Wednesday, April 27, 2016

Cosmic High Fives to High Tides

by Drew Martin
As a kid, my grandparents’ vacation spot at a creek near Williamsburg, Virginia was the most remote place in the world for me. My relatives had either small cabins or trailers in the thick woods and we were miles from civilization. We fished, spent time on boats, or shot at cans with BB guns or sling shots during the day and played cards at night. It was a place to kick back and spend time with relatives, but I never imagined I would have a profound, worldly feeling there. 

On a recent trip to Virginia, my brother arranged for my family stay at his own cabin on the same creek. I got up before everyone else in the morning and went out to his dock. A text from him (he was back at his house in Richmond) mentioned it must be high tide. I said it was and sent him a picture of his dock (top). Then I ran with my older son to our old stomping grounds at the creek. The water was the highest I had ever seen it so I texted my brother and asked if it had something to do with global warming. He mentioned that the tides vary with many factors. 

In that moment I looked around this marshy area of the meandering creek and I felt like I was on top of an observatory mountain in Hawaii. All of these years I never thought about how perfect of a place this was to observe the influence of the moon and sun's tractive force on Earth. Unlike the ocean coasts, the creek is undisturbed by crashing waves and blasts of wind. And unlike a steady flow of a river, the creek flows in different directions based whether it was rising towards high tide or lowering towards a low tide.

I had always felt overwhelmed by nature at the creek but that was because of water snakes, spiders, and ticks - this was a cosmic connection. Since our recent stay at the creek, I have looked at a lot of detailed tidal charts of that area and compared them to moon phase charts. What I keep reading is that tides are a "Complex Phenomenon."

They are barely noticeable at the equator and range only about a foot at high sea, but in other areas they are extreme. At the Bay of Fundy, Nova Scotia, Canada tides vary as much as 44.6 feet!

Although the time between high tides is (normally*) constant, around 12 hours and 25 minutes, the heights of high and low tides vary because of orbital path of the moon and how it aligns with the sun - the sun adds to the moon's gravitational effect. The bulge of the Earth's body of water is greatest when there is a new moon (when the sun and moon are aligned on one side of the Earth) or during the full moon (when the moon is on opposite side of Earth from sun). 
The gravitational force of the moon is actually only one ten-millionth of the Earth’s gravity [to things on Earth]. And the sun's gravitational force is only 46% of the moon, but when combined with the Earth’s centrifugal force created by the spin of the Earth, the effects are increased. The Proxigean Spring Tides are the highest because they are when the moon is closest to the Earth. Neap tides, the weakest, happen during quarter moon phases, which diminish global water bulging because the sun and moon are perpendicular with respect to Earth. The illustration here (bottom) is much simplified but is one of the better ones I found online.

*Click here to read more about this "Complex Phenomenon", which includes such morsels:

The world ocean is a complex dynamical system. The natural velocity of a water disturbance depends on the depth and salinity of the water at each point it passes. When bodies of land circumscribe bodies of water, they produce a collection of resonating systems that favor water oscillations with certain frequencies over others. From among the 300+ harmonics that can be measured, every port and coastal location has its own unique signature depending on its latitude, longitude, water depth and salinity. The result is that the 'two high two low' tide rule can be strongly modified so that the time between successive high tides can be greater than or less that 12 hours in many cases. The result is that for some locations, there can be days when only one high tide occurs.