Moons and Tides

In the recent film, "Antonia's Line," there is a character who can be heard howling at the full moon every month. She lives in a small village, and most of the townspeople who are awakened by her cries at night think she's crazy. But others accept this woman's need to acknowledge the power of the full moon -- knowing full well that they, too, feel the influence of the lunar cycle. Perhaps the full moon makes us all a little crazy.

While culturally we may not be well-attuned to the cycles of nature, it is a rare person who does not notice the changing shape of the moon. The lunar cycle lasts 29.5 days, during which the moon not only completes a full rotation on its axis but also makes a complete revolution around the earth.

What we see as the changing shape of the moon is actually a change in the part of the moon illuminated by the sun. Over the course of a lunar month, the moon appears to change from full to waxing gibbous (a crescent missing on the left), to first quarter (half moon, round on the right), to waxing crescent (open side on the left), to new (can't see it at all), and back around to full, the open and rounded parts falling on opposite sides for the second half of the cycle.

The moon's gravitational force is only one ten-millionth of that of the earth, yet it is strong enough to create tides in the earth's bodies of water (and to a lesser extent in the atmosphere and even on land). This, combined with the force created from the earth's revolution and the gravitational attraction of the sun, creates two bulging masses of ocean water (one balances the other) that move over the earth: tides.

The speed and height of a tide can be affected by local geographic features such as water depth, land formations and wind. While in the open ocean, the difference between high and low tide is only about one foot, on the land the difference can be more than 20 feet. A spring tide, during which there is the greatest difference of height between low and high, occurs when the sun, moon and earth are in line (full and new moons). When the sun and moon are at right angles (first- and last-quarter moons), a neap tide occurs, and the difference between the high and low is at a minimum.

Those of us who frequent rivers, whether as boaters, swimmers or fishermen, know full well that we must plan our activities carefully around the tides. I have been known to tell the story of an excursion 15 years ago that resulted in my parents dragging my sister and me ini a canoe through the mud of the South River. We hadn't consulted the tide chart and ended up beached in the riverbed at low tide.

There is a widespread tradition in this country of naming the full moons, usually to correspond with events occurring in the natural world. The sixth full moon of the year, that which falls closest to the summer solstice, was known in Colonial times as the rose moon (when the roses were in bloom) or the strawberry moon (when strawberries were ready for harvest). Colonial Americans also referred to the sixth full moon as the flower moon or the honey moon. (Is that why there are so many June weddings?)

This year the sixth full moon falls on June 30. Because it will be the second full moon in the month (we had another June 1), this will also be a blue moon. Despite what we mean by the phrase "once in a blue moon," a blue moon occurs more often than one would think. In fact, there will be two blue moons in 1996.

Full-moon summer nights are a great time to explore the rivers. Plan your trip carefully, wear a PFD (personal flotation device), and be sure to consult your tide chart!

by Kezia Bacon
June 1996

Kezia Bacon of Marshfield is the Assistant Director of the North and South Rivers Watershed Association (NSRWA).