Gravitation

The Pull of the Moon

To get the whole picture of the tides in action, you must first look at why tides occur. You may not think about the moon every day or even look at the moon every night. But every day and every night the moon is hard at work "pulling" at our planet. That pull is the moon's gravitational attraction to Earth. The gravity of the moon pulls on every particle of the Earth; however, the pull on liquids is more noticeable than its pull on solids. Liquids move more easily than solids so we see the effects of the moon's gravitational pull on the Earth's liquids, such as ocean water, more than we see its pull on the Earth's solids, such as our trees and homes. Remember, the Earth's gravity is also "pulling" on objects so the Earth is able to "hold down" its solids. But the Earth's gravity cannot keep its water from sloshing around a bit while the moon's gravity is pulling at the same time.

The moon pulls the water on the side of Earth closest to the moon more strongly than it pulls on Earth itself. When the ocean water bulges toward the moon, these bulges are the high tides. High tides occur on the side of the Earth closest to the moon (direct high tides) and at the same time, high tides also occur on the opposite side of the Earth - farthest from the moon (indirect high tides.) How do high tides happen on the side farthest from the moon at the same time the tides are high on the side closest to the moon? Easy. Remember, the moon's pull is stronger on the side of the Earth closest to the moon. On the other side of the Earth, farthest from the moon, there is a centrifugal reaction due to the Earth's rotation that also draws the water upward from the Earth's surface. These two bulges of water on opposite sides of Earth are where you will see high tides. Low tides occur in places on the Earth's surface which are off to the side: farthest from an imaginary line connecting the centers of the Earth and moon.

The Tug of the Sun

Now that you have an image of the moon pulling at our planet, imagine the sun tugging the same way. The sun's gravitational tug on the planet works the same way, but less powerfully. Although the sun is much larger than the moon, it is also much farther away, so its pull is not as strong. Even though the gravitational forces of the moon and the sun are constantly pulling at the Earth, the moon's pull has more effect on the tides.

But picture this. What happens when the moon and sun are lined up on either side of the Earth, with the moon pulling and the sun tugging, together, at the same time? You can imagine this powerful tug-o'war creates the highest tides of all on the sides of Earth where the sun and the moon are playing together to pull at the Earth. This alignment occurs every 14 days. You may know of it because you see it at times of the new moon and the full moon. The time when the Earth, the sun and the moon are aligned is the time when the high tides are especially high and the low tides are especially low.

Twice a month, when the tidal range is filled with more extreme highs and lows, the tides are called spring tides. This name has nothing to do with the spring season. The name, in this case, has to do with the "springing" (as in "to leap up") action of the tides.

When the moon and the sun are not aligned with the Earth (during half-moon phases), also twice a month, the tides have the lowest tidal range. The high tides are not as high and the low tides are not as low. This is because the sun's tug is canceling out some of the moon's pull and they are actually working against each other. At these times, the tides are called neap tides. During neap tides ,the range of high and low tides are not as dramatic as the range of high and low tides during the spring tides.