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Monday September 25, 2017

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7 things to know about the longest day of the year

The summer solstice is upon us: June 20th and the 21st were the longest days of 2017 for anyone living north of the equator. Technically speaking, the summer solstice occurs when the sun is directly overhead the Tropic of Cancer, or 23.5° north latitude.

It might seem like a day to celebrate, but it actually signals the moment the sun's path stops moving northward in the sky, and the start of days becoming steadily shorter as the slow march towards winter begins. However, we won't notice the days becoming shorter for a while. The shortest day of the year isn't until Thursday, December 21, known as the winter solstice.

So between March and September, Earth’s Northern Hemisphere gets more exposure to direct sunlight over the course of a day. The rest of the year, the Southern Hemisphere gets more. It’s the reason for the seasons.

Ever since the Earth has had liquid oceans and a moon, its rotation has been gradually slowing over time due to tidal friction. That means over very, very long periods of time the days have been getting steadily longer. About 4.5 billion years ago, it took the Earth just six hours to complete one rotation. About 350 million years ago, it took 23 hours. Today, of course, it takes about 24 hours. And the days will gradually get longer still.

Given that, you'd think 2017 would be the longest day in all of history. But while it's certainly up there, it doesn't quite take top honors.

That's because tidal friction isn’t the only thing affecting Earth’s rotation  there are a few countervailing factors. The melting of glacial ice, which has been occurring since the end of the last ice age 12,000 years ago and is now ramping up because of global warming , is actually speeding up Earth’s rotation very slightly, shortening the days by a few fractions of a millisecond. Likewise, geologic activity in the planet’s core, earthquakes, ocean currents, and seasonal wind changes can also speed up or slow down Earth’s rotation.

When you put all these factors together, scientists have estimated that the longest day in Earth’s history likely occurred back in 1912. That year’s summer solstice was the longest period of daylight the Northern Hemisphere has ever seen (and, conversely, the 1912 winter solstice was the longest night we’ve ever seen).

Eventually, the effects of tidal friction should overcome all those other factors, and Earth’s days will get longer and longer as its rotation keeps slowing (forcing timekeepers to add leap seconds to the calendar periodically).Which means that in the future, there will be plenty of summer solstices that set new records as the "longest day in Earth's history."

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