Why the day is 24 hours long: Astrophysicists reveal why Earth's day was a constant 19.5 hours for over a billion years

A team of astrophysicists at the University of Toronto (U of T) has revealed how the slow and steady lengthening of Earth's day caused by the tidal pull of the moon was halted for over a billion years.

They show that from approximately two billion years ago until 600 million years ago, an atmospheric tide driven by the sun countered the effect of the moon, keeping Earth's rotational rate steady and the length of day at a constant 19.5 hours. Without this billion-year pause in the slowing of our planet's rotation, our current 24-hour day would stretch to over 60 hours. The study describing the result, "Why the day is 24 hours long; the history of Earth's atmospheric thermal tide, composition, and mean temperature," was published today in the journal Science Advances . Drawing on geological evidence and using atmospheric research tools, the scientists show that the tidal stalemate between the sun and moon resulted from the incidental but enormously consequential link between the atmosphere's temperature and Earth's rotational rate.

The paper's authors include Norman Murray, a theoretical astrophysicist with U of T's Canadian Institute for Theoretical Astrophysics (CITA); graduate student Hanbo Wu, CITA and Department of Physics, U of T; Kristen Menou, David A. Dunlap Department of Astronomy & Astrophysics and Department of Physical & Environmental Sciences, University of Toronto Scarborough; Jeremy Laconte, Laboratoire d'astrophysique de Bordeaux and and a former CITA postdoctoral fellow; and Christopher Lee, Department of Physics, U of T. When the moon first formed some 4.5 billion years ago, the day was less than 10 hours long. But since then, the moon's gravitational pull on the Earth has been slowing our planet's rotation, resulting in an increasingly longer day. Today, it continues to lengthen at a rate of some 1.7 milliseconds every century.

A team of astrophysicists have shown that from approximately two billion years ago until 600 million years ago, an atmospheric tide driven by the sun countered the effect of the moon, keeping Earth's rotational rate steady and the length of day at a constant 19.5 hours. Credit: Kevin M. Gill Read more

A power spectrum of the Earth's atmosphere. The x-axis is wavelength, e.g. 5 is a wavelength of one fifth of the circumference of the Earth for a wave traveling west to east, and -5 means the same, but for waves traveling east to west. The y-axis is frequency, in cycles per day, e.g. 2 means two cycles per day, or 12 hours. The thin horizontal brown lines show the Solar forcing at one, two, three, etc. cycles per day (24 hours, 12 hours, 8 hours period, and so forth). Credit: Sakazaki & Hamilton Read more

Murray and his collaborators relied on geologic evidence in their study, like these samples from a tidal estuary that reveal the cycle of spring and neap tides. Credit: G.E. Williams Read more