A lunar day is how long it takes for one point on the Earth to make one complete rotation and end up at the same point in relation to the moon. The reason that a lunar day is longer than a normal hour day is because the moon rotates around the Earth in the same direction that the Earth is spinning. Tides are very long waves that move across the oceans.
They are caused by the gravitational forces exerted on the earth by the moon, and to a lesser extent, the sun. When the highest point in the wave, or the crest, reaches a coast, the coast experiences a high tide.
When the lowest point, or the trough, reaches a coast, the coast experiences a low tide. Imagine the ocean is shaped like a football pointing at the moon. 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.
Arrows represent the force of the moon's gravitational pull on Earth. To get the tidal force—the force that causes the tides—we subtract this average gravitational pull on Earth from the gravitational pull at each location on Earth.
The result of the tidal force is a stretching and squashing of Earth. This is what causes the two tidal bulges. Arrows represent the tidal force. It's what's left over after removing the moon's average gravitational pull on the whole planet from the moon's specific gravitational pull at each location on Earth. These two bulges explain why in one day there are two high tides and two low tides, as the Earth's surface rotates through each of the bulges once a day. The Sun causes tides just like the moon does, although they are somewhat smaller.
When the earth, moon, and Sun line up—which happens at times of full moon or new moon—the lunar and solar tides reinforce each other, leading to more extreme tides, called spring tides.
When lunar and solar tides act against each other, the result is unusually small tides, called neap tides. There is a new moon or a full moon about every two weeks, so that's how often we see large spring tides. When the gravitational pull of the Sun and moon are combined, you get more extreme high and low tides.
This explains high and low tides that happen about every two weeks. Note: this figure is not to scale. The Sun is much bigger and farther away. Our sun is 27 million times larger than our moon. Based on its mass, the sun's gravitational attraction to the Earth is more than times greater than that of the moon to the Earth.
If tidal forces were based solely on comparative masses, the sun should have a tide-generating force that is 27 million times greater than that of the moon. However, the sun is times further from the Earth than is the moon. Thus, its tide-generating force is reduced by 3 , or about 59 million times less than the moon.
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