In a sense, it already has. A day on Earth is actually 23 hours and 56 minutes long, but we live our lives according to a 24-hour clock. So when we flip the clocks ahead one hour in the Fall and one hour in the Spring, we're playing with Mother Nature's timekeeping conventions and giving ourselves an extra minute of daylight at night during which we otherwise wouldn't have any.
And it turns out there's evidence that flipping time zones may not be such a good idea after all: studies show that such changes can contribute to heart attacks for people who work night shifts or sleep less than six hours per day.
But what would happen if the Earth really did begin spinning backwards?
The first thing to consider is whether the reversal would happen suddenly or gradually. The answer is unclear. One school of thought, advanced by scientist Albert Einstein, suggests that if Earth's spin were to reverse, the change would happen rather slowly over thousands of years. Another school of thought says that Earth's spin could reverse quickly enough to cause stormy weather for us all within a few weeks. It is not known how fast the earth spins now, but scientists agree that if it ever stopped spinning completely—which it won't—it would take about one week for the earth to break up into pieces so small they couldn't be distinguished from space dust.
Though as predicted some things wouldn't change. The sun would still rise in the east and set in the west. Two objects dropped off of a tall building would hit the ground at the same time no matter which direction they were dropped. And unfortunately, gravity would still pull us toward the center of the Earth.
The world's oceans would move to new locations on Earth, though. Where they went depends on which theory is correct about how fast Earth spins. If it spins slowly, then our day would get longer by 500 milliseconds (thousandths of a second) every year until it reached an extra day every 18 months or so. This would be really cool if it was true, but we know it isn't true and will never happen.
But what about the weather? How would the rotation of Earth change the weather? Scientists disagree about this too. One theory suggests that while Earth's rotation would continue, its axis would tilt by a few degrees and we'd experience more extreme climates than we do now. Another theory says that although our day would get longer, our days wouldn't vary by more than a few minutes from one year to the next. If this is the case, then winter wouldn't be much colder than winter today.
Now on to the space stuff. If Earth's rotation reverses, then the days on our calendar would become shorter. The months would continue to have only just over 30 days, but leap years would be skipped. Years with an odd number of months would be skipped while years with an even-numbered month count would count forward by one day. If years were precisely divisible by 100, there wouldn't be one every fourth year; instead there would be one every 100th year—and this is where things get interesting (and bizarre).
Some people believe that for this reason, light from distant stars will reach us more often in the future than it does today. Astronomers call this the "starlight crisis". If the length of Earth's day were shortened enough, then light reaching Earth from stars becomes temporarily blocked by our atmosphere.
It gets stranger. Some scientists believe that if this ever happened, it might trigger the formation of new stars. They say that if too many new stars were created in too short a period of time, they could send Earth into an extinction crisis that would wipe out the human race by the 22nd Century.
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