Zoom

by Bob Berman

Suddenly, without preamble, a yellow dust devil appeared perhaps forty yards in front of me, and I slammed on the brakes, creating a competing dust cloud. It was a dead ringer for a tornado, a miniature version. I got out and had to crane my neck to see how very high it towered into the blue cloudless sky. And now it was joined by a twin to my right. Swirling crazily, they both moved ahead at maybe walking speed and showed no sign of dissipating. Each was perhaps six feet thick. In the unchanging sameness of the desert, where everything else was utterly motionless and even the wind was perfectly calm, this sudden lively animation was startling. The fierce whirlwinds were not just surreal. To tell the truth, they were downright spooky.

  Unlike tornadoes, dust devils develop from the ground up. They favor dry places, such as deserts, and do not form from clouds. Indeed, like the pair I was now observing, they usually materialize beneath calm, cloudless skies.1

  I knew that they could reach above the tallest skyscrapers, but these towered perhaps three hundred feet. Thirty stories.

  In the dry, very thin air of Mars, the sudden materialization of dust devils marching across the chocolate-orange soil seems like the work of spirits. In fact, these dust devils are referred to in Arabic as jinni, which means “demon” and which was the origin of our word genie. They abruptly give that lifeless red planet the brash hint that, yes, the hand of nature still stirs even there, where Earth is a mere dot in the sky.

  Those bizarre “spirits” may even be benevolent. On March 12, 2005, technicians monitoring the Mars rover Spirit found that a fortunate encounter with a dust devil had blown off the thick dust on its solar panels, which had choked off much of the power supply. Now, suddenly, electricity generation dramatically increased. Expanded science projects were joyously scheduled. Previously, another rover, Opportunity, had also had its solar panels mysteriously cleaned of accumulated dust, and a dust devil was likewise assumed to have been the cause.

  I got the sudden urge to step into one. Would it be dangerous? How fast were those winds, exactly?2

  I’d heard that dust devils sometimes throw jackrabbits into the air. But the only truly scary story I’d ever encountered was that of three children who sat in an inflatable playhouse just outside El Paso, Texas, in 2010. The trio were taken into the sky, playhouse and all, carried over a fence and three houses, and then deposited on the ground without serious injury.

  The impulse was irresistible. It was my investigative duty as a science journalist, I rationalized. I trotted clumsily across the sands toward the nearest dust devil, my feet sinking with each step, but the whirlwind moved away like a mischievous jinni. It kept eluding me, and then the farther one dissipated abruptly, as if in a dream.

  When I finally turned back toward where I thought the car and dirt road were, there was no trace of either. They must be hidden in a depression, I thought. With the lone dust devil snaking away, the silence of the harsh, sunlit desert was overwhelming.

  I stood, mesmerized by the isolation. I felt quarantined from the human race.

  Anyone who has been to a desert knows its hypnotic appeal. In 2006 I had gone to the Sahara to meet the moon’s shadow, but that total solar eclipse wound up as merely the initial enticement for me; the desert’s magic only grew. And much earlier, as a twenty-two-year-old wandering the world with a backpack, I spent a couple of weeks in the great vast desert of southeastern Iran, between Kerman and Zahedan, where the nightly skies are as inky black and star-filled as I imagine they are on the far side of the moon.3 I had also loved the Thar Desert, with its herds of wild camels and friendly people, in the Rajasthan wastelands of western India. Every desert is unique. This one, the Atacama, is special in several ways.

  For starters, it’s the driest of them all. In some sections no measurable rain has fallen for the past five years. There is thus not the slightest trace of even scrub vegetation. The cold, rich South Pacific, with its famous Humboldt Current, laps at the desert’s beaches, where penguin colonies nest in its protected bays, while the forbidding peaks of the Andes abruptly define its eastern edge. These mountains are the culprits that manufacture the aridity. The prevailing easterly winds are forced to rise, cool, and dump their moisture on southern Bolivia and northern Argentina. At night, nearly continuous lightning over the Andes hovers above the unseen border between the two countries. When the air descends from the Andes it is bone dry.

  Lack of rainfall and vegetation are the calling cards of most deserts, but they also hold one other feature in common: the classic stage set of blue sky and fierce sun. With no trees to cast shadows, the Atacama offers no relief.

  Standing amid a 360-degree panorama of stark, sandy, sunlit isolation, I realized I’d overlooked the most central “action figure” of all, the one whose natural motion rules everything else. The sun.

  The author appears lost in the desert. On all the world’s ergs, sand moves in a precise, mathematical way.

  For most of us, it is sometimes factored into our plans: Should we cancel our beach trip if it’s cloudy? But it’s relatively rare for the sun to modify our behavior in modern times. We mostly ignore it. Even science nerds are only vaguely aware of its various cycles and quirks.

  But here in the desert there is nothing else. The sun calls all the shots. And if you’re stuck without a means of exit, it ultimately decides whether you live or die.

  Its most basic animation is the day-night rhythm. This remains steady throughout our lives but is much less uniform over the lifetime of our world. When the first dinosaurs walked through the Meadowlands of New Jersey—then part of the supercontinent Pangaea—the year had four hundred days.

  Not impressed? Then look back further, to when life first appeared. Earth spun much faster then. The environment was truly alien, unrecognizable as the precursor to today’s world. The air had no oxygen. The sun was 30 percent dimmer and daily crossed from horizon to horizon in five hours. It visibly moved. Shadows perceptibly shifted, as they do in time-lapse nature photography.

  As the sun spins once a month, its surface pulses up and down like that of a subwoofer. (Matt Francis)

  The moon’s tidal tug creates an oceanic bulge beneath it and a second bulge on the exact opposite side of Earth. These bulges travel around as our planet spins, exerting a bit of torque as countless tons of seawater smash into coastlines to deliver the “High tide!” news to bathers and gulls. Continuously lengthening our days by slowing our spin, the moon’s tides make the sun move ever more sluggishly across the sky.

  We’re reminded of this every year or two when scientists announce the insertion of a “leap second” into the final minute of June or December. Television stations give this job to their meteorologists, who explain that extra seconds are needed because our planet is winding down and will ultimately make each rotation, each of our days, forty days long in the far future.

  But if you’re a serious card-carrying geek you’ve surely stopped in your tracks, grabbed your calculator, and said to everyone within earshot, “Wait a minute! They add a second every year or two? Earth can’t be slowing that quickly. It just can’t!” You go tap-tap-tap on the instrument’s keys and realize that if our planet’s day was really growing a second longer every couple of years, we’d have come to a frozen halt billions of years ago. Something doesn’t add up. Something about the rotation of our world simply doesn’t make sense.

  Because the media always get this wrong, here’s the real scoop. The answer involves beauty. Poetry, even. After all, a watch set to the right time is a device synchronized with Earth’s rotation. It lets Orion and the Dog Star march to the beat of the timepieces on our wrists or, more likely these days, the überprecise digital time on our smartphones, whose signals are periodically synchronized with atomic clocks even if we don’t care about such accuracy.

  In the 1950s an important decision was made, an agreement between every nation on our whirling planet. It was, simply, that Earth’s spin rather than vibrating quartz crystals or any other timekeeping method would dictate the time.
This meant we needed two parallel monitoring systems kept in sync with each other. One is our planet’s spin, constantly scrutinized by an agency in France called, not surprisingly, the International Earth Rotation Service.

  The other system requires the careful daily marking off of 86,400 seconds, each precisely defined. These official ticktocks are counted by forcing the nucleus of the cesium 133 atom to maintain a particular spin direction, which it does only when bathed in 9,192,631,770 microwave pulses per second. Any other frequency changes the cesium. So an atomic clock is simply a vacuum chamber where a fountain of gaseous cesium atoms are bathed in microwaves and the state of the cesium is continuously monitored. That’s the story. A servomechanism slightly varies the microwave frequency if required. An official second is thus 9,192,631,770 microwaves, just what’s needed to maintain cesium 133 in a fixed condition. That exact number of microwave pulses is the definition of a second.

  The official second remains constant. Earth, alas, does not. Along with spin irregularities not fully understood, observations of the stars show that our planet’s day becomes one seven hundredth of a second longer after each century has passed.

  This may seem too trifling to matter at all. Compared to the day you were born, the day you start receiving Social Security checks is one thousandth of a second longer. Sure, this adds up, but it’s way too little to require meddling with clocks every year or two. So again, why those leap seconds?

  Here’s the explanation that, guaranteed, nobody on your block knows.

  When the current system was set in place in the 1950s, astronomers had been using earth-rotation data collected over the previous three hundred years. The official length of a day was codified in 1900. But during those centuries of observation, a day’s length slowly grew. Careful analysis now shows that a day was exactly 86,400 seconds long in 1820. Before that each day was shorter. Since then it’s been longer.

  We generally labor under the illusion that 86,400 seconds make up a day. But this hasn’t been true for nearly two hundred years. A modern day is 86,400.002 seconds long. So we messed up. When the current system was put into place a half century ago, we could have then defined each second a little differently by adding a couple hundred more of those microwave beats to each official second. Who would care? Then our clocks would almost never need leap seconds. But we didn’t. So every year or two now, the little daily error accumulates enough so that we must take care of the accrued discrepancy.

  To sum up, the real problem is not that Earth is slowing, which happens too gradually to matter much. It’s that each of our current days is longer than a day was in 1820, upon which our timekeeping system is, bewilderingly, based.

  Because we foolishly designed the “second” around the 1820 data, we now need to compensate for the difference between a day now and a day when James Monroe was president. That means adding a second every five hundred days or so. It’s a “patch” to keep Earth-spin time and atomic-seconds time in agreement.4

  As the Earth slows, the sun moves more leisurely across the sky. Its gradual slowdown is of course unnoticed in human lifetimes. Instead the dominant rhythm that affects us is its position in the sky. The sun is low and feeble during winter and high and fierce in summer. The daily light-darkness ratio—winter’s short days and long nights—is also critical. Other than that, most folks are oblivious to the sun’s motion. How many people even realize that throughout the Northern Hemisphere, in the United States, Europe, China, and so on, the sun always moves to the right? Meaning the sun rises diagonally upward to the right, then moves directly rightward at midday, and sets by slinking rightward into the western horizon.

  Equatorial residents view something different. There the sun rises straight up until it gets overhead. Then, through the afternoon, it drops straight down like a lead ball. Because of this, it quickly buries itself below the horizon after sunset. Twilight in the tropics is always short. In the Southern Hemisphere, the sun moves leftward during the day. It’s a quick way of knowing where you are in case you’re ever shanghaied and wake up on another continent.

  Can you handle one more solar oddity? Over the course of a year, day and night are not balanced. Thanks to our atmosphere, which bends light, the sun seems to sit on the horizon when it’s actually already set. At that point we see a ghost, a solar phantom. This air trickery, refraction, grants most locations seven minutes of extra daily sunlight. It’s why days and nights are not equal at the equinoxes: sun dominates.

  This undeserved sunshine adds up. We enjoy forty extra hours of sunlight annually. The year is not even close to a fifty-fifty day-night mix.

  On top of that, as we all know, sunset is never followed by sudden blackness. On the moon, yes, but not here. Refraction delivers its enchanting gift of twilight. Its brightest portion bestows yet another hour of useful light split between dawn and dusk.

  The brightest afterglow is called civil twilight. Although it sounds vague, the term twilight is precisely, legally defined, dictated by the sun’s unseen motion below the horizon. In the evening it’s the interval between sunset and the time when the sun has sunk six degrees, or a dozen sun widths. Civil twilight lasts about a half hour in most places. At its conclusion, according to many municipal ordinances, streetlights must be on.5

  But the bottom-line sun motion is its speed as it crosses the sky. Most people don’t know about angles or degrees, so let’s simply use the sun’s own width as a measuring tool. Think about all the sunsets you’ve watched. How long does it take the sun to move a distance equivalent to its own diameter? Or ponder the moon instead, which moves at the same visible speed. The answer:

  Crossing the sky, the sun traverses its own width in exactly two minutes.

  During a sunset, because the sun slides into the horizon at an angle, the interval from first contact to complete disappearance is about three minutes. This is right on the borderline of perceptible motion. The sun appears to move at the same speed as the minute hand of a kitchen clock when viewed from a few feet away.

  Our final desert-motion phenom is its most renowned specialty: the mirage. As we all know, mirages are common on hot surfaces, such as a highway on a summer afternoon. The culprit is the changing speed of light. Despite its reputation as a constant, light travels more slowly through cool air. But the hot air above a summer road or broiling sand lets light move faster right there, closer to its vacuum speed, and this change bends, or refracts, images hitting it. The result is a mirror effect. The air reflects the sky, perfectly mimicking a puddle of water.

  But finding any movement was an impossible job when I was in the desert. Nothing budged once those dust devils died. The absence of flowing water, moving clouds, circling birds, buzzing insects, or rustling leaves makes the desert visually frozen. A still photograph. Its landscape offers the antithesis of animation.

  But later there came a few hot afternoon gusts. Bits of sand blew momentarily. The still life came alive. Clearly the dunes migrate over time. And when it comes to shifting sands, only one person is associated with their vagaries. British brigadier Ralph Alger Bagnold.

  He was the archetypical English stiff-upper-lip, military-cum-Renaissance man. Bagnold was born in 1896, son of a derring-do colonel in the Royal Engineers who gloriously participated in the 1884–85 rescue expedition that attempted to free Major General Charles George Gordon from Khartoum. His sister was Enid Bagnold, who wrote the bestselling 1935 novel National Velvet.

  Armed with this odd genetic pedigree, Bagnold attended Malvern College, joined the Royal Engineers, as his dad did, and received medals for serving in the miserable World War I French trenches for three years. After the war Bagnold studied engineering and earned a master of arts degree at Cambridge University. He returned to active duty in 1921 and then got swept into his lifelong calling. He served in Cairo and the Thar wastelands of northwestern India, and at both places he spent every spare minute exploring the desert.

  Bagnold described his extensive excursions in his book Libyan
Sands: Travel in a Dead World (1935). He developed a special type of compass that would not go awry around the iron ore often buried in arid regions. It was he who discovered that one really could drive a car across the Sahara as long as you let most of the air out of the tires and kept punching the gas pedal when the sands got deep. You got the feeling this was knowledge gained the hard way.

  Although a third of the world’s deserts are covered with sand, there has been very little research into these ergs, as sand-covered desert areas are, oddly, called—probably because it’s hard to travel there or even reach many of them, and at that point it’s very slow going to make much physical progress. Bagnold changed that with his still-definitive book, published in 1941, The Physics of Blown Sand and Desert Dunes, which is every bit as tedious as it sounds. After the first two or three chapters I discovered that it is not a page-turner, despite the Amazon five-star rating that lured me to purchase it. But no one to this day has improved on its revelations. Bagnold used wind-tunnel experiments to predict sand movement and confirmed these expectations with extensive observations in the Libyan desert.

  Basically, sand is characterized by its size rather than its composition. Bagnold defines sand as any particle between 0.02 millimeter and 1.0 millimeter in diameter, although later experts generously expanded the upper range by more than 50 percent, to 1.6 millimeters—a fifteenth of an inch. Size matters, because sand is defined as consisting of grains small enough to be moved by the wind but too heavy to remain in suspension in the air, as dust and silt do. Particles too heavy to be blown by wind are classified as pebbles or gravel. If it’s smaller than a thousandth of a millimeter, a particle essentially remains suspended in the atmosphere and scarcely falls at all. But then it’s called smoke or dust, not sand. It’s not rocket science.

  Although sand can be composed of nearly anything, most of it is quartz, essentially because quartz is common and, Bagnold explained, “resistant to both mechanical and chemical breakup into smaller sizes.”