Tuesday, March 20, 2012

Let’s Talk Science! Gravity, the moon, and all this talk of King Tides

By Brook Beeler, Environmental Educator, Office of Communication and Education

Here at Ecology we are dedicated to understanding the effects of climate change in Washington. One of the ways to visualize what sea level rise could look like is to document high tides. Spring tides occur naturally when the sun and the moon align, causing an increased gravitational pull on the Earth’s oceans. Note that spring tides are the scientific term commonly used to describe tides and don’t refer to the season. The very highest spring tides are called “King Tides”. Ecology’s King Tide photo initiativeis an exciting interactive way to get citizens engaged in science. But how many of us can actually explain how tides work? It turns out that many adults know it has something to do with the moon, but explaining the process can be complicated.

Let’s start with gravity. All objects that have mass have a force between them. There is gravitational force between me and my laptop, me and my coffee table, and me and the chair across the room. It is also important to remember that the gravitational force gets weaker with distance. The closer I am to the chair the greater the pull of the gravitational force. As I move further away, that pull decreases. Note gravitational force is pull, not push. When talking about tides, we’re referring to the force of the moon’s pull.

So what is the moon pulling exactly? If the moon is pulling the Earth’s water towards itself, why is there also a bulge on the opposite side of earth-moon alignment? A common misconception among many folks attributes the second bulge to the earth’s rotational force. To be honest, when researching this blog, my mind was a bit boggled. I wasn’t “getting” it either. Then, the January issue of Science & Children published by the National Science Teachers Association landed on my door. 

Here is what I learned: It helps to imagine the moon exerting its force on three things. 1) It pulls the water closest to the moon, 2) It pulls the Earth, and finally, 3) It pulls the water farthest from the moon. Remember the strength of the moon’s pull is weaker on the “things” it is farther from. Therefore water closest to the moon receives the strongest pull causing a water bulge. The Earth receives a medium pull, separating it from the water farthest from the moon, causing a water bulge. The water farthest from the moon receives a small pull, but not enough to diminish the water bulge. Now add the Earth, its continents and ocean basins rotating. Twice a day the continents and ocean basins pass through the gravitational pull causing the bulge and water slowly rises then falls. Boom, tides!

Now add the sun’s gravitational pull. When the sun isn’t in alignment with the earth and moon the gravitational pulls cancel each other out. These are called neap tides. When the sun is in alignment it reinforces the moon’s pull causing spring tides, the highest “King Tides”. That is where our climate connection comes in. Earth and moon interactions, in the form of “King Tides”, can provide a glimpse of what sea level rise caused by climate change might look like in Washington. King tides are a natural part of the earth's tidal cycle and are not a result of climate change but they do provide a view of the locally highest tides, which may occur more frequently as sea levels rise due to the effects of climate change.

Illustration credits: NSTA Journal Science & Children

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