Changing Planet

Low Tide at Guernsey

The British Isles are known to have some of the wildest variations in tidal heights on Earth — note the stains on the seawall in St. Peter Port, Guernsey Island. Photo by the authorLow Tide at Guernsey

It was the incomparable Sir Isaac Newton who demonstrated that the gravitation of the Moon (and somewhat less, the gravitation of the Sun) created the tides on Earth.

The universal law of gravitation, formally introduced by Isaac Newton in his definitive treatise, “Philosophiae Naturalis Principia Mathematica” (“Mathematical Principles of Natural Philosophy,” 1687), explained that the force between two bodies is proportional to the product of their masses and inversely proportional to the square of their separation. Because of the immense disparity in the masses of the Sun and Moon (a ratio of 27 million) and in spite of the inverse square of the inverse of their relative distances squared (1/395^2), the Sun-Earth attraction is 174 times greater than the Moon-Earth attraction.

However, the distance between two diametrically opposite locations on the Earth (i.e. the diameter of the earth) compared with the distance between the Sun and the Earth is a miniscule 0.000 043, whereas the same two diametrically opposite locations on the Earth compared with the distance between Moon and the Earth is 0.016 06. In light of these two figures, the Sun’s effect on Earthly tides compared with that of the Moon is approximately (0.016 06/0.000 043) x 174 = 0.46. The Sun’s effect is only 46% of the Moon’s.

The British Isles are known to have some of the wildest variations in tidal heights on Earth (note the stains on the seawall in St. Peter Port, Guernsey Island). Two other area that sees such drastic difference in tides are the Bay of Fundy in Canada and Collier Bay in down under northwestern Australia. Record differences between high and low tides of up to 15 m (50 ft.) have been recorded at these three sites. They occur when the Moon is in its perigee (closest point in its slightly elliptical orbit around the Earth), when the Earth is in its perigee around the Sun, and when the three heavenly bodies are co-linear.

The earth of course rotates around its own axis in 24 hours, but the moon takes a relatively long time (28 days) to orbit the earth. Accordingly, a high tide in Guernsey would have occurred when the moon is directly above the longitude of the Isle of Guernsey, i.e. making a 90° angle relative to the horizon. At the same time, the longitude diametrically opposite that of Guernsey — somewhere in the Pacific Ocean — is farther from the moon than is be the rest of the earth. There the tide is also be at its highest. Twelve hours later, when the moon is above the latter longitude, high tides will again occur at these two spots. Conversely, low tide occurs, as in this featured photo, when the moon is above the eastern (and western) horizon, i.e. 0° and 180°. In short, the sea “bulges” twice during the day, 12 hours apart, when the moon is closest and when it is farthest from the same meridian. In this photo, the moon would have been at 0° (or at 180°). This photo was shot on 6 September 2014 at 9:00 am.

The picturesque Island of Guernsey, although geographically closer to France, has been staunchly British through the ages. Its diverse culture, however, reflects the best of both countries. Pastel-colored houses rise layer upon layer behind St. Peter Port, the island’s picturesque main town, where goldsmiths, silversmiths, wood carvers and clockmakers feature their ware.

A favorite son of Guernsey, Victor Hugo, French author and poet, spent 15 years in exile in St. Peter Port. It was there that he wrote his masterpiece, “Les Misérables” (1862). Although he is known to the world as a surpassing writer, he prided himself more with his passion for interior and landscape design, embodied in his home, Hauteville House.

Acknowledgment: I am grateful to Aussie friend Chris Jules who pointed out the large tidal variation in northwestern Australia. He wrote, “On a Kimberley spring tide, the difference between a high and a low tide can be as much as 14.1m as measured in Collier Bay near Yule Entrance (-0.4m – 13.7m range).”

Bulent Atalay, a scientist, artist and author, has been described by NPR, PBS and the Washington Post as a “Modern Renaissance Man.” He is the author of two successful books on the intersection of art, science and mathematics, with Leonardo, the pre-eminent Renaissance man, serving as the foil. His best selling book, "Math and the Mona Lisa," (Smithsonian Books, 2004) has appeared in 13 languages. Professor Atalay's academic background is in theoretical physics. He travels around the world lecturing at academic institutions and on cruise ships on the "A-subjects," art, archaeology, astrophysics, atomic physics and Ataturk, confessing that he knows much less about the "B-subjects," business, banking, biology and botany... He is the President of the Ataturk Society of America (ASA), dedicated to promoting Ataturk's ideals of science and reason over dogma and superstition, of a secular state with full equality of genders. For more details click on Bulent Atalay
  • Will green

    Actually the greatest differences of tides are in the Bristol channel. Mainly because of the funnel like geography

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