December 6, 2021

I, Science

The science magazine of Imperial College

The Greenwich Prime Meridian: Jessica Norris explains the history and science behind this special landmark

Greenwich meridian Jessica Norris 1200wThis isn’t just a line across a courtyard. This is the prime meridian line, and the home of Greenwich Mean Time (GMT), the world’s timekeeping centre.

This imaginary line, running from North to South Pole with longitude 0°, marks the start of every new day. From here the earth divides into east and west, just as the equator denotes north and south. It not only enables us to navigate, but has kept the world’s clocks ticking to the same 24 hour clock for over 125 years. Though, new technology threatens to redefine time.

GMT was first proposed by Royal Astronomers to equate the sun movements to local time. Later, GMT found its way into everyday use following the need for a standardised time, due to the rise of the railway. After gaining worldwide approval in 1884, time zones around the world have been measured in relation to a reference time generated from this line.

Since 1960, atomic clocks have taken over. Using the reaction of caesium atoms, these clocks can measure the length of every second with extreme accuracy. Since then we have also realised that the Earth’s rotation slows two thousandths of a second every day. This means atomic time and Earth time, may slowly drift apart; so leap seconds have been added to keep them in sync. As such, a compromised version of GMT, Co-ordinated Universal Time, UCT, controls time.

French scientists have recently suggested a new system based entirely on atomic clocks without reference to Earth’s rotations. This would eliminate the need for leap seconds, but would also cut long held ties between the rising of the sun and time, rendering GMT obsolete. While their proposition has yet to come to fruition, it reminds us that the Greenwich Meridian line unlike the equator is an arbitrary concept. It is not determined by nature, but is proposed by science and debated by governments.

Jessica Norris is studying for an MSc in Science Communication