Because the moon follows an elliptical path around the Earth, the distance between them varies by about 31, miles over the course of a month. Once a month, when the moon is closest to the Earth at perigee , tide-generating forces are higher than usual, producing above-average ranges in the tides.
About two weeks later, when the moon is farthest from the Earth at apogee , the lunar tide-raising force is smaller, and the tidal ranges are less than average. A similar situation occurs between the Earth and the sun. When the Earth is closest to the sun perihelion , which occurs about January 2 of each calendar year, the tidal ranges are enhanced. When the Earth is furthest from the sun aphelion , around July 2, the tidal ranges are reduced Sumich, J.
Because of the tidal force, the water on the side of the moon always wants to bulge out toward the moon. This bulge is what we call a high tide. As your part of the Earth rotates into this bulge of water, you might experience a high tide. An illustration of the tidal force, viewed from Earth's North Pole. Water bulges toward the moon because of gravitational pull. Note: The moon is not actually this close to Earth.
One thing to note, however, is that this is just an explanation of the tidal force—not the actual tides. In real life, the Earth isn't a global ocean, covered in an even layer of water. There are seven continents, and that land gets in the way. The continents prevent the water from perfectly following the moon's pull. That's why in some places, the difference between high and low tide isn't very big, and in other places, the difference is drastic.
The tidal force causes water to bulge toward the moon and on the side opposite the moon. These bulges represent high tides. If the moon's gravity is pulling the oceans toward it, how can the ocean also bulge on the side of Earth away from the moon? It does seem a little weird. It's all because the tidal force is a differential force—meaning that it comes from differences in gravity over Earth's surface.
Here's how it works:. On the side of Earth that is directly facing the moon, the moon's gravitational pull is the strongest. The water on that side is pulled strongly in the direction of the moon. On the side of Earth farthest from the moon, the moon's gravitational pull is at its weakest.
At the center of Earth is approximately the average of the moon's gravitational pull on the whole planet. They are pulling in the same direction. USA TODAY had a great explanation of the lunar influence on tides back in "During both new and full moon phases, the gravitational attraction of the moon is in line with the gravitational pull of the sun, producing high tides and low tides.
Those high highs and low lows are about the same at both the full and the new moon when the moon is dark. The next new moon is Sept. Now, how about those supermoons that caused a buzz — and some spectacular photography — all summer? Shipman said they also have an influence on tides, more even that a traditional full or new moon: "A supermoon happens when the moon is near the closest point in its orbit.
The moon only comes a few percent closer at some points in its elliptical orbit around the earth.
0コメント