Book Readings: Week 6

For my Marine Resources class (GEOG 646) at SF State this semester I am reading The Sea Around Us, by Rachel Carson (1951), and The Tide: The Science and Stories Behind the Greatest Force on Earth, by Hugh Aldersey-Williams (2016). Please check out my first Book Reading blog post to read about why I chose these two books and why I chose to blog about them concurrently.

In The Tide this week, Aldersey-Williams starts off by talking about the River Thames. In particular, he points out that London must have been an unlikely port city, since sailing up the Thames could be difficult work thanks to the tides. Even traveling downstream and out to sea on a single ebb tide would be impossible; indeed, sailor's called traveling down the Thames and to the sea "more than a tide's work."

The author then talks about Charles Dickens, and how the fact that he spent his life close to the Thames influenced his writing. Two of Dickens' characters in Our Mutual Friend take advantage of the ebb tide to transport a dead body downstream, and in Great Expectations, Pip imagines being carried away by a strong spring tide. It was a good reminder that for many people, the action of the tides serves as a "backdrop" that helps to shape and inform our daily lives.

Aldersey-Williams then talks about how the Greenwich Observatory, from which we get Greenwich Mean Time, was not only a place where astronomer John Flamsteed and other scholars determined the position of many stars, but a place where these scholars were also able to make direct observations of the tides. Measurements of the orbit of the moon were also made at the observatory, and these measurements as well as Flamsteed's tidal observations would later inform Isaac Newton's work:

Newton had determined that the moon played a far greater role in contributing to the force of the tides than the sun or any other cosmic body, and he regularly asked Flamsteed for his data so that he could refined his model of how the moon influenced the tides. Aldersey-Williams claims that when Newton published the Principia Mathematica, many people who picked up his work did so in order to get a better understanding of the tides in particular.

From here, I moved on to Carson's The Sea Around Us. Last week, I had began the middle portion of the book, titled "The Restless Sea." This week, I finish that section.

Carson discusses the tides and how they're formed in the final part of "The Restless Sea." She begins by talking about the Straits of Messina between Sicily and mainland Italy, one of the few places in the Mediterranean where significant tidal phenomena exist, and talks about how the one of the whirlpools there is connected to the myth of Scylla and Charybdis. I noted that this was strikingly similar to what Aldersey-Williams discussed regarding the Straits of Messina quite early on in his book. Indeed, the way Carson talks about the tides reminded me a lot of The Tide, and knowing that Aldersey-Williams had referenced The Sea Around Us in the beginning of his book, it wouldn't surprise me if the tidal phenomena Carson chose to describe in her work influenced what Aldersey-Williams chose to discuss in his.

One of Carson's points I found important was the apparent paradox of the tides, in that although they are ultimately governed by large, cosmic forces (the gravitational pull of the moon, and to a lesser extent that of the sun), how the tides actually manifest themselves at the local level depends so much on small-scale factors, such as the slope of a beach and the general bathymetry of the local ocean, how open or enclosed a river mouth draining into the sea might be, and how much the tides' power might "buffered" by wetlands or barrier islands.

Probably my favorite part of this section of The Sea Around Us was Carson's description of how friction caused by the tidal forces is actually slowing down the earth's rotation at a steady rate, although that rate is one we cannot discern on a day-to-day basis (our 24-hour day is thought to be about a few seconds longer than it was several millenia in the past). The earth used to rotate much faster on its axis, completing a full rotation in about four hours instead of twenty-four. As the Earth's rotation slows, the moon's centrifugal force causes it to move further and further away from the Earth. According to Carson, the force of the tides decrease as the Earth's rotation slows, and millions of years into the future, the Earth will eventually rotate so slowly that a single day will be as long as a month, and at this point, there will no longer be any tides on Earth.

Carson then moves on to describe some marine organisms that time certain activities to the tides, such as the grunion, a small shiny fish that times its spawning activity on the beaches of California so that their mating and egg-laying occurs during the ebb of spring tides, in the spring and summer months. It was interesting to read about how difficult it has been for scientists to determine exactly why and how certain organisms "know" when a certain tide occurs, and why some organisms may choose to take advantage of spring tides while others choose neap tides.

To this point, Carson ends by talking about a small worm now called Symsagittifera roscoffensis, but which was called Convoluta roscoffensis when The Sea Around Us was written. The small worm has a symbiotic relationship with a unicellular green algae that produces all the food the worm needs via photosynthesis. I looked up the worms online and they are a stunning bright green and lovely to look at. What makes the worm even more fascinating is that at ebb tide, S. roscoffensis rises from intertidal sands so that it can get access to light and photosynthesize the food it needs. As the tide returns, the worms sink back into the sand so that they don't get washed out to the deep ocean.

How does the worm "know" to do this? Carson mentions that when a colony of the worms were taken from the water and placed in tanks with sand for research, the worms would rise from the sands and then sink back into them in time with the tides, even though there was no tidal action in the tank, and even though there was no way for the worms to perceive the sunlight outside. How these tiny worms have the rhythm of the tide imprinted into their bodies is a mystery.

I'll be blogging again about both books at the same time next week. Stay tuned!

References:

• Aldersey-Williams, H. (2016). The tide: The science and stories behind the greatest force on Earth. New York, NY: W. W. Norton & Company, Inc.
• Carson, R. L. (1951). The sea around us (1989 ed.). New York, NY: Oxford University Press, Inc.