what causes tides

For this part of the project my partner, Sarah, and I were looking at the connection between the tidal action on the earth and its connection to the moon. One of our primary questions was whether or not you could predict the tides by simply knowing the phase of the moon. We discovered that knowing the phase is an important part of several different pieces of information required to predict the tides.

Several different things are responsible for the tidal forces on Earth, but the two main astronomical reasons are the force of gravity exerted by the moon and sun upon the earth and centrifugal forces produced by the revolutions of the earth and moon and earth and sun around their common centers of gravity (NOAA).

I will attempt to explain the centrifugal forces first since it helps to clarify the effects of the solar and lunar gravitational forces. The common center of mass, or barycenter, of the earth and moon is not at the center of the earth. Instead the center of mass is approximately 1068 miles below the surface of the earth’s crust (on average), always facing the moon along an imaginary line that connects the two. The centrifugal force is always in the opposite direction of the moon’s gravitational pull and thus is pulling away from the barycenter in an equal amount of force. This keeps everything in balance (NOAA).

The best analogy I can give is that of an Olympic hammer thrower. When the Olympian spins the ball and chain around, centrifugal force is being exerted on the ball at the end of the chain, which is pulling on the athlete. The athlete in turn has to exert at least the opposite amount of force in the opposite direction to keep the ball from pulling him off balance.

This is the same thing that happens with the earth and moon. They are both exerting force on each other and are both exerting opposite force in opposition to each other. Since we are only concerned with the barycenter below the earth’s crust, that is the only one that I will pay any attention to. The effect this force has on the earth is going to be constant on all points on Earth. However, the gravitational effects of the moon and sun may be different at different times on the earth because the magnitude of the gravitational forces of the various attracting bodies are at either farther or closer positions in relation to the Earth. One of Newton’s laws of Gravity can be expressed here as the distance away from the objects to the second power over the tide raising force or gravity to the third power (NOAA).

The effect of the Moon’s gravity on the ocean’s of the Earth is very minute, but is significant enough to create bulges that are responsible for the tides. The gravity of the Earth is what keeps everything on the Earth from flying off into space. This means that the Earth’s gravitational force is greater then the centrifugal force produced by the Earth. The oceans are no exception to this rule, however the nature of the water that makes up the oceans makes it possible for the Moon to move the water just slightly enough to make tidal forces. The tractive force that actually makes the tides does not come from the moon pulling directly or perpendicularly to the oceans. The force comes from when the moon pulls horizontially across the plane, causing the water to bulge and produce the tides (NOAA).

These two forces in coordination with each other super-impose themselves over the Earth and create the force necessary to produce tidal action. This is done in a couple of different ways: the net or differential force and the tractive force, which is explained above.

Tidal Inequality is the variation in the range of the tides. This is where the lunar phases play their most significant role in the tidal shift. The lunar phase effect is responsible for the neap and spring tides. The spring tides have nothing to do with the season; instead it refers to the fact that tides are usually a little higher at this time or “springing out”. Neap tides are weaker tides and reflect the Greek definition of the word meaning “scanty” (NOAA).

Spring tides occur when the moon is at the new and full phase and neap tides occur at the first quarter and third quarter phases. The lunar and solar gravities reinforce each other during the spring tide and thus have a greater effect upon the tides. At neap tide the moon and sun run perpendicular to each other resulting in some what of a nullifying effect. This results in lower or weaker tides (NOAA).

Another factor that determines the tidal differential is the parallax effect. This is when the sun or moon or both are either closer or farther, in regards to the distance from the earth. When the moon is at its closest position it is at a position called perigee, its furtest is apogee. Similarly when the earth is at its closest to the sun and then farthest away is called perihelion and aphelion, respectively. The effects of these forces are quite simple to explain. When the moon or sun is closer, the amount of gravitational force they can exert on the earth will be greater, thus resulting in stronger or higher tides (NOAA).

Thirdly there is the lunar declination effect. The moon does not travel around an elliptic directly around the earth’s equator, instead it travels on a path that is about five degrees declination either above or below the equator, depending on how you look at it. When the moon is at high declination, diurnal tides are the result; half-way between the equator and the highest point of declination results in mixed tides, and when the moon is directly over the equator semi-diurnal tides are the result (NOAA).