The tablets, which document the precise time of solar and lunar eclipses about 2,500 years ago, show the Earth is spinning slightly slower than at the time of the Babylonian astronomical observations.
If an accurate clock had been running for the past million days - about the time of the beginnings of Babylonian interest in astronomy - it would now be running several hours faster than the actual time judged by the positions of the Sun or stars.
Astronomers who have analysed the solar and lunar eclipses described in tablets in the British Museum calculate that the gradual slowing of the Earth's rotational spin means that day-length has been gradually increasing by about 1.7 milliseconds a century. The Babylonian day was one-twentieth of a second shorter than today's day.
Richard Stephenson of Durham University and Leslie Morrison at the Royal Greenwich Observatory have in addition analysed Chinese documents dating from between AD500 and AD1200 which also demonstrate that the days are getting longer. They will present their findings at next week's meeting of the Royal Astronomical Society at Leicester University.
Mr Morrison said the Earth's spin was slowing down because of the drag imposed on the planet by the Moon. Ultimately the days would get so long that they would coincide with a lunar month.Reuse content