Why We Have Tides

In an effort to tie astronomy and meteorology together, we will discuss the forces that create the tides on Earth.

A brief equation that can explain what celestial bodies have the most influence on tides. Simply put, tide generating force = mass/(distance)^3

So how much influence does the sun have? The mass of the sun is roughly 27 million times heavier than the moon, but its 390 times further away (390 cubed is 59 million). So, the sun's tide-generating force is about half of that from the moon (46%).

Tides are based on gravitational attractive force. Distance between two objects is more critical than their masses.

The strongest gravitational force the moon exerts on Earth is on the side of the planet directly facing the moon. So, this force of gravity more or less pulls the water towards the moon on this "near side" of Earth.

But, there are two different tidal bulges at any one time. The second bulge is not formed by gravity. It's actually caused by inertia. Basically the spinning of the globe throws the water towards space on the opposite side of where the moon is. The moon's gravity is too weak to counter act this inertia. Are you with me? The two bulges stay directly aligned with the moon as the earth rotates on its axis.

Why do high tides occur 12 hours and 25 minutes apart instead of just 12 hour apart? The reason is the lunar day. The moon rotates around the earth in the same direction that the earth spins. So, in one spin of the earth (24 hours) the moon has moved in the sky. So the earth has to spin for another 50 minutes for the lunar day to be completed.

As we know the position of the moon plays an important role. But, the position of the sun with both respect to the earth and moon play a role as well. As we talked about earlier, the force that the sun exerts on the oceans is about half of the force from the moon. But, when the earth, moon, and sun are all aligned, the tide is usually high. i.e. the sun and moon work in tandem. This set up occurs twice a month, once during a full moon, and once during a new moon.

Roughly one week after the new and full moon, the sun and moon are at a 90-degree angle from one another. In this set up, the sun's gravitational pull partially cancels out the moon's pull, the result is neap tides.

One more factor to consider is the fact that the moon's orbit around the earth is elliptical. The distance between the earth and moon changes by about 31,000 miles during one orbit around the earth. When the moon is at it's closest, its called perigee. It's farthest distance from the earth is apogee. When the full moon or new moon line up with the perigee, you get an extremely high tide. You will frequently hear this referred to as a "astronomically high tide". Like we saw earlier, distance is crucial when calculating the tidal force.

So why do some places have a higher high tide than others? This has to do with the landmasses on earth. if Earth just had a few small landmasses, then high tides would be nearly uniform at all locations across the globe. But, the large landmasses make it harder for water to move around freely. In short, large landmasses usually have larger difference between high and low tides, Meanwhile, most small islands have a small difference between high and low tide.

The shape of bays, inlets and estuaries can magnify the intensity of the tides. Funnel-shaped bays are particularly notorious for magnifying tidal variation. The most notable example of this is the Bay of Fundy in Nova Scotia, which experiences a nearly 50-foot swing between tides. Concave shaped coastal areas also have a higher than normal tide. e.g. the coast from South Carolina to northern Florida is a concave shape. The difference from high tide to low tide in Savannah is 7-8 feet.

On the flip side areas with very shallow water and convex coastlines have a much lower tide cycle, like the Pamlico south of, North Carolina, and the Laguna Madre in south Texas.

One of my favorite ways to view current tide levels is through NOAA's tides and currents page.. Here you can follow along and see how the tide is running with comparison to normal. When astronomically high tides are in play, you can easily see that on the realtime plots.

Accuweather.com has forecast high and low tide information for many locations

Speaking of apogee and perigee. The earth's orbit around the sun is important too! The earths orbit around the sun is not completely circular. It is elliptical. The distance from the earth to the sun varies from 152.6 million km at the farthest to 147.5 million km at the closest. When the earth is the closest to the sun, its called perihelion. This occurs around January 2nd.

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