Water on Earth  The Science of Tides

a.  Why the tides are later each day

As the earth rotates under the two ocean bulges, we would expect that each point on earth should experience two high tides and two low tides every day. That is the case in most places - though there are exceptions, as we shall soon see. However, the two high tides do not come exactly 12 hours apart, as we would expect, but come a little later each day.

As the earth rotates in its 24-hour day, the moon is also revolving around the earth - in the same direction. So, when the earth gets to the same spot it was in 24 hours ago, the moon has moved ahead.  How far has the moon moved?  See the diagram to the right.

Remember that it takes 28 days for the moon to go around the earth. In one day the moon has moved 1/28 of the way around the earth. So when the earth gets to the spot it was in 24 hours ago, the earth still has to rotate 1/28 of a day to catch up with the moon. 1/28 of 24 hours = 24/28 hours = 51minutes. So the tides come about 51 minutes later each day.  A person standing at one spot on the earth would see the same high tide not 24 hours later, but about 24 hours 51 minutes later.

Now, remember that there are (usually) two high tides each day. If the moon stood still they would be 12 hours apart, but since the moon revolves around the earth, the two high tides come about 12 hours and 25.5 minutes apart.