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  • How Snowflakes Grow

    How Snowflakes Grow
    2026/03/29
    You’ve probably heard that no two snowflakes are alike. And mostly, that’s true. Snowflakes aren’t formed from freezing water, but when water vapor freezes, skipping the liquid stage altogether. This allows the water molecules to organize themselves into the snowflake’s delicate structure. Snow crystals come in more than 30 basic shapes, with the classic star-shaped snowflake forming best in temperatures around 5 degrees Fahrenheit. These begin when water vapor freezes around a nucleus like a dust particle. This forms a single hexagonal ice crystal, which is heavier than air and so begins to fall. As it does, the ice crystal grows. The original hexagon sprouts hexagons on each corner. These grow hexagons on their corners. And so on. But the exact form that the growing snowflake takes is influenced by changing humidity and temperature levels in the air it falls through. And because no two flakes follow the exact same path from sky to Earth, no two grow in the exact same conditions, and therefore no two are the same. The only exception is when scientists make them in a laboratory, where they can control the conditions so precisely, they can produce identical flakes. So if you find yourself out on a snowy day, take a moment to marvel at the thousands of snowflakes you can see falling, the millions in your yard, or billions in a field. Like people, no two are or ever will be the same.
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    2 分
  • Winter Sun Close Encounter

    Winter Sun Close Encounter
    2026/03/29
    Each winter in the Northern Hemisphere, it feels like Earth is drifting farther away from the warmth of the Sun. In fact, it’s just the opposite. When we measure the size of the Sun in our sky, we find that it actually appears 3.4% larger in winter—because it’s 3.4% closer. More than 3 million miles closer. The Sun is about 95 million miles away in summer, but just 92 million in winter. And this is because Earth’s orbit is elliptical, with Earth following the closer part of its path during the northern winter. You’d expect the nearer Sun to provide more solar energy to our planet and you’d be right: almost 7% more. But its effect is greatly minimized by the 23-degree tilt of Earth’s axis, turning the Northern Hemisphere away from the Sun in the winter. At the same time, in the Southern Hemisphere summer, that extra solar energy is mostly absorbed by the huge southern oceans. The effects may be minor today, but that won’t always be the case. The gravity of Saturn and Jupiter pulls Earth through a long cycle of orbital stances, from nearly circular, to more elliptical. We’re now as close to circular as our orbit ever gets. In about 200,000 years, we’ll be at our elliptical maximum. Then, Earth will receive 23% more solar radiation when closest to the Sun. This varying solar energy will have significant impacts on weather and climate.
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    2 分
  • Just a Second

    Just a Second
    2026/03/29
    How long is a second? That depends on how you measure it. In a previous EarthDate, we talked about how people divided the day into 24 hours, then hours and minutes into 60. That made the second 1/86,400th of a day. But how long is a day? Turns out, that’s not constant. Earth wobbles slightly on its axis, changing its rotation time. So scientists settled on a year, reasoning the day’s length would average out. But which year? Those aren’t constant either. So they agreed on a specific year: 1900. A second then, was measured as 1/31,556,925.9747th of the year 1900. Not a very elegant solution. So scientists looked for a new measurement. They found an isotope that changes magnetic orientation when, and only when, it’s hit with microwave energy of a very specific frequency. This relationship was so stable, and the procedure so repeatable, that they used it to set the new standard of time: 1 second equals 9,192,631,770 cycles of the microwave energy that switches the atomic state of cesium-133. That may not be any more elegant. But it is very precise—down to the nanosecond.
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    2 分
  • How 10 Fingers Became 12 Hours

    How 10 Fingers Became 12 Hours
    2026/03/28
    We have 10 fingers, so we’ve based our common counting systems on 10. Why then, do we divide the day into 12 hours and the hour into 60 minutes? To understand, we need to go back 3000 years. Babylonians also used their hands to count, but wanting to count higher than 10, they devised a different system. They used their thumb to count the three segments of their four fingers to get 12. They marked that 12 by raising a finger on the other hand. Twelve times five fingers is 60. 2500 years ago, when people started using sundials, it seemed only natural to divide the day in 12. Egyptian astronomers then found 12 stars to mark the passing of time in the night sky, making a 24-hour cycle. Early Greek mathematicians realized they could divide a circle into six equilateral triangles like a sliced pizza. Around 2200 years ago, the first Greek astronomer to describe a round Earth wanted a system to navigate it. He took that 6-part circle and divided each part by 60 to get 360 degrees. Another Greek divided those degrees further, into 60 minute parts, and those into 60 secondary parts. A few centuries later, these geographic minutes and seconds were applied to the 24-hour day. But it would take another thousand years before we could accurately measure that second, which we’ll cover on another EarthDate.
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    2 分
  • Wondrous Waterfalls

    Wondrous Waterfalls
    2026/03/28
    The waterfall. In concept, it’s simple: just water… falling. But in life, waterfalls are a wonder. Beautiful, frightening, powerful, calming, deafening, soothing—sometimes all at once. They’ve been inspiring humans for millennia. When water flows over a wide shelf, it’s called a block falls. The widest is Victoria Falls in Zambia, over one mile across. Called “smoke that thunders” in the local language, it can be heard 25 miles away. Human tools have been found there from 50,000 years ago. The tallest falls are in Venezuela. Angel Falls cascade down the face of a half-mile-high rock shelf in a horsetail plume that falls so far, the water turns to mist before it can reach the ground. There, the river reforms to continue its path. Yosemite Falls in California is 200 feet shorter but nearly as spectacular. But by far, the biggest waterfall in the world is “out of sight”: Water falling into water under more than half a mile of ocean. Off the coast of Greenland, a huge torrent of dense, very cold water surges over a shelf into the warmer North Atlantic, plummeting 11,000 feet. Its flow is 30 times that of all the rivers that flow into the Atlantic, an astonishing 1.3 trillion gallons per second! Next time you get out into nature, make it a point to take in a waterfall.
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    2 分
  • Under Greenland’s Ice Sheet

    Under Greenland’s Ice Sheet
    2026/03/27
    Greenland is Earth’s largest island, and most of it is covered by ice up to 2 miles thick. Scientists decided to investigate the age of this ice sheet and were surprised to find that not that long ago, there wasn’t one. They used ice cores to date different areas, then flew planes over it with special ice-penetrating radar. The radar identified bands of softer and harder ice and layers within them, which they matched to the ice cores to determine different ages of ice across the island. Turns out Greenland’s ice accumulated in three separate events: the oldest beginning 130,000 years ago; the middle one during the last Ice Age; and the most recent just 12,000 years ago. Equally interesting, the radar revealed the bedrock beneath the ice sheet to find that a giant rift as deep as the Grand Canyon and even longer, at almost 500 miles, splits the island and exits north into the Arctic Sea. It’s possible this was formed by massive floods as much as 2 million years ago. They also found a huge impact crater that appears to have been made by a mile-wide meteorite, which might be the one that caused the lingering Ice Age discussed in a previous episode. It’s unclear when it struck Earth, but it was at least 12,000 years ago, since the young ice above it is undisturbed. This new technology has given us a remarkable first look at Greenland’s landscape, otherwise hidden beneath the ice.
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    2 分
  • Platinum Record for the Ice Age

    Platinum Record for the Ice Age
    2026/03/27
    Earth has experienced dozens of ice ages in the past few million years. At the coldest point of the most recent one, ice sheets over a thousand feet thick covered much of North America. Then, 15,000 years ago, the world began to warm, and ice started melting—until 13,000 years ago, when Earth plunged back into the Ice Age for a thousand more years. Scientists thought this may have been caused by trillions of gallons of cold glacial meltwater flooding out into oceans, interrupting the warmer surface currents that heat the continents. But recently, they found platinum in sediments in several places around the world, all correlating to that time when Earth began cooling again. Some thought this may have been caused by a meteorite impact dispersing platinum into the atmosphere. While some meteorites contain platinum, most are also high in iridium. But they found no iridium in these deposits. A debate began, sending teams to analyze sediments around the world. Many of them showed a platinum spike from that same period. The search is now on for the impact zone of a huge, rare magmatic iron meteorite, which contains little iridium. If and when they find it, you’ll hear about it on a future EarthDate. After that 1200-year cold snap, Earth warmed dramatically—in as few as 40 years. And stable mild temperatures for the last 12,000 years have allowed the flourishing of all civilization on Earth.
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    2 分
  • Death Apples in Paradise

    Death Apples in Paradise
    2026/03/27
    Imagine walking down a beautiful tropical beach. You spot a fruit on the sand, like a plum or small ripe apple. It smells sweet, so you wash it in the water and you’re about to take a bite… But wait! You may have in your hand a Death Apple, the fruit of the manchineel, a tree that’s relatively common to the tropical beaches of the Americas. Despite its good looks, the manchineel is listed in the Guinness Book of Records as the most dangerous tree in the world. Every part of it is highly toxic. The juice of its fruit, had you eaten it, damages the entire digestive tract. It burns the throat and causes it to swell dangerously, and produces severe abdominal pain, vomiting, and bleeding that can lead to fatal dehydration. Its sap is even worse. A single drop can burn the skin and blind the eyes. And it’s water soluble. Rainwater dripping off the tree can cause painful welts or burn the paint off a car. Breathing air laced with the sap after a rain or smoke when the wood is burned can asphyxiate. And if the sap reaches the bloodstream, it can kill. Ponce de León, the famous explorer, is thought to have died from a native arrow dipped in the sap of the Manchineel. Habitat loss and efforts to eradicate it are reducing the manchineel’s numbers. But, if you’re on a Caribbean or Central American beach and see trees with red markers on the trunks, be sure to give them a very wide berth. The Death Apple is truly forbidden fruit.
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    2 分