Moon's birth changed the length of days on Earth
Courtesy: New Scientist

THE collision that formed our moon may have defined the length of our planet's day and set the direction in which it spins.

The moon is widely thought to have formed after an object roughly the size of Mars crashed into the Earth 4.5 billion years ago, throwing up a cloud of debris that eventually coalesced into a rocky sphere. Robin Canup of the Southwest Research Institute in Boulder, Colorado, wanted to find out if this process was influenced by the spin of the Earth at the time - something previous models of the moon's formation did not take into account.

Canup built a computer model that used as many as 120,000 pieces of virtual rock to simulate the two colliding bodies. Her model showed that the Earth's rotation beforehand may have been very different to what it is today. Prior to the impact, the Earth's axis of rotation may have been steeply tilted, and the planet would have spun much faster, with a day lasting as little as 4 hours. The model also showed that the direction in which the Earth spins could have been reversed by the impact.

If the Earth had previously rotated in this way, its current spin and that of the moon can be accounted for, Canup says. What's more, if the Earth had once spun faster, enough material would have been thrown up by the impact to make the moon the size it is.