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Why does the tidal bulge extend in 2 directions?Why does the tidal bulge extend in the...

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faithlovehope | Student | (Level 2) Honors

Posted February 28, 2010 at 12:20 PM via web

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Why does the tidal bulge extend in 2 directions?

Why does the tidal bulge extend in the direction away from the Moon as well as in the direction toward the Moon? Why does the Earth have 2 tidal bulges instead of just one, on the side closest to the Moon?

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enotechris | College Teacher | (Level 2) Senior Educator

Posted March 1, 2010 at 12:22 AM (Answer #1)

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The Earth-Moon system, or the "double planet" nature of these two bodies, is unique in the Solar System, and may be quite rare as planetary bodies go.  The reason is that the size of the Moon relative to the Earth is quite large; other planets may have "moons," but they tend to be very small in comparison and exert minimal gravitational influence upon their primaries.

This is not the case with the Moon!  The center of gravity between Earth and Moon is some distance below the Earth's crust.  Wherever the Moon is, that section of Earth lying closest to it is subject to its gravitational force.  So although we on Earth perceive the "tide" moving in and out, what's really happening is that the water is following the Moon as it orbits Earth.  So this "bulge" or tide is between the center of gravity and the Moon, and it is subject to the most amount of gravitational force from the Moon.  However, exactly on the opposite side of Earth, there is also a bulge!   This is a result of that area being "left behind."  The force of gravity gets weaker the further the two masses are; in other words, that area on the other side of the Earth is subject to the least amount of gravitational force from the Moon, and trails behind.  Picture the waters of Earth forming an oval around the planet, with one end of the oval always pointing towards the Moon, and the other always pointing away.  As Earth rotates, the oval stays in place, and the waters appear to rise and fall relative to a spot on shore.

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