Little Big Questions - Why Is The Earth Round?

We're smart people. We live in a world where information is literally at our fingertips, and pretty much any answer can be found online, yet how much do we educated people know about facts and phenomenons we take for granted?

Source: Wikipedia

Sure, nobody's asking you to know everything - with Google, after all, you don't have to. Yet there are some things we do take for granted. Imagine you were alone in the middle of a green field - no internet, no help - and a curious child asks you a simple question: why is the Earth round? What would you say?

These little questions in life that have big implications - this is what this series is about.

I've asked a few friends to answer a simple question: Why is the Earth Round?

Here are some of their answers:

And here are some of their (and other) answers, quoted:

• Andrei said: "The Earth is round because it's on an elliptical path around the Sun. Moreover, it is not completely round, it's bulging at the equator a little ( good point!). Additionally to this trajectory, the Earth has its own gravitational field that is generated by the molten core at the center. Since gravitational field lines are scattered in all directions and strength decreases with the square of distance, the only form that can result in time is the sphere or spheroid."
• Laura said: "Was it shaped by the rotation movement? I'm inclined to think it's got something to do with the force of gravity but I couldn't really tell you why I think so, because I can't really grasp the mechanics. I mean, a spherical shape is the only one that would permit its specific kind of motion, so it sort of makes sense, but I don't know why?" ( followed by a sad frowny face, of course)
• Luca believes that because the Earth formed by more and more atoms gathering on top of each other, "and since the only logical shape they could form up in space, with the force and gravity and so on, is a sphere", you have a round Earth.
• C.C. believes something similar too: the shape is due to the "larger nucleus of particles, they all clumped together in one spot, and since more particles=more gravity and more gravity=more particles join the party, then the crust cooled, and was shaped by the rotation of the planet around the sun".
• Dorky McDorkins (she asked!). thinks that the shape is due to the "relationship between the Earth and the Moon" and because of "gravity and the rotation and revolution" and the "pull between them", they both became round.
• Catalina decided to troll me and state God just likes balls - though she did make a point out of the fact that technically it's not round (you devil).
• Ozana thinks it's magnetic: "I suppose that has to do with the density of the materials its made of, and an attraction between molecules. Like when you put two magnets side by side, with opposite polarities facing each end. It just sticks together that way"

Some of these guys and gals got pretty close, some were a bit off. But you've probably noticed I highlighted all the occasions when the Earth's gravity field was mentioned - and I'll tell you why. You see, there's actually one single answer to why the Earth is round.

The answer is very simple: Gravity.

No, not the John Mayer song. (via Vimeo)

Gravity is the force exerted by every body with mass. Every object in our universe has gravity - from the Earth to the Sun, down to yourself and your mouse and keyboard - and even the smallest atom. An each and every one of these bodies attracts each other with a gravitational force which depends on their mass. 

Earth's pull on you is of course much stronger than the keyboard's pull on you - or indeed, even your pull on the Earth.  And the entire planet generates this gravity field, every single atom combined - not just the molten core, as Andrei mentioned (but fret not, dude - you were pretty close!).

But wait a minute - the Earth isn't one solid object. It's actually pretty fluid and grainy, when you analyze it. It's composed of a solid outer shell - the crust, as well as a mantle and a core. The mantle is highly viscous, and the core is molten, a very loose liquid ( although, the inner core is a solid mass floating right in the center of the liquid outer core).

The Earth's composition - via Wikipedia

So the Earth is essentially, not so stiff or rigid - it's layers upon layers of viscous stuff covered in an egg-shell like thin crust. It's billions upon billions of tons of mass - of stuff.

As all this mass and stuff pulls on the mass around it, it organizes into the most efficient shape: a sphere. ( congrats to my friends who figured this one out too!)

This happens because gravity spreads out in a spherical shape - it tends to have the same intensity in all directions. This has the result of pulling fluid bodies with enough strength into itself that it shapes them into spheroids. In fact, the fluidity of the body becomes non sequitur after a certain size: eventually all large stellar bodies pull themselves into a sphere.

Asteroids lack the necessary mass to pull themselves into a sphere - unlike Mars.

The opposite can be observed with lighter stellar objects, such as asteroids. Those who are under about 1000 km in diameter lack the gravitational "umph" to spherize themselves and overcome the "strength" of the rock matter itself, keeping it in shape.

And that's why the Earth is round. But is it really?

Well, no. Saying that the Earth is round is actually shorthand for "the Earth is an oblate spheroid". Because the planet is spinning on its axis every 24 hours (that's 1,664 km/hour), it bulges at the Equator, the point where the centrifugal force of the planet is strongest. This bulge makes up for a roughly 43 km difference between the equatorial and polar diameters - a difference of 0,033% when you do the math.

The bulge isn't visible to the unaided eye, though.

So then, the Earth is not round, but an oblate spheroid then - except, not really. Things are a bit more complicated because of the Sun and Moon, whose gravity stretches and bulges the Earth's surface towards them as well. The fun bit is realizing that these bulges travel along the planet's surface, following the transit of these stellar bodies across our sky. All these movements translate to about 1 meter of tides in the Ocean and 30cm of shift in solid earth. There's also the unevenness caused by the weight of the continents, upheaval due to tectonic forces and more - but it simply is more convenient to disregard these minor variations when talking about the grand scheme of things.

The Moon stretches the Earth. Via hurricainescience.org

So it's kind of an oblate spheroid. But wait a minute - it does have very shapely geological figures. What about mountains, valleys, peaks and such? Planet Earth isn't exactly smooth -  how rough is it? Can we measure it?

Why, yes we can. Earth's deepest point is the 11km deep Mariana trench, whereas its highest peak is Mount Everest, roughly 8.9km, giving us about 20km of difference between the two. But hold up - that 20km figure is not entirely accurate. See, the Ocean covers 70% of the planet's surface, and it's way smoother than any land mass. This means we can't take into account anything below the ocean since it's covered by this huge, smooth surface of water, leaving us a maximum roughness of 9km above sea level and a minimum of 0 km at sea level. To translate this into terms we can understand, let's scale down the Earth to the size of a billiard ball.

Smoother than a Billiard ball, if you're too lazy to read.

If you ran your finger over such a scaled down Earth, your finger couldn't pick up the imperfections. The World Pool-Billiard Association defines an acceptable ball as a pool ball that is 2.25 inches in diameter, and has a tolerance of +/- 0.005 inches - translated to Earth's scale, that means a maximum feature height of 28 km. Hey, the maximum roughness was 9 km...

Well, that's well within pool tolerances!

So - that does it. We know that the Earth is smooth, slightly squashed and round. Or is it?

There's a curious group of people calling themselves the Flat Earth Society who maintain that the Earth is actually a flat disc continually moving upwards in space. They also believe that the disc is enclosed by an impossible to cross 45-meter high wall of ice keeping all the ocean's water in ( we non Flat-Earthers call this the continent of Antarctica and crossing it is like totally a thing that happens), that the Sun and Moon are about 50 km in size and orbit the disc at a few thousand kilometers, and the continual upward movement of the disc gives us gravity - oh, and of course, that the Apollo Moon Landing was a hoax. Could they be right?

Also, could somebody get them a 4th grade science book?

How do we know that the Earth is round?

The ancient Greeks proposed that the Earth may be spherical as early as the 6th century BC, with Aristotle theorizing this in 330 BC. Being explorers and colonists, they noticed that the observable altitude of circumpolar stars changed drastically between their colonies. Sailors noticed that less elevated land was also less visible in the distance than higher elevated land at the same distance, and they blamed the Earth's curvature for this. Aristotle had a few convincing arguments beyond these astronomical hints, including the very obvious fact that the shadow of the Earth on the Moon during eclipses is round.

Eratosthenes was kind of a boss.

Eratosthenes, the inventor of geography, even went so far as to calculate the diameter of the spherical Earth. He knew  that in Syene the Sun was directly overhead at the summer solstice whereas in Alexandria it still cast a shadow. Using the differing angles the shadows made as the basis of his trigonometric calculations he estimated a circumference of around 250,000 stades - which roughly translates to only a 10% error in calculation compared to the actual size of the planet, and provided some of the first real proof of the Earth's sphericity. 

By the way, he freaking did this in 240BC. 

A more practical demonstration of Earth's spherical nature was achieved by Ferdinand Magellan and Juan Sebastian Elcano's expedition's circumnavigation - a feat which has since been repeated countless times.

He did this between 1519-1522.

But let's fast forward to modern times. We have all the technology we need to confirm this otherwise well-proven ancient fact. The Foucault Pendulum experiment proves the Earth's spherical nature when analyzing the shadow it casts over the Moon, because of the consistent oval-shaped (and not round) shadow it casts on our spherical satellite. 

Airplane rides and other high-altitude vehicles routinely circle the Earth - a feat impossible if it were not spherical - and on high altitude flights the curvature is immediately visible if you literally just look out the window.

Timezones are yet another phenomenon that provide solid evidence for the round shape of our planet, because of the occurrence of the day/night cycle and that half the planet is in sunlight while the other experiences night time at the same point in the planet's orbit around the sun. This also means that the sun is lower in the sky as you travel away from the tropics, as does the variation of daylight between summer and winter.

Earth as seen from the International Space Station - courtesy of NASA.

But perhaps the biggest proof is something we humans have been doing for well over half a century - space travel. The advent of satellites, space stations and manned space missions have provided a plethora of direct and indirect proof of this - to such an extend that you have to question how Flat Earth society followers find their evidence to suggest the contrary. GPS, cell phones and internet access wouldn't be possible without geostationary satellites, and these objects wouldn't be possible themselves without a round Earth. I'm not even going to mention photographs or video taken from space.

Do the stick shadow experiment that the Greeks performed millennia ago and see for yourself. Check out the International Space Station's live stream and watch our beautiful blue marble rotate majestically below the station.

Just get yo self some knowledge!

A special, kind "thank you" to all the friends that agreed to contribute to this article! I love ya faces.