Every day, the Earth spins around once its axis, making sunrises & sunsets a daily feature of life on the planet. It has done so since it formed before 4.6 billion years, & it will continue to do so until the world ends, likely when the sun swells into the red giant star & swallows the planet. But it rotates at all, why?
The Earth formed out of a disk of dust & gas that swirled around the newborn sun. In this spinning disk, bits of rock & dust stuck together to form the Earth. As it grew, the space rocks continued colliding with the nascent planet, exerting forces that sent it spinning, explained Smadar Naoz, an astrophysicist at the University of California (UOC), Los Angeles. Because all the debris in the early solar system was rotating around the sun in roughly, the same direction, the collisions also spun the Earth & most everything else in the solar system in that direction.
But why was the solar system spinning in the 1st place? The sun & the solar system, formed when the cloud of gas & dust collapsed due to its own weight. The most quantity of the gas condensed to become the sun, while remaining material went into the surrounding, planet-forming disk. Before it collapsed, the gas molecules & dust particles were moving all over the place, but at a certain point, some gas & dust happened to shift a bit more in 1 particular direction, setting its spin in motion. When the gas cloud then collapsed, the cloud’s rotation sped up — just as figure skaters spin faster when they tuck their arms & legs in.
Because there is not much in space to slow things down, once something starts rotating, it usually keeps going. The rotating baby solar system, in this case, had lots of what’s called angular momentum, a quantity which describes the tendency of an object to keep spinning. As a result, all of the planets likely spun in the same direction when the solar system formed.
Nowadays, however, some planets have put their own spin on their motion. Venus rotates in the opposite direction as Earth, & Uranus’ spin axis is inclined 90 degrees. Scientists are not sure how these planets got this way, but they have some ideas. For the Venus, maybe a collision caused its rotation to flip. Or just like the other planets, maybe it began rotating. Over time, the sun’s gravitational tug on Venus’ thick clouds, combined with friction between the planet’s core & mantle, caused the spin to flip. A 2001 study published in Nature suggested that gravitational interactions with the sun & other factors might have caused Venus’ spin to slow down & reverse.
In the case of Uranus, scientists have suggested that collisions — one huge crash with a big rock or maybe a 1-2punch with two different objects — knocked it off kilter, Scientific American reported.
Despite these kinds of the disturbances, everything in space rotates in one direction or another. “Rotating is the fundamental behavior of objects in the universe,” Naoz said.
Asteroids rotate. Stars rotate. Galaxies rotate (it takes 230 million years to complete one circuit around the Milky Way for the solar system, according to the NASA). Some of the fastest things in this universe are dense, whirling objects called pulsars, which are the corpses of massive stars. Some pulsars, which have a diameter about the size of a city, can spin 100’s of times per second. The fastest one announced in Science in 2006 & dubbed Tarzan 5ad, rotates 716 times per second.
Black holes can be even faster. One, called GRS 1915+105, may be spinning anywhere between 920 & 1,150 times per second, a 2006 study in the Astrophysical Journal found.
But things slow down, too. When the sun formed, it spun once around its axis every 4 days, Naoz said. But today, it takes about to a month for the sun to spin once, she said. Naoz said, its magnetic field interacts with the solar wind to slow its rotation.
Even Earth’s rotation decelerates. Gravity from the moon pulls on the Earth in a way that ever so slightly slows it down. A 2016 analysis in the journal Proceedings of the Royal Society A of ancient eclipses showed that the Earth’s rotation slowed by 1.78 milliseconds over a century.
So, while the sun will rise tomorrow, it just may be a tad late than today.