This is a fascinating account of the modern tools we use to keep track of time, and the growing problems we face as our drive towards time accuracy conflicts increasingly with the imperfections of our terrestrial and celestrial home.
The frequency of the transition of strontium, for example, is 444,779,044,095,486.71 Hz. A strontium clock developed in the US would only have lost a second since the earth began: it is accurate to a second in five billion years.
The scientists at NPL reckon optical clocks that keep time to within one second in 14 billion years are on the horizon – that’s longer than the universe has been around.