Swimming in the Sahara

Leonardo’s Rule of Trees

Leonardo da Vinci was one of our most brilliant thinkers. He’s known as an inventor, scientist, mathematician, architect, sculptor and, of course, painter of the most famous portrait in the world, the Mona Lisa. But unless you’re also a painter, you probably haven’t heard of Leonardo’s Rule of Trees. Intent to paint them realistically, he carefully analyzed trees and recognized a pattern, which he encoded in this formula: The thickness of the limbs that grow from a trunk, when combined, equals the thickness of the trunk. The thickness of all the branches that grow from each limb, equals the thickness of that limb. Trees are fractal, meaning the pattern established in their larger branches repeats in smaller ones—so Leonardo’s rule applies all the way to the twigs. Using his formula to design his trees, his results were so convincing that other painters adopted his rule, too—and have used it since. Recently, scientists wanted to figure out why this pattern exists. One researcher posited that trees evolved this way to withstand force, from wind or the weight of fruits. And in wind tunnel tests, this held up. Other scientists, who hoped Leonardo’s rule might also hold for the vascular pattern of trees, found it did not. But he never intended it to—only to depict their outer structure. Which, 500 years later, it still does remarkably well.

If Earth Had No Moon

What if Earth didn’t have a moon? Well, nearly everything about our home planet would be different—so much, we’d hardly recognize it. Our Moon is a quarter the size of Earth, far bigger than most planets’ moons. Scientists think it formed when a smaller planet crashed into Earth 4 billion years ago, knocking off a chunk that eventually became our Moon. Its large size is key to its dramatic effects on Earth. The gravity of the Moon pulls on the oceans to produce tides. Tidal range would be only a third its size otherwise. Tides define coastal ecosystems and influence what kind of life can live there. Tides also shape ocean currents—like the ocean conveyor we discussed on another EarthDate. Currents moderate global weather and temperature, helping create the stable climate that has allowed human civilization to thrive. Tidal friction has also slowed Earth’s rotation. Billions of years ago, a day on Earth lasted just 10 hours. Without the Moon, it would be even shorter than that! The Moon also stabilizes the tilt of Earth on its axis, which stabilizes our seasons. Planets without moons have much more dramatic seasonal variations. Adapting to erratic seasons and temperatures, and days as short as 6 hours—the plants and animals that could live on Earth would likely be very different. Without the Moon, humans might not be here at all!

Swimming in the Sahara

In 1933, Hungarian explorer László Almásy set out across the Sahara in search of a legendary lost city. What he discovered was far more magical and, at first, completely unbelievable. In a remote ravine, Almásy found a shallow cave. Inside he saw cave paintings of human figures with outstretched arms and legs. They appeared to be flying. But no, Almásy realized, they were swimming—in the Sahara! Other cave paintings showed antelope, giraffe, elephants and lions alongside more human figures. Almásy realized they depicted a Sahara of grasslands with abundant water. His theories were met with skepticism but sparked further investigation. And geologic studies eventually proved him right! Those cave paintings dated from what’s now called the African Humid Period, which lasted from the end of the last Ice Age until 5,000 years ago.  It was caused by a thousand years of African monsoons, brought on by changes in the wobble of Earth’s axis. These greened the Sahara and formed vast lakes.& More than 150 excavations uncovered the remains of aquatic species like hippos, crocodiles and more than 30 types of fish. Further investigation showed the Sahara has alternated between desert and wet grasslands every 20,000 years or so—for hundreds of thousands of years.

Solar Maximum Is Heating Up

Millions of miles from Earth, the fastest object that humans have ever built is circling the Sun at 430,000 miles per hour, and nearer than any spacecraft has ever dared go. It’s the Parker space probe, and NASA sent it there in 2018 with a mission: to study solar activity. That’s because, in 2024, the Sun is predicted to reach its next solar maximum. It happens when the Sun’s poles flip and its magnetic fields go “crazy.” This produces coronal mass ejections—burning gas that erupts into the Sun’s atmosphere, sending waves of electromagnetic energy toward Earth. This is often called “solar weather.” In 1921, a solar maximum disrupted our nascent electric grid, as we described in another EarthDate. Now, a hundred years later, our electrical networks are vast and more vital—both on Earth and in our atmosphere, where thousands of satellites orbit. Normally, a solar maximum occurs every 11 years or so, but this one’s coming sooner than expected, which has governments and companies rushing to prepare our technology to avoid the worst problems. But we still may see errors and slowing of GPS, interrupted satellite internet and communications, and reduced cellular coverage. Meanwhile, the Parker probe will track the Sun’s activity through the solar maximum and for years beyond to help us weather … the solar weather.

Life in the Fiery Fields

“Campi Flegrei” in Italian means “fiery fields.” That may not sound like a place you’d want to live, but 360,000 Italians do. Italy is the only country in Europe with active volcanoes on land. The “fiery fields,” just west of Naples, are actually a caldera—the depression left when a volcano erupts and collapses. At the center of the caldera is the Pozzuoli Bay. The coast around it has been densely populated for many centuries. The volcano has had two major eruptions in the distant past, much larger than well-known Vesuvius, and smaller ones more recently. Every day, residents get geologic signals that they might be tempting fate: Gas bubbles up in the waters of the bay. The land rapidly rises and sinks in response to shifting magma beneath—in the last 50 years, some areas have risen more than 10 feet. Roads and buildings that once disappeared under water sometimes emerge again, including Roman ruins. And there are frequent small earthquakes. These and other signs of volcanism are monitored closely by the Italian government. If it looked like the fiery fields were about to get fiery again, half a million people would quickly evacuate, following a well-established plan. Why do they stay? Because it’s home, and has been for generations. Because of the sea, the sun and some of the best food in Italy—in a setting that would get any geologist fired up!