Episodes

Space is black—so why is the sky blue…and the sunset red? It has to do with the properties of sunlight and Earth’s atmosphere. Sunlight is made up of many wavelengths of visible light. Combined, we see them as white. But individually they’re a rainbow of colors, from violet and blue to orange and red. When sunlight enters Earth’s atmosphere, it collides with molecules of oxygen, nitrogen, and other particulates, which scatter the light into different colors. Violet and blue light have the shorte...

To make it through winter, trees have developed two very different strategies. Deciduous trees, whose broad leaves are too delicate to survive the freeze, pull their sugars back into the body of the tree and let their leaves die and fall, as we explored in a previous EarthDate. Coniferous trees—like pine, spruce, fir, and cedar—have a few different solutions. First, their “leaves” are needles, adapted to the cold. They’re thick, have less surface area, and are coated with a waxy substance called...

In other EarthDates, we’ve talked about how water can shape Earth’s surface. It erodes mountains, slowly leveling them over millions of years. It fills aquifers, and when those are drained too much or too rapidly, the surface of the land above them can sink. But another way water can shape the land is simply by weighting it down. And when billions of tons of water evaporate from a mountain range or inundate a city, the land can rise or fall. California suffered a severe drought from 2011 to 2015...

In the 1970s, engineers drilled a series of wells off the U.S. Atlantic Coast, looking for oil. Instead they found water. Fresh water. Puzzled, they wondered if they’d found isolated pockets or a continuous aquifer. Thirty years later, a new technology arrived to help answer this question. Electromagnetic imaging could read the electrical conductivities of subsurface fluids to identify them. Saltwater is very conductive. Oil and fresh water are not. They surveyed the same area, and what they fou...

Within a tiny salmon egg, molecules of magnetite are already gathering. Once the egg hatches and the salmon begins to grow, it moves downriver, eating larger prey, taking in iron from its environment and its food, to synthesize more magnetite to store in its body—particularly in the sinus bones within its skull. Once it’s large enough to enter the ocean, its scales change from green and brown to the silver and gray of the open sea. The magnetite in its nose begins to line up into chains that can...

Think of Earth as a rubber ball. If you push in at one place, the surface will indent. Surrounding that indentation, the ball will bulge outward. Near the end of the last Ice Age, just 20,000 years ago, continental ice sheets covered 25 percent of Earth’s landmass, pushing down on Earth’s surface with as much as 150 tons per square foot of pressure. This depressed the surface and pushed up the land around the perimeter. During the last 15,000 years, the Ice Age ended and glaciers have naturally ...

You may remember from an earlier EarthDate that seawater is so salty, drinking it pulls water from the rest of your body, which can lead to dehydration and even death. To keep their ships stocked with fresh water, sailors have relied on innovation and technology for centuries. Early on, they realized they could funnel rainwater from their sails into storage—once the rain had washed away the ocean spray. Greek sailors then discovered they could hang sheep pelts in the cool night air to absorb wat...

The sun may appear constant but, like most things in the universe, it’s constantly in flux—cycling between an active state, called a solar maximum, and a more restful one, called a solar minimum. It alternates regularly between these about every 11 years. During solar maxima, sunspots increase but so do solar flares, making these times of increased irradiance. Higher-than-normal electromagnetic radiation causes geomagnetic storms on Earth that impact GPS systems, satellites, and power grids. Cha...

What makes leaves change colors so dramatically in the fall? Three things: leaf pigments, the weather, and the length of the days, called the photoperiod. To protect themselves from freezing, broad-leafed trees in temperate areas must harvest the sugar from their leaves for the winter. This starts when shorter days signal trees to slow the production of chlorophyll. As photosynthesis uses up the remaining green chlorophyll, yellow pigments that are always present in the leaves show through. Mild...

Earlier on EarthDate, we talked about the three North Poles. Of these, geographic north, the tip of Earth’s axis, defines latitude and longitude on our maps, while magnetic north is what compass needles point toward. Unfortunately, geographic north and magnetic north are not in the same place. So the north of our maps and the north of our compasses don’t match. The difference between them is called declination. To complicate matters, declination varies depending on your location. Compass navigat...

The human body is amazingly capable…despite the fact that, in many places, it’s engineered all wrong. Take for example, our lower leg. Our foot has 26 bones, and our ankle, 7. Far more moving parts than needed to walk or run, making us much more prone to injury. How did this happen? Well, we evolved from earlier hominins who evaded predators and found food in the trees. Those ancestors needed a grasping foot, like a chimpanzee’s—with a big toe like an opposable thumb and a flexible ankle like a ...

The most abundant element, making up an incredible 75 percent of the known mass of the universe, is hydrogen. It also might be the energy fuel of the future. For more than 4 billion years, hydrogen has fueled our sun, a nuclear fusion reactor that fuses hydrogen into helium to light up our solar system. Hydrogen has also fueled our rockets. It’s very lightweight, a huge advantage for space travel. Under extreme pressure, or at extremely low temperatures, it becomes liquid. Without a spark and an...

It might surprise you to know there are actually three North Poles: the geographic, the magnetic, and the geomagnetic. The one we most commonly think about is the Geographic North Pole—the tip of the axis that the globe spins around. It’s in the middle of the Arctic Sea, on pack ice, above more than 2 miles of water. During the summer, when the pole is tilted toward the sun, it’s 24-hour daylight. In the fall, the sun finally sets—once, and only once, per year. That brings on 6 months of night, ...

The Grand Canyon is so grand it can be seen from space. At more than a mile deep and nearly 300 miles long, it could hold all the world’s river water and still be only half full. And its colossal size is an evolving mystery. Early geologists could not believe such a comparatively small river could carve something so immense. So they looked more closely… And discovered that a myriad of geological processes have combined to form the canyon through time. One of the more dramatic is giant floods, va...

Since the Roman Empire, troops have used homing pigeons to carry messages from the front lines to command posts. They remained popular for long-distance communication, even after the telegraph was invented in 1844. In World Wars I and II, homing pigeons could operate faster than wires could be strung and farther than the troops’ radio signals. In one famous account, an infantry unit trapped behind enemy lines released three pigeons, but all were shot down. Despite her injuries, one took flight a...

Earth is mostly covered with water, and most of that is ocean. Only 2.5 percent is fresh water. Of that, 99 percent is locked up in glaciers and underground aquifers. That leaves just 1 percent of Earth’s fresh water on the surface. From all that salt water, how does this tiny fraction of surface fresh water come to be? It’s a process of natural distillation. Heat and wind turn seawater into water vapor. In the phase change from liquid to gas, water leaves salt and all impurities behind. In the ...

In October 1964, three young thieves cased the American Museum of Natural History in New York City. They returned that night to scale the museum wall, climb through a bathroom window, and steal 22 of the most precious jewels in the world. Among them were the Eagle Diamond, the DeLong Star Ruby, and, most famous of all, the Star of India sapphire. Sapphires are a variety of corundum, the third-hardest mineral. Pure corundum is clear, but when colored blue by titanium impurities, it’s called a sap...

If you’ve been to Iceland, you know it doesn’t have much ice. In fact, there’s so much grass that on maps it’s colored green. On the other hand, you probably know that Greenland is covered in glaciers. So why is the green one Iceland and the white one Greenland? Legend has it that the Vikings who discovered Iceland wanted to protect it from settlement, so gave it an unflattering name. But it was actually a matter of perspective. The first explorer to Iceland had a terrible trip. His daughter die...

We often think that evolution takes thousands of years. But in rare cases where humans impact small populations, adaptation can work much faster. Take the case of the tuskless elephant. Nearly all male elephants and most females have tusks. These are just elongated lateral incisors that grow outward once the elephant loses its baby teeth. But a small percentage of elephants are born without these teeth and never develop tusks. In 1919, the South African government brought trophy hunters to the E...

This 4th of July, try regaling your fellow revelers with some firework history and, yes, geology. They’ll probably know that fireworks originated in China. But they likely won’t know they started as simple bamboo sticks thrown into a fire. The air inside the hollow stalks expanded, then exploded, making a “crack” that the ancient Chinese used to ward off evil spirits. A few centuries later, legend has it that a kitchen recipe gone awry combined charcoal, saltpeter, and sulfur. Who knows what foo...

Leonardo da Vinci once said: “We know more about the movement of celestial bodies than about the soil underfoot.” He was right 500 years ago, and he’s still right today. That’s partly because the study of space is mysterious and cool, and there really wasn’t much interest in studying dirt. Until recently... We now know that about 90 percent of all land-based species live in the soil, not on it. Most of these are microscopic, but they’re incredibly plentiful: there are more microbes in a handful ...

We talked about how a fungal network connects plants underground. But did you know it allows them to communicate? Specifically, they exchange sugars, and chemical and electrical signals, with each other. The largest trees are now known to “mother” the surrounding forest. They give their sugars back to the entire soil community, to support neighboring plants and trees. They’ve even been shown to preferentially identify young trees within their own species, and send them a larger serving of sugars...

If you’re driving while listening to this, please think of your vehicle not as a mere car or SUV, but as a starship cruiser! Here’s why: the iron it’s made of came from the heart of a distant star. Stars begin their lives as giant balls of gas, mostly hydrogen, the first element on the periodic table, with one proton. Under the force of the proto-star’s enormous gravity, hydrogen atoms fuse together to produce helium, with two protons. This nuclear fusion releases a huge surge of energy, and the...

Why did the ancient Greeks build their cities in earthquake zones? For several—very good—reasons. Greece and Turkey lie along massive fault zones. Faults, when they move, create earthquakes. Spring water tends to follow these fault systems, and the Greeks, following the water, did, too. Fault lines also create cliffs, which provided natural defenses for the cities. And fault zones tend to form surface depressions where soils can accumulate, making them good for agriculture. So the faults gave Gr...