The day is something which appears to be very concrete among all objects in this world. Midnight comes right after 11:59; time moves for 24 hours, and then the process repeats itself. However, the Earth itself doesn’t revolve uniformly.
The length of an Earth day can vary by a few milliseconds over periods of decades. The change is far too small for people to notice in everyday life, but precise measurements of Earth’s rotation can detect it. Scientists have long suspected that processes deep inside the planet are responsible for these fluctuations.
