Why do we always think of North as up?

Hi! It might be a dumb question but it’s been in my mind for a while. We are convinced that North is up and South is down because that’s the way maps have been for many many years, but we don’t really know which way is actually up, it could be east or northwest, etc, right? Because there isn’t a real orientation/position in space, there’s no fixed up or down, but... doesn’t the way the Earth rotate determine in a way which way is up? How do those two things related to each other? Or is there no connection at all? Thank you!
"The Blue Marble" is a famous photograph of the Earth taken on December 7, 1972, by the crew of the Apollo 17 spacecraft en route to the Moon at a distance of about 29,000 kilometres (18,000 mi). It shows Africa, Antarctica, and the Arabian Peninsula. In this version, it has been flipped upside down, with South at the top of the image. Image credit:  NASA

"The Blue Marble" is a famous photograph of the Earth taken on December 7, 1972, by the crew of the Apollo 17 spacecraft en route to the Moon at a distance of about 29,000 kilometres (18,000 mi). It shows Africa, Antarctica, and the Arabian Peninsula. In this version, it has been flipped upside down, with South at the top of the image. Image credit: NASA

You’re right that the way we draw our maps with North pointing up and South pointing down is largely arbitrary, and indeed there are a number of maps with the Southerly direction at the top rather than at the bottom, and they’re good fun to look at However, there are good reasons to say that a Northerly or Southerly direction should be “up”, and these reasons extend beyond just the rotation of the Earth.

The rotation of the Earth is a good starting place, though - the rotation axis of the Earth goes more or less through the North and South magnetic poles of the Earth. The magnetic North & South poles wander a little, so some years they’re closer to the rotation axis than others. Fixing the rotation of the Earth as a cardinal direction makes good sense, and is what we’ve done - East and West point 90 degrees from North and South.

There’s one more reason to put North as up, and it’s a physics convention. Most of the time, when we’re talking about rotation, we say that the direction of the rotation axis is actually just in one direction, rather than having to indicate both North and South. If we do this, it allows us to encode both the axis of rotation, and the direction of rotation at the same time. The way we determine which of North or South should be “the direction”, we use what’s called the “right hand rule”. You curl your fingers in the direction of rotation, and your thumb points in the direction of the rotation axis. In the Earth’s case, we rotate towards the East, so your thumb will point in the direction of North.

A drawing of the solar system shows Pluto's tilted orbit. Pluto's orbital path angles 17 degrees above the line, or plane, where the eight planets orbit. Credits:  NASA

A drawing of the solar system shows Pluto's tilted orbit. Pluto's orbital path angles 17 degrees above the line, or plane, where the eight planets orbit. Credits: NASA

However, if you’re thinking of orientations beyond just the Earth’s own rotation, while it’s true that there’s no way to set an entirely objective zero point from which to measure other positions, and a sphere doesn’t have much intrinsic orientation to it, we can still do relative positions pretty well. And on the scale of our solar system, we have a pretty solid alignment going on. All the major planets in our solar system trace oval paths around the Sun as they go about their respective years. Not only do they orbit around the Sun in the same direction, they all tend to point their rotation axes in the same direction (notable exceptions here are Venus and Uranus). On top of all that, the ovals are almost perfectly aligned in a flat plane. If we take our same physics convention and use the rotation of the planets around the Sun to tell us which direction we’re going to point up, our Planet Earth based North is more or less pointing in the right direction. Our planet’s spin is not perfectly aligned with the “up” out of the solar system, but tilted by 23 degrees, a feature of our planet responsible for our seasons. This tilt is why many globes are set at an angle - they’re mimicking the tilt of our planet relative to the “up” defined by our solar system.

So the North is up convention is partially mapmakers, partially the spin of our Earth, and partially physics notation, but there are definite ties between all of them.


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Is there a universal addressing scheme?

Is there a universal addressing scheme for the universe? (e.g. Quadrants… etc.) If so, what is the central reference point, or zero?

We’ve come up with a few different coordinate systems for the Universe, but none of them are really “universal”. By that, I mean that you wouldn’t be able to use them no matter where you are in the universe - most of them only work conveniently from Earth.

Starting small, we’ve got one coordinate system to describe our solar system, with our sun at the center. This coordinate system is easiest to visualize as if you had a birds eye view of our solar system, watching all the planets go around the sun. Every coordinate system needs three coordinates to describe a location - normally we think of up/down, right/left, and forward/back. The three coordinates here are distance outwards from the sun (in any direction), the position of the object in a circle around the sun at that distance (usually measured as an angle from a line drawn from the position of the Earth in September to the sun), and distance away from the plane described by the orbits of the planets. Perhaps unsurprisingly, this is only really a useful coordinate system for objects in our solar system.


We also have a galactic coordinate system, which is not centered on the center of the galaxy as you might expect, but instead, it’s still centered on the location of the sun. The second coordinate here is the distance “up” out of the disk of the Milky Way, and the third is distance from the sun once again. The disk of the Milky Way is not in line with the disk of our solar system, which you can spot if you go outside in a dark sky. The Milky Way usually goes North-South in the sky, and all the planets (and the moon) will always be found in a line that goes East-West. This coordinate system is generally considered to be confusing to work with, and is mostly not used, unless you’re trying to map the Milky Way itself.

By and large, what astronomers use to describe the locations of things in the sky is actually a projection of the Earth’s latitude and longitude lines onto the sky. Take the line between the north and south poles, and extend them out into the sky - those are the northern and southern celestial poles. (The north celestial pole points almost exactly to the North Star.) The equator, expanded outwards in a plane from the surface of the earth, describes the Celestial Equator. The units we then use to describe the positions of objects are right ascension, declination, and distance (if we have it). Declination is similar to latitude - it describes the angle above or below the celestial equator. Right Ascension is like longitude, but instead of being measured in degrees, is measured in hours, minutes, and seconds. We do this because we know how fast the earth rotates - 360 degrees in 24 hours. This means that the earth rotates through 15 degrees in an hour, and we can easily tell how long we need to wait for another object to be overhead. If an object at the 0 hour coordinate is overhead right now, objects at the 3 hour line will be overhead in three hours. This coordinate doesn’t tell you anything about the absolute position of an object in the Universe, but it’s very good at describing where that object appears to be placed in the sky.



Using the earth as the zero point starts to make more sense once you start looking at things that are very far away, because at that point, you start looking increasingly far back in time. Looking very far away in the universe is looking backwards through shells of time, and since we are observing from Earth, those shells are by definition centered on the Earth - the objects that we see as five billion years old are being viewed at that age because it took light that long to get here. The speed of light is the same in every direction, so you’ll see the same age in every direction at that distance, from the perspective of the Earth. This would be true of anyone observing from anywhere in the universe, but they might see a slightly different set of galaxies.

Because we only have coordinate systems that are centered on the earth or the sun, if you were elsewhere in the galaxy, or elsewhere in the universe, you’d have to constantly convert your location into where you would appear to be (from the perspective of the earth or the sun) if you wanted to use one of these systems. I suspect that whenever we do start exploring outside our solar system, we will come up with another coordinate system that’s convenient for keeping track of our spacecraft, but we’ll keep all the old ones we’ve come up with “for historical reasons”.

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