Why does earth rotate in a clockwise direction? Did a large meteor collide with earth causing earth to rotate? If that is what happened, could another, larger meteor cause earth to reverse its rotation?

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    $\begingroup$ The earth spins clockwise because "clockwise" was first defined as the direction the shadow on a sundial moves, which is caused by the Earth's rotation. ;) $\endgroup$ – Mason Wheeler May 1 '14 at 0:06
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    $\begingroup$ Also, we in southern hemisphere know that all you northern hemispheries are wrong - the earth spins anti-clockwise. We just have to put up with your clocks because of cultural imperialism and all that :P $\endgroup$ – naught101 May 1 '14 at 6:34
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    $\begingroup$ Are you asking why Earth spins at all, or why it spins in a specific direction? $\endgroup$ – Daniel Griscom May 6 '16 at 11:40
  • $\begingroup$ @DanielGriscom, Both? $\endgroup$ – Pacerier Aug 23 '17 at 12:39

Earth's Spin

Earth rotating clockwise is the result of a chain reaction that started when Earth's star formed as the result gas clouds collapsing. During the collapse of the gas, one direction was shorter and a disc formed. Due to the law of conservation of angular momentum, the disc gained an overall spin, which was passed to all the objects of notable mass within its solar system; these objects are commonly known as planets. As a result, all planets within a given solar system have the same spin as the star in it to start. That said, the axis of a planet may do a 180-degree flip on its axis at some point, and if this happens, like Venus, it would spin counter to its native spin; in fact, in the case of Earth, it appears given there appears to be evidence that it's has flipped in the past, that the Earth's axis has already flipped at least twice, since it's current back in sync with the Sun's spin.

Might be worth noting that all rotating bodies that rotate clockwise when viewed from the southern hemisphere rotate and counter-clockwise when viewed from the northern hemisphere. Also, since the Sun is not solid on the surface, it does not rotate as a solid body; meaning it rotates faster at its equator and slower at its poles.

Earth's Axis

As for the reasoning behind, the axis of the rotation, as pointed out in other answers, this is the result of Earth's past impacts with objects of enough mass to produce the shift from a zero offset from the stars axis, to the current axis of the rotation. Future impacts would be able to change the rotation.

Will the Earth's axis flip?

Scientists have found evidence that the Earth might have flipped over in the past, completely shifting the orientation of its poles. The theory has been around for years; that a large mountain range or supervolcano might unbalance the spinning Earth. Over the course of millions of years, the Earth would change the orientation of its axis until the object was balanced at the equator again.

Current Axis of the Planets in Earth's solar system

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    $\begingroup$ NB some planets do exhibit retrograde spin - even within our solar system. Venus is a counter-example; Uranus is ambiguous. Source. $\endgroup$ – kaberett Apr 30 '14 at 21:46
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    $\begingroup$ +1 @kaberett: Awesome, thanks; reviewed another source - "Why Venus Spins the Wrong Way" - and updated the answer too. $\endgroup$ – blunders Apr 30 '14 at 21:56
  • $\begingroup$ The source for the opening paragraph runs counter to current theory of terrestrial planet formation. It ignores that angular momentum is not conserved thanks to gravitational torques and collisions. Asteroids are also subject to the YORP effect. In fact, the obliquities of the asteroids is pretty close to randomly distributed. $\endgroup$ – David Hammen May 1 '14 at 14:44
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    $\begingroup$ @blunders - Lhotka, Souchay, and Shahsavari Obliquity, precession rate, and nutation coefficients for a set of 100 asteroids, A&A 556:A8 (2013) Those 100 asteroids have random obliquties. Obliquity ranges from 0 to 180 degrees. Slivan, Spin vector alignment of Koronis family asteroids, Nature 419:49 (2002). Most members of this family of asteroids have retrograde rotation. $\endgroup$ – David Hammen May 1 '14 at 15:02
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    $\begingroup$ Regarding the planets, Laskar and Robutel, The chaotic obliquity of the planets, Nature 361:608 (1993). "The obliquity of Mars is still in a large chaotic region, ranging from 0 to 60 deg. Mercury and Venus have been stabilized by tidal dissipation, and the earth may have been stabilized by capture of the moon. None of the obliquities of the terrestrial planets can therefore be considered as primordial." (Emphasis mine). $\endgroup$ – David Hammen May 1 '14 at 15:03

If you ascribe to the giant impact hypothesis, which most scientists do, whatever rotation the Earth had prior to the formation of the Moon was lost thanks to that 'giant impact'. That was a massive impact involving a Mars-sized body colliding with the proto-Earth. A "little" impact wouldn't change the Earth's rotation by much at all. Note very well: Something that could entirely wipe out life as we know it would still be "little" in this regard. Another big oblique whack by a Mars-sized body would be needed to make a sizable dent in the Earth's rotation axis.

If you look at the nine eight planets, it's only Jupiter that reliably retains its primordial rotation. Whatever rotation rate / rotation axis Mercury and Venus had in the distant past has been wiped out thanks to tidal interactions between those planets, the Sun, and Jupiter (a.k.a. the 600 pound gorilla). Mars' rotation is notoriously chaotic. Uranus has been tilted by ~90 degrees by something, either a big whack or (once again) tidal interactions. Neptune and Saturn have obliquities of about 30 degrees. Those deviations may be a result of tidal interactions, or maybe of a lesser whack. Scientists don't know. There are lots of different hypotheses, but all have problems and none has taken hold yet.

The Earth? Whatever formed the Moon, whether by giant impact or something else, it almost certainly wreaked havoc with the Earth's primordial rotation.

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    $\begingroup$ "If you look at the (nine) eight planets" Ya, I'm still really missing Pluto too. :'( $\endgroup$ – Azzie Rogers May 1 '14 at 14:35
  • $\begingroup$ I don't think tidal forces can explain the tilt of Uranus or the retrograde rotation of Venus. $\endgroup$ – equant Jan 24 '15 at 6:05

Looking down from the North Pole, it spins anti-clockwise: https://en.wikipedia.org/wiki/Earth%27s_rotation. The spin is a relic from the angular momentum of the primordial cloud which became our solar system. If the lunar impact hypothesis is correct, this would have been altered significantly at the time.

  • $\begingroup$ Please don't repeat other answers $\endgroup$ – Jan Doggen Jul 29 '16 at 9:03
