This section is from the "The New Student's Reference Work Volume 5: How And Why Stories" by Elinor Atkinson.
Did you ever go to the sea shore for a vacation? And did you build forts and dig caves in the sand on the beach? Then, when you went to play the next morning, you found the beach smooth. Your forts and caves were gone. Grown people told you the tide came up and washed everything away. A tide is the rising and falling of the ocean along the shore. The moon pulls the water up and then lets it go again, so it falls back. See if you can understand it.
You know how the earth pulls the apple down? It pulls everything on or near its surface toward its own center. And everything near enough to be pulled, pulls back as hard as it can. The earth pulls the moon, and the moon pulls the earth. Although it is much smaller than the earth it is just the right size, for its distance away, to keep from falling into or away from the earth. We cannot see its pulling power on the solid parts of the earth. But the ocean is made of water. A slope of land, a brisk wind, many things set water in motion. It feels the pulling power of the moon. Whenever the moon rises over the ocean, it pulls the water that is just under it. So, a great wave, or tide, travels under the moon across the wide sea. When the shore is reached this wave rises higher against the rocks, or spreads over level sand beaches. When the moon sets, the wave goes back to the old level.
We know this is so because the tide always comes up with the rising, and goes out, or ebbs, with the setting moon. If the moon stood still, and always rose and set at the same hours, we could not be so sure that it had so much to do with the tides. But as the moon travels around the earth in twenty-eight days, it rises nearly half an hour later at any given place, every day of its journey. The tides rise just so much later every day, too.
The sun also makes tides. But the sun is so very, very far away, that its pull on our waters is very much less than the moon's. We would hardly notice it if it wasn't for one thing. Sometimes, for a few days in every month, when the sun and the moon are both on the same side of the earth, they pull together. Then the tides rise the highest of all. About two weeks later in the month, the sun is on one side of the earth and the moon is on the opposite side. Then they pull away from each other. The moon wins, in this tug of war, but it cannot pull the water nearly so high. Twice in the month the moon's and sun's pulls are at right angles to each other. Then the tides are just of moderate height. If you live near a seashore, make a record of tide soundings or points reached by the tide every day for a month, with the time of the rising of the tide and the moon, and find out for yourself how the moon pulls the sea.
This picture shows how the moon attracts or pulls toward itself the water on the side of the earth nearest it, causing a high tide. It also draws the earth away from the water on the other side, leaving a high tide there.
In this picture the moon and the sun being on the same side of the earth, both pulling the same way, a very high tide is caused, called spring tide.
When the moon is on one side of the earth and the sun on another, as shown here, each pulling toward itself, the attraction of the moon is weakened and the tides are not so strong. These lesser tides are called neap tides.