How Time Is Measured [4min]

American proposals to redefine how time is measured could force Greenwich Mean Time into irrelevance. According to the Observatory's Curator of Horology -- Jonathan Betts -- the meridian's role in providing the basis of time-keeping across the world is now under threat. The Prime Meridian, which runs through Greenwich in south-east London, became the basis for the world's time-keeping in 1884. The decision stemmed from the work 200 years previously of the first English Astronomer Royal, John Flamsteed, who calculated that the Earth rotated on its axis once every 24 hours. That discovery meant that time could be defined by the Sun's position relative to a point on the Earth -- in this case the meridian running through Greenwich. But it turned out that the Earth's rotation is ever so slightly slowing down and, since 1972, that anomaly has been corrected by adding a so-called leap second when necessary. Now US scientists want to add a leap second every 18 months or so.