It’s falling farther away at a rate of about 3.8 centimeters a year, or roughly the speed at which human fingernails grow, and the drag effect is weakening. The moon also used to sit much closer to Earth, so its gravitational tug was much stronger than it is today. That may not sound like much, but the effect adds up over billions of years, stretching out our days. This drag slows Earth’s rotation rate by about 1.78 milliseconds each century. But the water but lags slightly behind, dragging on the ocean floor. The gravitational tug of the moon raises the ocean tide, and as the moon orbits Earth pulls the tide along with it. What’s been influencing the length of Earth’s days?īut back when dinosaurs lumbered across the place where Madison stands today, Earth spun faster around its axis.Įarth’s spin has been slowing down for billions of years, explains Kurt Lambeck, a geophysicist from the Australian National University in Canberra, primarily because of the moon. In Alaska, it won’t drop below the horizon, and in Wisconsin, Meyers will be able to linger in solstice daylight for a full 15 hours and 22 minutes. The sun also takes longer to travel across the sky on the June solstice than on any other day of the year. At that time, someone in Havana, Cuba, will feel the sun beating straight down on them. On the summer solstice-June 21 in the north this year-the sun’s path through the sky arcs higher than on any other day of the year, hovering directly overhead anyone standing at the Tropic of Cancer at noon. As Earth trundles along the second half of its orbit, the Southern Hemisphere faces the sun more directly, and it’s time for people in cities like Sydney and Buenos Aires to pull out the popsicles and sunblock. That’s when it’s summertime in places such as Stockholm, Moscow, and Toronto. That’s about the angle between Spock’s fingers in the Vulcan salute.īecause of that tilt, the Northern Hemisphere sits a little bit closer to the sun than the Southern Hemisphere for half the year. What is a solstice, anyway?Įarth’s axis is tilted about 23 degrees relative to its orbital path around the sun. So, what’s been altering Earth’s days over geologic time, and does this mean anything for solstices past and future? Here’s the breakdown. “So if I just wait a few hundred million years,” he says, “I’ll get that extra hour back!” And changes in the gravitational dance between our planet and the moon are causing Earth’s day to get ever so slightly longer each year. Meyers and a colleague recently showed that 1.4 billion years ago, a single “day” lasted only about 18 hours.
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