If asked what causes the tides most people would answer immediately, "the moon". They would be correct, however the general mechanism is generally not understood. Further you might know that it is the gravitational attraction of the moon which causes the ocean to be attracted towards the moon thus causing a tide, again you would be correct...however...

There are two facts which you might ask yourself to test your understanding of how tides work.

First, the gravitational attraction on a mass on the surface of the earth due to the sun is about 50 times stronger than that of the moon. (The force is proportional to the mass of the sun and inversely proportional to the square of the distance) Why then should the moon cause the tides, it has a relatively small contribution to the gravitational force on a parcel of water?

Second, why do we get two (almost) tides per day? Given that the moon travels around the earth in about 28 days then its motion across our sky is mostly due to the earths rotation which of course spins around once per day. Thus wouldn't we expect only one tide per day. ie: if the oceans bulge towards the moon then the bulge moves around the earth once per day.

A correct understanding of the tides explains both the above.

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