8 Things You Need to Know About Earth

It's probably best that we don't think too much about the Earth. After all, it's a tiny orb spinning more than 1000 mph at the equator while simultaneously zipping through space at 67,000 miles per hour. It circles a mysterious, 10,000°F fusion reactor that's more than 100 times its size, and spends most of its orbit narrowly (in a cosmic sense) avoiding collisions with giant chunks of rock that could practically wipe its surface clean. But if you're feeling brave, here are a few things you might not know about Earth. Mental Floss spoke to Josh Willis, a climate scientist at NASA's Jet Propulsion Laboratory, about the planet we call home.


The Earth orbits the Sun at approximately 93 million miles. As you probably know, at this distance it takes one year for the Earth to complete a revolution, and 24 hours to complete one rotation. The surface of the Earth has temperatures ranging from -126°F to 136°F. The planet is about 7900 miles in diameter (though the deepest we've ever drilled is 7.6 miles). There are 332,519,000 cubic miles of water on the planet, which is enough that, if the water broke from the Earth and organized itself into a sphere, it would have a diameter of 860 miles—about 40 percent that of the Moon.


The first photograph of Earth from space was taken in 1946. It's a grainy, black-and-white shot of a tiny slice of our world, curved with the ink of space as a backdrop. In 1960, weather satellites began sending photographs back to Earth, images that were still hideously deformed but scientifically valuable, especially for meteorologists, who now had stunning views of cloud systems from which to work. NASA's ATS-III satellite in 1967 returned the first color images of the full Earth. Now at last, we could see our living world, ringed in space and wrapped in billowing clouds.

On Christmas Eve, 1968, Apollo 8 astronaut William Anders sent back "Earthrise," a now-iconic photograph of a fragile cerulean orb rising over the lunar surface. But the most famous photograph of the Earth, by far, was taken about four years later, on December 7, 1972: the "Blue Marble." You've probably seen it countless times, enough that when you think of the Earth, that's what you think of. You may be less familiar with how astronaut Harrison Schmitt described the sight to Mission Control: "I'll tell you, if there ever was a fragile-appearing piece of blue in space, it's the Earth right now."


The Earth is the first planet, moving outward from the Sun, that possesses a moon. We call our moon "The Moon" (which will be a real headache centuries from now, when we've colonized the solar system). Every 27.32 days, the Moon completes an orbit of the Earth, which is why it has phases. When the Earth is between the Sun and the Moon, we see the Moon in full illumination (a round orb). As it circles the Earth, less and less of its visible surface is illuminated, until at last the Moon is between the Sun and the Earth. At that point, the "far side" of the Moon is in full illumination, and from our perspective, the Moon is receiving no light at all. The cycle then repeats itself, with more of its disc being illuminated as the month elapses, until it is again full. Because the length of the Moon's orbit is just shy of a month, every so often a month (which, itself, derives from the word "moon") has two full Moons, the second of which is colloquially called a Blue Moon.

The moon does spin, but in synchronous rotation with the Earth. In other words, it spins at the same speed as its orbit. As a result, the Earth only ever gets to see one side of our only natural satellite. The best guess for the origin of the Moon involves an object the size of Mars smashing into the Earth 4.5 billion years ago, sending debris into space. This debris organized itself into a molten form of the alabaster orb we know and love. Within 100 million years, an early crust had begun to form. Today, the Moon influences the tides of the ocean and eases our axial wobble, keeping things (more or less) nice and stable—a perfect condition for life.


When it comes to life, there are a lot of maybes in the solar system. Maybe Mars supported life billions of years ago. Maybe Europa is teeming with life today. The problem is that there is no evidence anywhere of anything that wiggles, walks, or swims … except for one place. Earth is the only body in the universe known to harbor life. And it has been tough going! Four billion years ago, the Earth's surface was sterilized during the Late Heavy Bombardment, when asteroids pilloried the inner solar system. To get some idea of what things must have been like during the LHB, look at the Moon. Most of its craters were formed during that time. Life survived on Earth in large part thanks to the hydrothermal vents at the bottom of the ocean.

There have been five mass extinctions on Earth, the worst of which (the Permian-Triassic, or "P-T Event") was 250 million years ago, wiping out 96 percent of sea species and nearly three-quarters of land vertebrates. Sixty-six million years ago, the Chicxulub impact wiped out 75 percent of all life, and ended the reign of the dinosaurs. Things recovered nicely, though, and today, biologists think there are 8.7 million species of life on Earth. That's not bad considering the universe's apparent hostility to life, and makes what we have going here all the more special and worth preserving. And we'd better get on it: Many scientists argue that we're in the midst of a sixth mass extinction—and we can only partially blame it on cats.


"Global warming is real, it's caused by people, and it's a big problem," Willis told Mental Floss. "Every year the impacts of human-caused climate change get bigger and bigger, and are felt more and more across the planet." We feel the effects of climate change today, but the worst is yet to come, both in terms of economic and social disruption. "Right now we have a choice about what kind of planet we want to have in the future. And the choice is: Do we want to continue to burn fossil fuels and heat up the Earth, or do we want to try and stabilize our climate and keep it more or less like we've had it for the last 10,000 years?"


Carl Sagan once observed that, to scale, the Earth's atmosphere is about as thick as the gloss coating on a globe. Our oceans, meanwhile, make Earth the only known planet with stable water at its surface. (Icy moons like Europa and Enceladus have subsurface oceans of liquid water, and Titan, in addition to a possible subsurface ocean of water, has vast lakes of liquid methane covering its surface.)

The problem is, we're causing those water levels to rise. NASA's Jason-3 spacecraft measures the height of the ocean with 1-inch accuracy. Every 10 days, it collects data on the entire ocean, revealing details about such things as ocean currents and how they change, tilts in the ocean's surface, and the average volume of the ocean. "The oceans are growing for two reasons," says Willis. "One is because they absorb heat trapped by the greenhouse gases, and the other is that the ice in places like Greenland and Antarctica and tiny glaciers all across the planet are all melting and adding extra water to the oceans. And so this satellite measures these things combined, and in a way it's really taking the pulse of our planet."

A decade ago, the ice sheets in Greenland and Antarctica were thought of as stable. They are the last remaining ice sheets that cover huge land masses, and today they are disappearing. In 50 years, their melting will be the dominant source of global sea level rise. "Every time a big discovery is made," says Willis, "it seems like the picture is worse than we thought it was. The possibility for really rapid ice loss and rapid sea level rise is greater than we thought."


The oceans remain a giant unknown for scientists. Knowing more about them would answer many of our questions about life and the life of the Earth. "Two-thirds of the planet is covered with water, and you can't see through it. And you can't shoot microwaves through it, and radio waves, and all the other kinds of things that we use even to measure other planets," Willis says. "If you probe the ocean, there are still a lot of big mysteries down there."

To understand how oceans really work would explain, for example, where the heat from global warming is going. Though the oceans absorb 95 percent of the heat trapped by greenhouse gases, it's still a mystery where that heat energy actually goes. Similar questions exist as to how the oceans interact with ice sheets.

Considering the stakes, it seems like an intense study of the Earth and its oceans is in order. And yet the same people who claim there isn't enough evidence to explain climate change want to slash the budgets of missions designed to find the requested evidence. Among the missions set to be killed are the PACE satellite, over a decade in development and designed to study the interaction of the ocean and the atmosphere, and the CLARREO pathfinder mission, which would cut the time necessary to predict climate change in half. (An extra 20 years to prepare for climate change would save the world $10 trillion.)


But it will take a concerted effort to change our behavior—before it's too late. "We think of global warming as something that happens in our cities, and it is happening there, but really 95 percent of the heat that's being trapped is going in the oceans. And I don't think people realize that. It just seems like, well, we're getting the brunt of global warming here in Los Angeles—but that's not true, really. It's the sea life and the oceans that are getting the brunt of the change," says Willis.

"One thing we should keep in mind is that all hope is not lost," he continues. "We are beginning to see changes in our economy, we're beginning to see the growth of renewable energy, and the strong desire to move to a fuel source that doesn't cook us, and I think that's a good thing. A lot of it happens at local and state levels now, but it's beginning to have an impact for real around the world."

Big Questions
What is Mercury in Retrograde, and Why Do We Blame Things On It?

Crashed computers, missed flights, tensions in your workplace—a person who subscribes to astrology would tell you to expect all this chaos and more when Mercury starts retrograding for the first time this year on Friday, March 23. But according to an astronomer, this common celestial phenomenon is no reason to stay cooped up at home for weeks at a time.

"We don't know of any physical mechanism that would cause things like power outages or personality changes in people," Dr. Mark Hammergren, an astronomer at Chicago's Adler Planetarium, tells Mental Floss. So if Mercury doesn’t throw business dealings and relationships out of whack when it appears to change direction in the sky, why are so many people convinced that it does?


Mercury retrograde—as it's technically called—was being written about in astrology circles as far back as the mid-18th century. The event was noted in British agricultural almanacs of the time, which farmers would read to sync their planting schedules to the patterns of the stars. During the spiritualism craze of the Victorian era, interest in astrology boomed, with many believing that the stars affected the Earth in a variety of (often inconvenient) ways. Late 19th-century publications like The Astrologer’s Magazine and The Science of the Stars connected Mercury retrograde with heavy rainfall. Characterizations of the happening as an "ill omen" also appeared in a handful of articles during that period, but its association with outright disaster wasn’t as prevalent then as it is today.

While other spiritualist hobbies like séances and crystal gazing gradually faded, astrology grew even more popular. By the 1970s, horoscopes were a newspaper mainstay and Mercury retrograde was a recurring player. Because the Roman god Mercury was said to govern travel, commerce, financial wealth, and communication, in astrological circles, Mercury the planet became linked to those matters as well.

"Don’t start anything when Mercury is retrograde," an April 1979 issue of The Baltimore Sun instructed its readers. "A large communications organization notes that magnetic storms, disrupting messages, are prolonged when Mercury appears to be going backwards. Mercury, of course, is the planet associated with communication." The power attributed to the event has become so overblown that today it's blamed for everything from digestive problems to broken washing machines.


Though hysteria around Mercury retrograde is stronger than ever, there's still zero evidence that it's something we should worry about. Even the flimsiest explanations, like the idea that the gravitational pull from Mercury influences the water in our bodies in the same way that the moon controls the tides, are easily deflated by science. "A car 20 feet away from you will exert a stronger pull of gravity than the planet Mercury does," Dr. Hammergren says.

To understand how little Mercury retrograde impacts life on Earth, it helps to learn the physical process behind the phenomenon. When the planet nearest to the Sun is retrograde, it appears to move "backwards" (east to west rather than west to east) across the sky. This apparent reversal in Mercury's orbit is actually just an illusion to the people viewing it from Earth. Picture Mercury and Earth circling the Sun like cars on a racetrack. A year on Mercury is shorter than a year on Earth (88 Earth days compared to 365), which means Mercury experiences four years in the time it takes us to finish one solar loop.

When the planets are next to one another on the same side of the Sun, Mercury looks like it's moving east to those of us on Earth. But when Mercury overtakes Earth and continues its orbit, its straight trajectory seems to change course. According to Dr. Hammergren, it's just a trick of perspective. "Same thing if you were passing a car on a highway, maybe going a little bit faster than they are," he says. "They're not really going backwards, they just appear to be going backwards relative to your motion."

Embedded from GIFY

Earth's orbit isn't identical to that of any other planet in the solar system, which means that all the planets appear to move backwards at varying points in time. Planets farther from the Sun than Earth have even more noticeable retrograde patterns because they're visible at night. But thanks to astrology, it's Mercury's retrograde motion that incites dread every few months.

Dr. Hammergren blames the superstition attached to Mercury, and astrology as a whole, on confirmation bias: "[Believers] will say, 'Aha! See, there's a shake-up in my workplace because Mercury's retrograde.'" He urges people to review the past year and see if the periods of their lives when Mercury was retrograde were especially catastrophic. They'll likely find that misinterpreted messages and technical problems are fairly common throughout the year. But as Dr. Hammergren says, when things go wrong and Mercury isn't retrograde, "we don't get that hashtag. It's called Monday."

This story originally ran in 2017.

science fiction
Why So Many Aliens in Pop Culture Look Familiar

Aliens have been depicted countless times in cinema, from Georges Méliès's A Trip to the Moon (1902) to James Cameron's Avatar (2009). But despite the advancements in special-effects technology over the past century, most aliens we see on screen still share a lot of similarities—mainly, they look, move, and interact with the world like humans do. Vox explains how the classic alien look came to be in their new video below.

When you picture an alien, you may imagine a being with reptilian skin or big, black eyes, but the basic components of a human body—two arms, two legs, and a head with a face—are likely all there. In reality, finding an intelligent creature that evolved all those same features on a planet millions of light-years away would be an extraordinary coincidence. If alien life does exist, it may not look like anything we've ever seen on Earth.

But when it comes to science fiction, accuracy isn't always the goal. Creating an alien character humans can relate to may take priority. Or, the alien's design may need to work as a suit that can be worn by human performers. The result is a version of extraterrestrial life that looks alien— but not too alien—to movie audiences.

So if aliens probably won't have four limbs, two eyes, and a mouth, what would they look like if we ever met them person? These experts have some theories.

[h/t Vox]


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