Ask Ethan: Are “North” and “South” totally arbitrary on Earth?

If you live on planet Earth, you’re probably familiar with our concepts of north and south. The tippy-top of the Earth — the North Pole — is the most northern point you can reach on our planet, while the very bottom of our planet is marked by the South Pole: the southernmost point on our surface, located in Antarctica. Geographically, the two poles are the locations where the axis of our planet’s rotation intersects the surface of the Earth itself. They are the only points at which, were you to stand on them, you would remain at the same location relative to Earth’s center even as the planet rotates.

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Our maps and globes always display north at the top and south at the bottom. But is that for any physical reason, or is it completely arbitrary? That’s what Jana Roose wants to know, asking:

“Could Australia be in the northern hemisphere? Does the universe have a definitive up or down, or could Australia be in the north and it’s a matter of opinion/historical habit?”

The concepts of north and south go way, way back in history: to decidedly pre-scientific times. So how well do our definitions hold up, given what we know today?

The Earth, moving in its orbit around the Sun and spinning on its axis, was not known to be spherical in nature until a little over 2000 years ago. Nevertheless, concepts like “north” and “south” have been around for far longer than that.

Linguistically, the origin of the word “north” is descended from the Proto-Indo-European language, where the root of the word north appears as *ner, which doesn’t technically mean north, but rather “left.” It’s theorized that the word “north” comes from *ner because when you face the rising Sun in the morning, north is to your left. (Since the Sun always rises in the east.) From this same language, the other three cardinal directions are also related to the position of the Sun.

We estimate that the Proto-Indo-European language was spoken from about 4500 BCE to 2500 BCE, so the word from which “north” derives, and its identification with the direction we know as north today, is at least some 4500 years old. It’s hard to claim that this identification, which humans have used ever since, doesn’t have at least some historical bias to it.

Whenever the Sun is seen rising over the horizon, we can be certain that the “left” direction when we’re facing the Sun points north, while the “right” direction points south. Over the entire world, this remains true, regardless of which hemisphere you’re in, as it’s a consequence of the direction that Earth spins relative to the Sun’s position.

After all, we didn’t even know the Earth was round until a little over 2000 years ago. Back in the 5th Century BCE, Herodotus — widely regarded as the first historian in European culture — took a visit to Egypt and inquired as to where the source of the Nile comes from. (The Nile River, of course, flows from south-to-north, emptying into the Mediterranean sea at the Nile Delta.)

He admitted that no one knew for certain, but gave two possibilities that he thought were likely, and then recounted a third possibility that the Egyptians related to him: that farther south, there were mountains, and these mountains gathered snow. As the seasons changed, the snows melted, and the melt from these snows fed the Nile River, serving as its source.

Although this is factually correct, Herodotus himself dismissed this possibility. As everyone knew, he explained, the Earth gets hotter the farther south you go, and northern Egypt was already ridiculously hot; too hot for the accumulation of snow. The idea that one could go farther south and encounter geographic features that were rich in snow was absurd, and therefore could not seriously be entertained as the source of the Nile River.

The Nile River, as viewed from south to north from above on the International Space Station. The source of the Nile was a mystery for millennia, but is now known to originate from mountains far to the south of the Nile Delta.

About 200 years after Herodotus, during the 3rd Century BCE, the shape of the Earth was truly demonstrated in a scientific fashion. Eratosthenes, a Greek living in Egypt, was familiar with how the Sun’s path changed in the sky over the course of a year. During the winter solstice, it reached its low point, with the shortest duration of daylight hours and the Sun never venturing very far from the southern horizon. But each day, subsequently, would get a little longer, with the Sun’s path rising higher and higher throughout the year until we reached the summer solstice, where the Sun would come closer to the zenith at high noon — directly overhead — than on any other day.

At his location in Alexandria, he knew that a vertical object in the ground cast its shortest shadow of the year on the summer solstice. But he was baffled at a report that he received from a city farther south: Syene, which corresponds to modern-day Aswan. There, he was told, that on the summer solstice, a vertical stick cast no shadow at all, and that if you looked down into a well at high noon on the solstice, your own head would block the reflection of the Sun itself.

At all locations located between the Tropic of Cancer and the Tropic of Capricorn in terms of latitude, there are two days per year where the Sun shines directly overhead at its zenith. In Hawaii, where this photo was taken, the phenomenon is known as Lahaina Noon.

What could be causing this? If the Sun were very far away, the rays that it emitted should all be approximately parallel to one another. If the Earth were completely flat — an assumption made by many at the time — then your location on its surface shouldn’t affect the angle at which the Sun’s rays struck it.

But if the Earth were curved instead, then you’d expect that the Sun’s rays would strike the surface at different angles at different positions. Moreover, there would be two different ways to think about your position:

1. longitudinally, where the Sun cast shadows that possess the same angles on the same dates, but the time at which those shadows appeared shortest/longest would vary between observers,
2. and latitudinally, where observers at different latitudes would see the Sun appear to reach a different maximum height above the horizon, even on the same dates.

Eratosthenes took measurements of the angle cast by a vertical stick at Alexandria on the summer solstice, and found that at its minimum, the angle made between the Sun’s rays and the stick was just 7.2 degrees. At Syene, then, where it was 0 degrees, he realized what had to come next.

If the Earth were perfectly flat, then the Sun’s rays would cast identical shadows at noon on the solstice everywhere on Earth (top), no matter where you were located. But if the Earth’s surface were curved (bottom), shadows at different locations would cast different shadows on the same day, depending on the angle that the Sun’s rays struck the object in question. By measuring the difference in shadow angle between two points on Earth’s surface, it became possible to measure the size of the Earth for the first time.

Alexandria was farther north than Syene, and the difference of 7.2 degrees in the shadows between them on the same date implied that a full circle — 360 degrees — would be traversed if you traveled 50 times whatever the distance was between Syene and Alexandria. Back then, distance was measured in stadia, and Eratosthenes reported the distance between the two cities was 5,000 stadia. (This, itself, was measured by camel, assuming a camel could travel a certain number of stadia in a day, multiplied by the number of days it took to make the journey.)

All told, then, it would take 250,000 stadia to encircle the Earth, which tells us the Earth’s circumference. If we assume that Eratosthenes — again, a Greek person living in Egypt — was using an Attic (Greek) stadium as his unit of measurement, which is 185 meters in today’s units, then his value for Earth’s circumference was 46,620 km: a value about 16% larger than the actual value. However, if we assume he was using an Egyptian stadium, which is only 157.5 meters, then his value for the Earth’s circumference was instead 39,375 km: only 2% smaller than the modern value we use today.

This 19th century reconstruction of Eratosthenes’ original map of the known world is the earliest map to recognize the Earth as having a spherical shape. The lines of latitude and longitude were inventions of Eratosthenes himself.

Why bother telling this story? Because the scientific contributions of Eratosthenes doesn’t end with the discovery of Earth’s round shape or with his measurement of Earth’s circumference. Instead, Eratosthenes went on to become the world’s first geographer. He invented the concepts of longitude and latitude. He mapped out over 200 locations relative to one another. He constructed the first models and maps that incorporate a spherical Earth. And he divided the world into five zones:

• two cold, polar zones at high latitudes, one in the north and one in the south;
• two temperate zones at the mid-latitudes, one in the north and one in the south,
• and one tropical zone at equatorial latitudes, extending a short distance into the north and south from the Earth’s mid-line.

Ever since that time, for more than 2000 years, we’ve recognized that our Earth is round, that it rotates on its axis, and that the “top” of the world is the north pole while the “bottom” of the world is the south pole.

Planet Earth, as viewed by NASA’s Messenger spacecraft as it departed from our location, clearly shows the spheroidal nature of our planet. This is an observation that cannot be made from a single vantage point on our surface, but there are many valid ways to measure the curvature of the Earth, all leading to the same conclusion.

All of this has emerged, of course, simply from human history. On Earth, most of the continental land mass is in the northern hemisphere, and most of the human population has inhabited the northern hemisphere for at least many thousands of years. As a result of this:

• our maps and globes are always oriented with north at the top and the south at the bottom,
• we define the motion of the bodies within our Solar System as though we’re looking “down” from above Earth’s north pole,
• we therefore think of the planets in our Solar System as revolving counterclockwise around the Sun, with Earth rotating counterclockwise and our Moon orbiting the Earth in a counterclockwise fashion,

and more. This arbitrary definition has now been extended to the Sun’s motion around the Milky Way galaxy as well, with “galactic north” being defined as the orientation where, when you look down upon it, the stars in the galaxy’s disk revolve counterclockwise around the galactic center.

A spiral galaxy like the Milky Way rotates as shown at right, not at left, indicating the presence of dark matter. Not only all galaxies, but clusters of galaxies and even the large-scale cosmic web all require dark matter to be cold and gravitating from very early times in the Universe. Modified gravity theories, although they cannot explain many of these phenomena very well, do an outstanding job at detailing the dynamics of spiral galaxies.

This perspective, which has always been completely arbitrary, has led to a series of self-reinforcing definitions. Earth doesn’t just have geographic poles set by our planet’s rotation about its axis, but also has magnetic poles, owing to electromagnetic processes happening in our planet’s metallic core. The magnetic poles are offset from the geographic poles by a few (about 7) degrees, but if you have a magnetized or simply magnetizable needle, it will line up with those magnetic field lines. One end of the needle will always point toward Earth’s magnetic north pole, while the other points toward Earth’s magnetic south pole.

Which is which?

Well, surprise surprise, we actually defined magnetism so that the magnetic pole that’s closest to our geographic north pole is defined as “magnetic north” as well. When the poles flip — which they routinely do on timescales of tens of thousands of years — our geographic and magnetic poles will become anti-aligned instead of aligned. This definition has propagated into all of electromagnetism, but much like the definition we use for geography, it’s completely arbitrary. One can argue quite easily that if human history had unfolded in an Afro-centric or Australo-centric fashion instead of a Eurasia-centric one, our definitions of north and south could have easily been reversed in every way imaginable.

The original “blue Marble” image, from Apollo 17, was actually snapped in the orientation shown here: where south is at the top and north is (invisibly) on the other side of the world. In space, as well as on Earth, our assignment of north as “up” and south as “down” are arbitrary.

Fortunately, whenever we quantify the motion of an object, we’re now well aware that there is no absolute frame-of-reference anywhere in the Universe. Objects are only in motion relative to one another and all frames-of-reference are arbitrary. Galaxies are just as likely to spin clockwise as counterclockwise. While planets tend to rotate and revolve in the same direction that their parent star rotates in, that’s a function of angular momentum being conserved from all the way back when stellar systems were forming from a protoplanetary disk. And once you venture beyond Earth, any Earth-centric definitions we have made become even less meaningful than the arbitrary meanings they currently have here.

In fact, whenever a spacecraft ventures beyond the Earth, our terrestrial definitions of orientation and navigation quickly become useless. If humanity ever becomes an interplanetary or interstellar species, we’ll undoubtedly leave our capricious definitions of north and south behind as relics of the past that have outlived their usefulness. In the meanwhile, we should all recognize that it’s for historical reasons — and not for any scientific ones — that we’ve defined north and south as we have. Those identifications may live on, but here on Earth, the only physically motivated one comes from gravitation: the center of the Earth is down!

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