Are “North” and “South” totally arbitrary on Earth?
This unusual view of the world is simply the conventional map of the world but with north and south reversed. Although this view is unfamiliar to most of us, it remains just as valid as the more typical view that equates “north” with “up.” Both are equally valid, as well as equally arbitrary.
On Earth, the geographic and magnetic north poles are nearly perfectly aligned, so you can use both the stars and a compass to find “north.” But you may also find “south” using the stars and/or a compass, leading Australians and others to wonder if Earth’s northern hemisphere bias is completely arbitrary. There are significant changes in direction, both on Earth and throughout the Universe, but the reasons aren’t as physics-based as you might think.
If you live on Earth, you’re undoubtedly familiar with the concepts of north and south. The North Pole, located in Antarctica, is the most northern point on our globe. The South Pole, located in Antarctica, is the most southern point on our planet. Geographically, the two poles are the points at which the axis of our planet’s rotation contacts the Earth’s surface. They are the only locations on Earth where, if you stood on them, you would remain in the same location relative to the planet’s center even while the planet rotates.
The north is always at the top of our maps and globes, while the south is always at the bottom. But is there any physical explanation for this, or is it purely arbitrary? Jana Roose wants to know, so she asks:
“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 conceptions of the north and south can be traced all the way back to pre-scientific times. So, given what we know now, how well do our definitions hold up?
The Earth was not known to be spherical in nature until around 2000 years ago, when it was moving in its orbit around the Sun and rotating on its axis. Nonetheless, notions such as “north” and “south” have been present for far longer. (Credit: Larry McNish/RASC Calgary)
Linguistically, the term “north” comes from the Proto-Indo-European language, where the root of the word appears as *ner, which technically does not mean “north,” but rather “left.” The word “north” is thought to be derived from *ner because when you face the rising Sun in the morning, north is to your left. (This is because the Sun always rises in the east.) The other three cardinal directions are also related to the position of the Sun in this language.
We estimate that the Proto-Indo-European language was spoken between 4500 and 2500 BCE, therefore the word “north” and its association with the direction we know as north today are at least 4500 years old. It’s difficult to argue that this identification, which humans have employed since ancient times, doesn’t have some historical bias to it.
While we observe the Sun rising above the horizon, we know that the “left” direction when facing the Sun points north and the “right” way points south. This is true around the world, regardless of which hemisphere you’re in, because it’s caused by the direction the Earth spins in relation to the Sun’s position. (Credit: Fyodor Yurchikhin/Russian Space Agency)
After all, humans didn’t even realize the Earth was round until about 2000 years ago. In the fifth century BCE, Herodotus, often recognized as the first historian in European culture, visited Egypt and inquired about the source of the Nile. (Of course, the Nile River runs from south to north, discharging into the Mediterranean Sea at the Nile Delta.)
He confessed that no one knew for sure, but he presented two alternatives that he thought were possible, and then repeated a third possibility that the Egyptians told him: that there were mountains farther south, and these mountains gathered snow. As the seasons changed, the snow melted, and the melted snow fed the Nile River, serving as its source.
Despite the fact that this is valid, Herodotus discounted the possibility. He said that the Earth becomes hotter the farther south you travel, and northern Egypt was already ridiculously hot; too hot for snow accumulation. The idea that one could go farther south and find snow-covered geographical features could not be taken seriously as the source of the Nile River.
The Nile River as seen from the International Space Station, looking south to north. For millennia, the source of the Nile was mysterious, but it is now understood to originate in mountains far to the south of the Nile Delta. (Credit: NASA/International Space Station)
Around 200 years after Herodotus, in the 3rd Century BCE, the shape of the Earth was scientifically demonstrated. Eratosthenes, a Greek living in Egypt, was well aware of how the Sun’s route in the sky changed over the course of a year. It reached its low point around the winter solstice, with the shortest daylight hours and the Sun never venturing far from the southern horizon. However, each day would become a little longer as the Sun’s path rose higher and higher throughout the year until we reached the summer solstice, when the Sun would be closer to the zenith at high noon — directly overhead — than on any other day.
He was aware that on the summer solstice, a vertical object on the ground cast the shortest shadow of the year. But he was puzzled by a report from a place farther south: Syene, which corresponds to modern-day Aswan. There, he was told, a vertical stick cast no shadow at all on the summer solstice, 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.
There are two days per year when the Sun shines directly overhead at its zenith at all locations between the Tropics of Cancer and Capricorn in terms of latitude. The occurrence is known as Lahaina Noon in Hawaii, where this picture was taken. (Credit: Daniel Ramirez/flickr)
What could be the source of this? If the Sun were really far away, the rays it emitted should be about parallel to one another. If the Earth were perfectly flat — as many assumed at the time — your location on its surface should have no effect on the angle at which the Sun’s rays strike it.
However, if the Earth were curved, the Sun’s rays would reach the surface at different angles at different points. Furthermore, there are two approaches to consider for your scenario:
- The Sun casts shadows with the same angles on the same dates, but the time at which those shadows appear shortest/longest varies between observers;
- and latitudinally, where observers at different latitudes see the Sun appear to reach a different maximum height above the horizon, even on the same dates.
On the summer solstice, Eratosthenes measured the angle cast by a vertical stick in Alexandria and discovered that the angle formed between the Sun’s rays and the stick was just 7.2 degrees. Then, at Syene, when the temperature was 0 degrees, he recognized what had to happen next.
If the Earth were exactly flat, the Sun’s rays would cast identical shadows at noon on the solstice everywhere (top), regardless of where you were. However, if the Earth’s surface were curved (bottom), shadows cast at different locations on the same day would cast different shadows based on the angle at which the Sun’s rays reach the object in question. For the first time, it was feasible to measure the size of the Earth by measuring the difference in shadow angle between two points on its surface. (Credit: E. Siegel/Beyond the Galaxy)
Alexandria was farther north than Syene, and the difference in shadows on the same date of 7.2 degrees meant that a full circle — 360 degrees — could be traversed if you went 50 times the distance between Syene and Alexandria. Distance was measured in stadia back then, and Eratosthenes claimed a distance of 5,000 stadia between the two cities. (This was calculated by assuming a camel could go a particular number of stadia per day multiplied by the number of days it took to complete the journey.)
Overall, it would take 250,000 stadia to encircle the Earth, which gives us the circumference. If we assume Eratosthenes — another Greek living in Egypt — was using an Attic (Greek) stadium as his unit of measurement, which is 185 meters in today’s units — then his number for Earth’s circumference was 46,620 km, which is nearly 16% higher than the real value. However, if we suppose he was using an Egyptian stadium, which is only 157.5 meters long, his number for the Earth’s circumference was 39,375 km, which is only 2% less than the present value.
This 19th-century restoration of Eratosthenes’ original map of the known world is the first to acknowledge the Earth’s spherical shape. Eratosthenes created the lines of latitude and longitude. (Credit: E. H. Bunbury, A history of ancient geography among the Greeks and Romans, from the earliest ages till the fall of the Roman Empire, 1879)
What’s the point of telling this story? Because Eratosthenes’ scientific achievements did not end with his discovery of the round shape of the Earth or his measurement of its circumference. On the other hand, Eratosthenes went on to become the world’s first geographer. He developed the ideas of longitude and latitude. He mapped approximately 200 places in relation to one another. He created the first models and maps showing a spherical Earth. And he classified the world into five regions:
- two cold polar zones at high latitudes, one in the north and one in the south;
- two temperate zones at mid-latitudes, one in the north and one in the south;
- and one tropical zone at equatorial latitudes, extending a short distance north and south from the Earth’s mid-line.
For more than 2000 years, we have understood that our Earth is spherical, revolves on its axis, and that the “top” of the world is the north pole and the “bottom” of the world is the south pole.
The rotating Earth as seen by NASA’s MESSENGER spacecraft as it left our planet’s orbit. Take what you know and excel at and apply it to something where your interests and talents can come together to make a real difference if you want to change the course of your life. (Credit: NASA/MESSENGER)
Of course, all of this has resulted from human history. The northern hemisphere has the majority of the continental land mass on Earth, and the northern hemisphere has been home to the majority of the human population for at least many thousands of years. This has resulted in:
- Our maps and globes are always orientated north to north and south to south.
- We describe the motion of our Solar System’s bodies as though we were looking “down” from above Earth’s north pole.
- As a result, we consider the planets in our Solar System to rotate counterclockwise around the Sun, with the Earth rotating counterclockwise and our Moon orbiting the Earth counterclockwise.
And much more This arbitrary description has now been extended to the Sun’s motion around the Milky Way galaxy, with “galactic north” defined as the orientation in which the stars in the galaxy’s disk revolve counterclockwise around the galactic center when viewed from above.
A spiral galaxy, such as the Milky Way, rotates as indicated on the right, rather than on the left, showing the presence of dark matter. Not only do all galaxies require dark matter to be cold and gravitating, but so do clusters of galaxies and even the large-scale cosmic web. (Credit: Ingo Berg/Wikimedia Commons; Acknowledgement: E. Siegel)
This wholly arbitrary perspective has resulted in a series of self-reinforcing definitions. Earth not only has geographic poles determined by our planet’s rotation around its axis, but it also has magnetic poles caused by electromagnetic processes occurring in our planet’s iron core. The magnetic poles are around 7 degrees offset from the geographic poles, but if you have a magnetized or just magnetizable needle, it will line up with those magnetic field lines. The needle’s one end will always point toward the Earth’s magnetic north pole, while the other will always point toward the Earth’s magnetic south pole.
Which is which?
Surprise, surprise, we defined magnetism in such a way that the magnetic pole nearest to our geographic north pole is also defined as “magnetic north.” When the poles flip, which happens on tens of thousands of year timescales, our geographic and magnetic poles become anti-aligned instead of aligned. This notion has spread across electromagnetism, although it is completely arbitrary, just like the one we use for geography. One may easily argue that if human history had developed in an Afrocentric or Australocentric manner rather than a Eurasiacentric 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 taken in the orientation shown below, with south at the top and north (invisibly) on the opposite side of the earth. Our classifications of north as “up” and south as “down” in space and on Earth are arbitrary.(Credit: NASA/Lunar and Planetary Institute)
Fortunately, anytime we measure an object’s motion, we are now well aware that there is no absolute frame of reference anywhere in the Universe. Objects move exclusively in relation to one another, and all frames of reference are arbitrary. Galaxies can spin either clockwise or counterclockwise. While planets tend to rotate and circle in the same direction as their parent star, this is due to angular momentum being conserved since the formation of stellar systems from a protoplanetary disk. And after you leave Earth, whatever Earth-centric definitions we’ve created become even less relevant than the arbitrary meanings they already have.
Indeed, once a spacecraft beyond the Earth, our terrestrial conceptions of orientation and navigation rapidly become outdated. If mankind ever becomes an interplanetary or interstellar race, our artificial notions of north and south will be left behind as relics of the past that have outlived their usefulness. Meanwhile, we should all acknowledge that we have defined north and south as we have for historical reasons, not scientific ones. Those identifications may carry on, but here on Earth, the only one that is physically motivated is gravitation: the Earth’s center is down!