Image is a drawing of Pytheas

Source: Pytheas, Thames Estuary

The moon is also important to us because it creates tides. Tides are daily movements of ocean water that change the level of the ocean's surface.

Tides are causes by:

  1. the exertion of gravitational forces by the moon and sun
  2. the rotation of the Earth

The relationship between the phases of the moon and the tides was first discovered more than 2,000 years ago by a Greek explorer named Pytheas.

However, Pytheas could not explain the reasons behind the relationship. It was not until Sir Isaac Newton published his theories on gravitational pull in 1687 that the relationship between the moon and tides were fully understood.

Gravitational forces from both the sun and moon continuously pull on the Earth. Even though the moon is much smaller than the sun, it is the moon's gravity that is the force behind the Earth's tides.

Image is cartoon of the sun and moon pulling creating a tide

Source: Tide of War, Tad Thomas, Infectious

Every particle on Earth is pulled on by the gravity of the moon. Because liquids move more easily, the pull of gravity is much more noticeable in liquids than in solids.

The moon's pull is strongest on the part of the Earth directly facing the moon. When that part of the Earth happens to include an ocean, the water there bulges toward the moon. At the same time, water on the opposite side of the Earth bulges due to inertia caused by the motion of the Earth and moon around each other. These bulges are what we call high tides. The figure below shows the position of the moon that causes the water to bulge.

Image is cartoon of the sun and moon pulling creating a tide

Notice that when high tides occur, water is drawn away from the area between the high tides, causing low tides. Two high tides and two low tides occur each day. The ocean is constantly cycling from high tide to low tide and then back to high tide. There is about 12 hours and 25 minutes between the two high tides.

The sun is the second factor that affects tides. However, the sun does not have as much influence on the tides as the moon does because it’s farther away. Tidal ranges are based on the position of the Earth, moon, and sun.

Spring tides occur when the Earth, the sun, and the moon are in a line. These are very strong tides and occur every 14 days. Spring tides occur during the full moon and the new moon. The diagrams below show the two situations in which spring tides occur. The name spring tide has nothing to do with the season.

The sun, Earth, and moon are at right angles during the moon’s two quarter phases. Neap tides which are very weak tides, occur during this time. During this time, the gravitational forces of the moon and sun are perpendicular to each other which cause the pulls to "cancel out" each other. The diagrams below show the two situations in which neap tides occur.

The animation below shows the revolution of the moon and spring vs. neap tides.

Source: Spring and Neap Tides, The Gate/James Irwin, Fitzgerald Marine Reserve