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If the moon causes the tides, then why are there two high tides and two low tides every day?

by Marshall Brain

July 2, 2009
2 Comments | Add Comment

You asked:
If the moon causes the tides, then why are there two high tides and two low tides every day? — Leigh, Raleigh, NC

Marshall Brain Answered:
A tide table like this one shows you exactly when the high tides an low tides will arrive every day, and it is true that, most days, there are two high tides and two low tides. There aren’t exactly two every day because the moon takes about 28 days to orbit the earth once, and most months have two or three more days than that. (The exact interval is 12 hours 25 minutes from one high tide to the next).

The moon is the source of the tides. It’s gravitational pull affects the level of the water in the oceans. In this case, a video paints a thousand words, as this video shows you exactly what is happening at high and low tides:

The Flash animation on this page os also very helpful.

For more info see What causes high tide and low tide? Why are there two tides each day?

 

Comments

2 Responses to “If the moon causes the tides, then why are there two high tides and two low tides every day?”

Wrong says:
September 23, 2009 at 4:28 pm

This explaination is half wrong, both the high tides are cause from the Moon(and Sun to a lesser extent), not the rotation of the Earth. If you draw the grvitaional vectors on the surface of the Earth cause by the Moon at various points, then subtract the gravitaional vector acting on the center of the Earth from each of those points you get the real reason.
The vectors where the moon is at zenith and nadir points directly away from the Earth(high tide). As you move 90 degrees away form these points the vectors start pointing towards the center of the Earth(low tides).

So to simplify the the explaination: The bulge on the side of the Earth facing the moon is obvious. The bulge on the opposite side is caused by the Earth being pulled towards the moon more than the water is. The water is further away on the far side of the Earth than the center of the Earth, so it experiences a weaker gravitaional force.

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