Moon & Tides

Gravitational Forces on Tides

The Sun & the Moon

Tides are controlled by the combined gravitational pull of the sun and moon. Because it is larger, the sun's gravitational force is 180 times stronger than the moon's. The moon's force, however, is twice as strong because it is much closer to the Earth.

The moon's influence on the world's oceans is strongest on the side of the Earth that is closest to the moon. As the moon rotates around the Earth, it pulls the water toward it creating a high tide. As gravitational force draws the water closer to the moon, inertia tries to keep the water in place. But the gravitational force is stronger and the water is pulled toward the moon, causing a "bulge" of water on the near side toward the moon.

On the opposite side of the Earth, farthest from the moon, the gravitational pull is weaker, however, inertia pulls the water away from the Earth, also forming a bulge. Think of two children, holding hands and spinning in a circle. As they rotate, they are both pulled outward.

The result of this tug of water between inertia and gravitational force is two tidal bulges - a higher one on the side of Earth closest to the Moon, and a lesser one on the side furthest from the Moon.

The positioning of the sun and the moon also has an effect on the height of the tidal swell. During a new or full moon phase, the sun and moon are in line, pulling on the earth's surface. This creates a higher level of tide - called the "spring tides" from the Welsh word "springan" meaning 'to bulge'.

During a first and last quarter moon phase, when the sun and moon are at right angles to the earth, a lower high tide results. These are called "neap tides" from the Anglo-Saxon term "neafte" meaning "scanty". (taken from 1914 edition of The Admiralty Manual of Navigation).

Perigee and Apogee 

Another influence on the height of tides is the elliptical path the moon takes during its revolution around the earth. When it is closer to Earth, it is referred to as a perigee moon. When the moon is furthest away, it is called an apogee moon. During apogee moon, high tides are not as significant.  

The moon in perigee appears larger and brighter than an apogee because it is, indeed, much closer. During this perigee moon phase, there can exist a 30-48% greater gravitational pull. Tides during this period would have a greater tidal range between high and low.

Learn why the Bay of Fundy has the highest tides in the world...