What Causes the Tides?

In most places, the oceans rise and fall every 12 hours and 26 minutes. These two daily “high tides” are caused by the gravity of the Moon, and also to a lesser degree by that of the Sun. One of the two high tides is usually greater than the other. Here's why.

The Moon's gravity pulls the oceans on the side of the Earth nearer to it more than it pulls the Earth itself, causing a bulge of water. On the opposite side (away from the Moon), the Earth is pulled more than the oceans, and this leaves behind a slightly smaller bulge of water. So the reason for the difference in the two high tide heights is that the force of gravity gets weaker the farther away a point on the Earth is from the Moon (the inverse square law of gravity). Since the Earth rotates once on its axis every 24 hours, its surface passes through these two bulges, or high tides, each day.

The Sun also has a tidal effect on Earth, but it is only half as much as the Moon's. Since the Sun is so far away, its force does not change very much from one side of the Earth to the other. So the greatest effect from the Sun is seen during New Moon, when the Sun and Moon are in a direct line as seen from Earth, and pulling together on the Earth's oceans. These high tides are called spring tides.

When the Sun and the Moon are at right angles and pulling in different directions, the lowest tides take place, called neap tides.

Activity

  1. What causes the tides?
    [anno: The tides are caused by the gravitational pull of the Moon and Sun on the Earth's oceans.]
  2. Of the two high tides that occur within a twenty-five hour period, why is one higher than the other?
    [anno: One tide is higher than the other because one side of the Earth is closer to the Moon at one time than the other side is. The side of the Earth that is closer to the Moon has a higher tide. The oceans on one side of the planet are closer to the Moon.]
  3. An ocean experiences a high tide approximately every 12 hours and 26 minutes. If the ocean experiences a high tide at noon on a Monday in a particular place, at what time will the first high tide occur on that Friday?
    [anno: The first high tide on that Friday will occur at 3:02 a.m.]