Look at the moon, and the last thing you think of is life. Suspended in space some 239,000 miles from Earth, it’s great for gazing at. But biologically, despite the recent discovery of water traces in shadowed craters, it’s a wasteland. Its air is unbreathable. Its temperatures are extreme: a blistering 243 degrees Fahrenheit during the month-long lunar day, followed by another month of deep-freeze night at 272 degrees below zero.
But that's the moon on the moon in isolation. The moon and Earth are linked in countless ways that influence life here: Astronomers, for example, tell us that without the moon’s stabilizing gravity, our planet would wobble erratically on its axis, creating climatic chaos. Other connections are less consequential and more reflective of our culture: moonshine, honeymoons and cows jumping over the moon. But the moon is connected to Earth’s biological life, too, in ways both literal and figurative, direct and indirect. Here are a few examples.
Each spring and fall, our skies fill with the beating wings of birds making their annual migrations — a phenomenon that has intrigued scientists for millennia. Aristotle was among the first to suggest an explanation for the birds’ mysterious seasonal appearances and disappearances. Some species, he thought, simply hid themselves in the ground until spring. “Swallows, for instance, have often been found in holes, quite denuded of their feathers,” he wrote.
Eclipsing all other far-fetched migration theories, though, was the one presented in 1703 in a booklet titled Whence Come the Stork and the Turtle, the Crane and the Swallow, When They Know and Observe the Appointed Time of Their Coming. Birds, suggested the publication’s author, fly to the moon to spend the winter.
It’s a funny proposition in light of modern science, but don’t laugh too hard. For the past 100 years, counting birds “on” the moon has been one of ornithology’s most important tools for calculating how many, and in which directions, birds migrate. The technique, called “moonwatching,” uses the disc of the full moon as a backlit random sample of the night sky. Although some birds migrate during the day, the vast majority — swallows, sparrows, herons, warblers, flycatchers, nuthatches, wrens, orioles and most others — make the trip at night.
Around the turn of the 20th century, astronomers using telescopes noticed that birds flying in front of a bright, full moon cast clearly discernible silhouettes. By counting the number of birds passing across the moon’s face in a given amount of time, and extrapolating that number based on the size of the full moon relative to the whole sky, scientists can calculate a fair approximation of the total numbers of birds traveling during a given period.
Today, radar is the primary data-gathering tool for studying night-migrating birds. Moonwatching is still used, too, often in conjunction with sophisticated radar and infrared sensors. Tech-equipped moonwatchers can not only count birds, but also can determine their distance, altitude, size and — in some cases — species.
You don’t need special training or high-tech equipment to try your own moonwatching, though. A clear night sky, full moon, watch, pencil and paper, and binoculars (or a telescope) will do. Ready? For 15 minutes, watch the moon carefully for the silhouettes of birds passing across it. Count them, and jot the number down. Give your eyes a few minutes’ rest, then repeat the process three more times. Add the four figures to get the number of birds you viewed per hour.
Of course, the moon occupies only a small portion of the night sky. Imagine a half-circle (180-degree) arc stretching from horizon to horizon. The moon takes up a little more than half a degree (0.581 to be specific, give or take a few hundredths) within the night sky, or about 1/347.4 of the entire visible sky dome. Multiply the number of birds you counted by 347.4. If you counted 20 “moon birds” in an hour, about 6,948 birds (20 times 347.4) traveled across your whole-sky field of view during that time.
That may sound like a lot of birds, but by some measures you had a slow night. At the height of autumn migration along the Gulf Coast, researchers have counted up to a million songbirds per mile passing in a five-hour period.
Among the world’s most spectacular insects are the moon moths — large, colorful and feathery-antennaed, their multicolored hindwings trailing long, delicate tails that flutter behind them in flight. Most moon moth species inhabit far-off places: Thailand, Madagascar, India, Japan and Africa. But the United States (or, at least, the eastern half) has its moon moth, too: the lovely pale-green, yellow- and lavender-tinged luna. Chances are, when you think of an attractive moth (as opposed to the ones that eat your clothes and invade your pantry), your mind’s eye ogles a luna.
But the luna’s beauty begins well before its fleeting week-long existence as a flying, mate-seeking adult. Like nearly all moths and butterflies, lunas go through the progression of life stages — egg, larva (caterpillar), pupa and adult — called metamorphosis. Females lay eggs, barely one twenty-fifth of an inch long, on the leaves of favored food trees — birch and alder in the North; hickory, sweet gum, persimmon and walnut in regions southward. Ten to 12 days later the eggs hatch, each yielding a bristly, quarter-inch baby caterpillar weighing a fraction of a gram. It’s a tiny but voracious leaf-eating machine so efficient that it must shed its skin three times over the next three to six weeks to make room for its ballooning body. In its final stage, a luna caterpillar is roughly the size of your thumb (picture a bright-green, yellow-striped Michelin Man lying on its side).
When the time comes to pupate, the luna larva turns a reddish brown and spins a wispy, walnut-size translucent cocoon around its hardening, compacted body. Here, nestled among sheltering leaves at the base of its food tree, the insect rests and develops before it emerges as an adult.
Moderating temperatures trigger most moths and butterflies to come out of this dormant stage — but not the luna. Instead, it “looks” for increasing hours of daylight — not an easy order for a creature with no functioning eyes. But the luna pupa possesses a clear, windowlike patch of cuticle on its head — a “moon roof” — that allows its brain to detect lengthening days through its cocoon. When that happens, the moth will exit its cocoon, crawl up a tree, and hang upside down while fluid pumps into and expands its delicate wings. But until that time comes, the luna pupa slumbers within its silken boudoir, waiting for the kiss of lingering light — truly a sleeping beauty.
Near the top of the tasty list among nature’s free-for-the-picking edibles are wild grapes, a late-summer and fall treat in most regions. The next time you notice a bunch of dark, just-ripe wild grapes dangling from a leafy vine, though, do not pop them into your mouth before taking a closer look. They might be a bad-tasting, dangerous look-alike: moonseed. Known as Canada moonseed, but distributed throughout the entire eastern third of the United States as well as southeastern Canada, the plant’s heart-shaped leaves and bluish-black fruit suggest wild grapes. But there are important differences: A moonseed vine lacks the grapevine’s curling tendrils, and its leaves are smooth-edged, not jagged-edged like grape leaves. Squeeze one of the plump berries. If you find several round or oval seeds inside, you’ve just squished a perfectly good grape. But if the fruit yields a single crescent-shaped kernel, like a miniature moon — you’d best seek a different vine for your snack. Moonseed berries contain a mix of toxic alkaloids that can cause severe stomach cramps and worse — several sources cite old reports of the deaths of children who mistook the fruit for grapes.
But that’s the dark side of the moonseed. On the sunny sides of forests and streams throughout its range, the plant provides an important — and safe — food source for many birds, including cedar waxwings, towhees, robins and thrashers.
Round and round and round we go, Earth and moon about the sun in timeless synchrony, each body’s gravity affecting the other’s in ways both subtle and not-so. Here on Earth, the moon’s pull, along with the far more distant sun’s lesser pull, sloshes our oceans in predictable rhythms we call tides. Twice a day the tides rise high and retreat low, alternately immersing and then exposing or partially exposing to air the restless edges of our world that are neither land nor sea: intertidal zones.
Here, within the areas between the highest of high tides and the lowest of low tides, vast quantities of marine organisms live, reproduce and die in cadence to the changes the moon’s gravity brings. It’s no easy place to survive, but eons of adaptation have produced diverse communities of resilient, prolific creatures. Housed within protective exoskeletons, snails, chitons and barnacles endure exposure to potentially drying wind and sun during receding tides. Plankton-feeding mole crabs burrow higher or lower on the beach to match the tides’ shifting food deliveries. Soft-bodied anemones, sponges and jellyfish absorb the rough tumble of restless surf.
Intertidal zones, say biologists, support the densest concentrations of marine life in our oceans; they are, in effect, protein factories. There can be as many as 7,000 mussels per square yard in a single colony. Countless organisms — mollusks, crustaceans, worms, fish, starfish, jellyfish, sluglike nudibranchs, algae, anemones, urchins and many more — flourish within an environment enriched by oxygenated water and literally awash in food; each species is both predator and prey. And the soup kitchen extends far beyond the intertidal zone. During the breeding season, tidal currents carry vast clouds of eggs and larvae from clams, worms, mussels, oysters and other creatures out to sea, providing sustenance to many deep-water species.
Of course, coastal habitats vary, from rocky shores and sandy beaches to salt marshes and estuarine bays. So, too, do the communities of species in each intertidal ecosystem. Geography plays a pivotal role in the tides themselves, too. In rocky, narrow-mouthed bays such as the Gulf of Maine’s Bay of Fundy, tides can rise 50 feet or more; along the Gulf of Mexico’s open sloping shoreline, the difference between high and low tide may be only a couple of feet.
But wherever water and terra firma overlap, the moon stirs an ancient, sustaining gumbo of living protein and nutrients. In the depressions and holes we call tidal pools, sunlight and evaporation concentrate the broth and tides bring a steady supply of new minerals and nutrients. The result 600 million-plus years ago may have been not just any old soup, but the primordial stuff itself. Biologists think that tide pools, as well as deep-sea geothermal vents, likely were the birthing grounds of our planet’s first living cells.
For centuries we humans have blamed the moon and its cycles for our wacky behaviors — the word “lunacy” is derived from the Latin word for moon, “luna.” Crime, emergency room admissions, depression — you name it, and we’ve pointed an accusing finger at the moon. Likewise, birth rates, stock market performance, dog bites and medical miracles have all been attributed to lunar influence. The jury is out on whether there is any substance behind such beliefs; for every study that says “yes,” another says “no.”
There is evidence, however, that lunar cycles have at least some effects on the behavior of certain insects, birds, fish and mammals. In some species the moon may cause hormonal changes that influence reproductive cycles. But in most cases, it’s the moon’s light that alters behavior. Most nocturnal creatures, except for predatory birds such as owls, are less active when the moon is full. Research has shown, for instance, that field mice eat more, badgers mate more often, and sea creatures forage more widely during relatively dark, new-moon periods. The reason is logical enough: Predators are less likely to catch you when it’s harder to see you.
An exception to this may be ruffed grouse, game birds that live in brushy habitat in much of the United States and Canada. If you’re a hiker or hunter, a grouse has probably set your heart to pounding when it burst from its hiding place with a loud whrrrrrrrr of wings. Ruffed grouse are normally shy and secretive and stay close to the nest, flying only when threatened. But in autumn — especially on nights of the full moon, according to many — the year’s broods suddenly disperse in what ornithologists call “crazy flights.” Young birds lose their sensible ground-hugging ways and take off in all directions — often flying headlong into buildings, trees and picture windows. Sheer lunar-cy.
As rare a treat as a rainbow is, its nighttime version — the moonbow — is rarer still. Little wonder, for conditions must be exactly right: a clear, dark night, heavy mist or raindrops in front of you, and a particularly bright full or near-full moon shining low in the sky behind you.
We humans have poor color vision at night, so a moonbow often appears to us as a ghostly white, gray or silver arc — itself a beautiful sight. But sometimes moonbows reveal their full spectrum. In 1879, Mark Twain reported while out to sea “a magnificent lunar rainbow — a complete arch, the colors part of the time as brilliant as if it were noonday.”
Witnessing a rain-spawned moonbow is a matter of luck, but in some places misty waterfalls offer predictable viewing. Moonbow watching was once a popular tourist attraction at Niagara Falls, but no longer — thanks to air pollution and artificial lights. Among the current hot spots for catching a moonbow are Africa’s Victoria Falls, Kentucky’s Cumberland Falls, and — in spring and early summer, when snow runoff is at its peak — Yosemite Falls in California.