Tides are caused by gravitational forces, which are based on the masses of and distances between objects. In Part 1 of this Snack, we explored solar tides. The force of gravity exerted by the sun pulls our oceans towards the sun. It’s easy to understand how this influence results in a tidal bulge, or high tide, on the sun side of the earth, but why is there also a high tide on the far side of the earth?
The force of gravity varies enough over the distance of the earth’s diameter that masses on either side of the earth experience different amounts of pull. While the gravitational force is proportional to the inverse of the distance squared between two objects, the tide-raising force is proportional to the inverse of the distance cubed. Therefore, the ocean on the side nearest the sun experiences the largest force, the ocean on the side farthest from the sun experiences the least force, and of course, the earth itself experiences a force somewhere in between, resulting in a “stretching out” of these three masses and tidal bulges on the near and far side of the planet.
As the earth rotates on its axis, it passes through two tidal bulges in one rotation (one day). As you revolve the earth around the sun (one year), the tidal bulges stay in line with the sun.
Just like the sun, the moon creates two tidal bulges on the earth—one on the side closest to the moon and one on the opposite side. These moon tides are created by the same gravity-based tide-raising forces that produce sun tides. Although the moon is much smaller than the sun, it is also much closer, so the moon’s tidal influence is twice that of the sun. As you rotate the paper earth model through a full day, each part of the earth rotates under the two tidal bulges, and therefore there are two high tides and two low tides per day on most parts of the earth. Watch our Dance of the Tides video for an illustration of this.
In Part 2 of this Snack, we looked at the influence of the sun and moon together.
During a new moon, the sun and moon are in line with the earth and their tidal influences add together, creating higher high tides and lower low tides. These additive tides are referred to as spring tides. Spring tides also occur during the full moon. Since the moon’s cycle takes about 28 days, spring tides occur every two weeks.
When the moon and the sun are at 90° to each other, during the first quarter or third quarter moon phases, their influences don’t add together, so the high tides and low tides are both less extreme. These tides are referred to as neap tides. Neap tides also occur every two weeks.
The sun and moon are the two main influences on the earth’s pattern of tides. Since the moon exerts the strongest influence because of its proximity, the main tidal pattern follows the phases of the moon.
As the earth rotates each day, the moon also moves in its orbit. It takes about a month (“moonth”) or 27.3 days for the moon to revolve around the earth. In one day, the moon moves 13°, viewed from the North Pole (13° per day = 360°/27.3 days) counterclockwise. Therefore, 24 hours after a new moon, the moon is 1/7 of the way to the first quarter. The larger tidal bulge follows the moon, so the earth has to rotate an additional 13° to reach high tide again. This takes about 54 minutes (approximately 24 hours x 13°/360°).
Thus, the high tide and all tides arrive about 54 minutes later each day.