Tides and rotation of the Earth–Moon system

The tides are a twice daily rise and fall of the ocean level. What causes the tides in the Earth’s oceans? The complete answer requires understanding the center of mass of the Earth–Moon system. Read the text aloud
Earth and Moon both rotate around their common center of massEven though the Earth is much more massive than the Moon, the Moon does not orbit around the center of the Earth. Instead, the Earth and the Moon both orbit around their combined center of mass. The system’s center of mass is located within the Earth itself, approximately 1,600 km below the surface of the Earth. The Earth travels in a nearly circular orbit around the system’s center of mass, as does the Moon—although the Moon’s orbit is much larger! (Note that the figure is not shown to scale; the Moon is located much further away.) Read the text aloud Show Real scale of the Earth–Moon system
High tide on Earth caused by Moon's gravitational pullConsider the side of the Earth facing the Moon. The Moon’s gravity exerts an attractive force on the water in the ocean. Unlike solid land, liquid water can flow in response to the Moon’s feeble gravity. As a result, the ocean surface rises a few meters in the “Moonward” direction. This causes one of the high tides on the Earth. But there are two tides per day; the second tide is trickier to explain. Read the text aloud Show Does the Sun produce tides?
High tide on Earth caused by centrifugal “force”The second tide is due mainly to Earth’s 28 day orbit around the Earth–Moon system’s center of mass. This 28 day rotation creates a “centrifugal” effect that tends to move ocean water radially outward and away from the Moon. The effect is similar to what happens if you swing a bucket of water around your head in a circle fast enough that the water stays in the bucket. This centrifugal effect is the cause of the second high tide—the tide that faces away from the Moon. Read the text aloud Show How long is the lunar month?
A more accurate picture is that the ocean surface is distorted into a slightly oval shape while the Earth rotates under the oval’s surface. Both tides lag slightly behind the passing of the Moon overhead due to friction and inertia of huge amounts of water sloshing back and forth between the continents. High tide occurs when a point on Earth’s surface passes through a bulge of the “oval.” Low tide occurs when that point passes in between the two bulges. Tides are highest near the equator and lowest near the poles. Read the text aloud Show What causes the highest tides?
Why is the center of mass of the Earth–Moon system located much closer to the Earth’s center than it is to the Moon’s center? Show
Does the Moon orbit around the center of the Earth? Show

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