Scientists have determined that some 1.4 billion years ago, an Earth day — that is, a full rotation around its axis — took 18 hours and 41 minutes, rather than the familiar 24 hours. The Guardian reports: According to fresh calculations, a day on Earth was a full five hours and fifteen minutes shorter a billion or so years ago, well before complex life spread around the planet. Scientists used a combination of astronomical theory and geochemical signatures buried in ancient rocks to show that 1.4bn years ago the Earth turned a full revolution on its axis every 18 hours and 41 minutes. The number means that, on average, the length of the day on Earth has grown by approximately one 74 thousandth of a second per year since Precambrian times, a trend that is expected to continue for millions, if not billions, of years more.

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Source: Slashdot – An Average Earth Day Used To Be Less Than 19 Hours Long