (the Prime Meridian) could be anywhere. At the time the decision was taken, Britain ruled the waves, and so the Royal Observatory at Greenwich became the reference point for longitude.


An international conference in 1884 officially established the Prime Meridian through Greenwich and ever since it has served as the reference line for Greenwich Mean Time (now known as Universal Time), from which all other time is measured.



If you visit the Royal Observatory, you can take a selfie of yourself standing on the Prime Meridian with one foot in the East and one foot in the West. And just think, without that important line, your smartphone would not now be able to tell you where you are and what time it is!