Our ancient ancestors divided their lives into days, following the natural rhythm of the rising and setting Sun. This system worked well for millennia but by the 20th century, scientists made a big discovery: the Earth is actually a terrible timekeeper.
Why? It turns out that no two rotations – no two days – are ever exactly the same length.
There are many factors that can affect the Earth’s spin, including earthquakes. The 2011 quake in Japan – the one that triggered the Fukushima nuclear accident – sped up Earth’s rotation by 1.8 millionths of a second.
The Moon’s gravity also contributes to all of this. It tries to hold our oceans in place, but the Earth continues to turn beneath them regardless. This tidal friction robs the Earth of a little rotational energy, meaning the day is getting longer by about two-thousandths of a second (2 milliseconds) per century.
Scientists have also discovered that so-called megastructures built by humans can also affect the Earth’s rotation. Take the 185m (about 600 feet) tall Three Gorges Dam. Spanning the Yangtze River in Hubei province, Central China, it is the largest dam in the world and is over 2,300m (7,500 feet) in length.
Its vital statistics are dizzying. It was made using 28 million cubic metres of concrete and enough steel to build 63 copies of the Eiffel Tower. It took 40,000 people 17 years to construct, at a total cost of $37 billion (£28 billion). The dam can hold 40 billion cubic metres of water – about 16 million Olympic-sized swimming pools.