# How do direction work in space because north,east,west and south are bonded to earth? How does a spacecraft guide itself in the unending space?

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How do direction work in space because north,east,west and south are bonded to earth? How does a spacecraft guide itself in the unending space?

In: Physics

Just like on earth n,w,e,s are meaningless without a reference. The same will apply to space.

Since things move in space you will need to use coordinates relative to some set objects. Say certain stars. We havent really begun space exploration to really hammer out a good system but we do use angles and distance that are relative to earth.

Directions only work with a reference point (even on earth – the reference point could be the geographic or magnetic poles)

So in space, a traveller would need reference points – possibly using the center of the galaxy or distant galaxies as reference points. Of course it wouldn’t be called N, S, E, W because there are 6 “cardinal directions”.

For travel within the solar system, the sun would be a reasonable reference point perhaps along with a few distant stars.

Ooh, I know this one. It’s called a [gimbal](https://en.m.wikipedia.org/wiki/Gimbal). The concept is used in [inertial navigation sysyems](https://en.m.wikipedia.org/wiki/Inertial_navigation_system). Basically, 3 gimbals provide your 3D reference (xyz) to orient yourself. The gimbals will always be spinning in the exact same orientation in space no matter how a spaceship flips and spins. There’s a scene in apollo 13 where they talk about [gimbal lock](https://en.m.wikipedia.org/wiki/Gimbal_lock), meaning they’re losing their ability to orient themselves because one of the gimbals is close to being “trapped” or “caught up” with another gimbal, losing orientation in that axis. [Here’s](https://youtu.be/OmCzZ-D8Wdk) a short video explaining it.

Edit: ~~Imagine two of the gimbals represent the xy-plane and its parallel with the Earth’s orbital plane around the sun. You can read the gimbals to tell you if you’re pointing “above” Earth’s plane of orbit or “below” Earth’s plane of orbit (assuming the North pole points “up” for us northern hemisphere dwellers).~~ I’m guessing, I shouldn’t do that.

More science related to gyroscopes and the relevant phenomenon with demonstrations you can see [here](https://youtu.be/XPUuF_dECVI?t=23m). See also 35:35 for another demo.

Edit: Silly me. Walter Lewin specifically talks about it in this video at 43:50. Watch that.

Edit: I’m an idiot. I’m talking about the gimbals like they’re spinning. They’re just the rings free to rotate and allow the central gyroscope to spin and maintain its initial position. Don’t trust everything anyone says.

Currently we map objects in the sky using polar coordinates. Two angles and a distance.

Usually we use Earth as the centre point (in fact the viewers position on earth) and we work out the angle the object is from the centre line of the sky (that we define) and then the angle off the horizon.

This is declination and right ascension.

It doesn’t make much sense for an interstellar space ship to use earth as the centre point. So we might use the centre of the galaxy. Then define 0 degrees as the line through the sun.

So the solar system would be at 0°,0°,25kly

Changing direction would also likely use angles. Similar to how boats do it. Change angle a by x° and angle b by y°.

I don’t know how actual space craft do it but there it’s precedent in fiction with star trek. At the end of an episode the captain might command the helm to set a course 120 mark 43. That’s your two angles relative to something (the ship, the galactic plane or something)

When away from the earth, stars serve as a suitable reference point. The north star is still in the same direction, even in space, and other stars become easier to use because you are no longer on the surface of a rotating sphere. Essentially, in space every star can be the north star.

Spacecraft are able to determine their position and orientation through a combination of on board sensors (like star sensors) and off board trackers (like radar). Beyond that, it is typical to describe their position and velocity as an orbit. These orbits can be described using a few variables that indicate the size, orientation, and direction of the orbit. These are called “Keplerian Elements.”

So, for example if you wanted to convey information about a satellite above the Earth, you wouldn’t say “It’s 500Km above the ground, moving 7km/s in the Northwest direction” but you could say, “The satellite’s orbit has a semimajor axis of 6800km, with an eccentricity of .01, inclination of 23 degrees…”

Of course, there are other ways of keeping track of and describing these, but that’s one of the most basic ways.

Play [Kerbal Space Program](https://www.kerbalspaceprogram.com/). Here is a helpful xkcd to help understand why it will help.

[https://xkcd.com/1356/](https://xkcd.com/1356/)

Also why you won’t be ready for that NASA position.

[https://xkcd.com/1244/](https://xkcd.com/1244/)

As an add-on to D1Foley’s comment, check out Quill18’s “Kerbal Space Program for Complete Beginners” series on youtube. He covers this stuff and does a preeeeeetty good job of it.

Aerospace engineer here!

The short answer is basically however you want it to!

The long answer is something called frames of reference.

A frame of reference, or reference frame, is how you determine your position and orientation relative to another object. On Earth we tend to use down as the direction earth is pulling us, up as the opposite and then north/south/east/west for planar (side to side, forward-back) directions. In space however, there is no absolute frame of reference.

You could be x miles from the earth and y miles from something else. (This also effects velocity but we won’t go into that unless someone asks).

So which reference frame do you use? Whichever one works best. Some times the math is easier if you use earth as a reference frame, sometimes it’s easier if you use the sun.

I actually work in the space industry, so I feel qualified to answer this. As other commenters have alluded to, there are two parts to this question: reference frame and navigation.
In science and engineering, when describing motion you need a base coordinate frame. To start, you need a fixed reference point and direction to base the coordinate frame on. The typical reference is the vernal equinox, which is an imaginary line pointing towards a distant star called Vega. For our purposes, the position of Vega is fixed, so it makes a good reference. From there we can build our axes, but this will depend on the physics involved.

For a low-earth orbit spacecraft we use the Earth-Centered Inertial frame (ECI), which has an origin at the center of the earth, x axis pointed towards vernal equinox, z-axis pointed through the north pole, and y axis perpendicular to both x and z.

A base reference frame should be “inertial,” or non-rotating and non-accelerating, in order to make the physics work out. For an interplanetary spacecraft, the ECI frame is NOT inertial, because it is fixed on the earth which is accelerating around the sun. In this case we define a different frame: sun-centered. In this case the origin is at the center of the sun, X-axis pointed towards vernal equinox, z axis perpendicular to the ecliptic (plane that Earth’s orbit makes around the sun), and y axis perpendicular to X and Z.

Now, for navigation: we use devices called Inertial Measuring Units, or IMUs, to constantly measure acceleration and rotation. Think of them as fancy accelerometers and gyroscopes like you have in your phone. If we know where we start, and we keep track of all the accelerations, we can figure out where we end up. The previously described reference frames give us the language to describe this (in terms of X, Y, and Z coordinates). We can improve knowledge of our position with dead reckoning, where we CHECK our distance and speed with radar measurements. If we send a signal to a spacecraft and it takes 20 minutes for that signal to get back to us, then by knowing the speed of light we can say exactly how far it has travelled, which makes the estimate we got from the IMU more accurate.

EDIT: I think forget what I said about Vega. The X axis is defined by the mean vernal equinox, which is when the southern and Northern hemispheres receive the same amount of light (around March 21st). At this point, you can draw a straight line from the sun though the center of the earth and that line will intersect Earth’s equator. Because of this, it is by definition perpendicular to the north pole.

Earth based directions (North/South/East/West/Up/Down) don’t work, so we create a new “frame of reference”.

A frame of reference is a way of looking at and measuring things. Walking around your neighborhood, you use N/S/E/W, but if you were walking on a huge cruise ship sailing through the ocean, you would use Fore/Aft/Port/Starboard, no matter which direction the boat was pointed. We would say we are moving towards the port side, even if the boat is moving west, so Pot is actually south. We would say we’re walking towards the Port side at 1.6 km/hour (1 miles/hour), even if the boat is moving forward through the ocean at 32 km/hour (20 mile/hour).

In the same way, we can create different frames of reference for outer space. One frame of reference when you are orbiting close to earth, another when you are far from earth and orbiting the Sun, another when getting close to the moon / Mars, etc…

A great and fun way to experience this is to play Kerbal Space Program.

Astronautical engineer here.

Spacecraft are equipped with a subsystem called Attitude Determination and Control System.

This subsystem can contain various tools including Star Trackers, Horizon Sensors, and Sun Sensors for navigation.

There are lots of stars in space, and a lot of them are so far away that they appear fixed, i.e. they do not seem to move.

A star tracker is basically a camera that scans the space for star patterns. Then it compares the image with the database to estimate its orientation.

Sun sensors find the Sun (obviously) and are generally used for solar panel pointing etc. Horizon sensors use infrared to find orientation based on the planet’s horizon line.

This is the navigation part. For control, there are reaction wheels, magnetorquers, reaction control thrusters, and more. RWs spin to generate a moment in the desired axis, so there are mostly 3 of them. Magnetorquers use magnetic field of the planet to change orientation. RTCs are small thrusters that are placed on large spacecraft to perform small correction/orientation maneuvers.

The best part of this question is the number of folks is the aerospace and astronautical field that are willing to chime in.

Thanks to all of you, I learnt more than I expected to. Much appreciated.

Easy, the enemy’s gate is “down”. I’m not a space engineer of any sort, but I can at least talk about the math that’s helpful here (linear algebra).

When you are walking around, you can talk about how things are *in front/behind* of you, *to the (right/left) side* of you, or *above/below* you. If you want to be clever, you can mix the descriptions too: “enemy ship at 2 o’clock!” means something is mostly to your right, but also a bit in front of you.

When you’re talking to someone else that isn’t facing the same direction, you can’t just use the forward/right descriptions anymore, so you have to pick something both of you understand. A nice one is to align to the Earth with North/East/South/West. Or, if you know what direction they’re facing, you may choose to use their perspective instead (“turn right on Maple, then turn left on Jefferson…”).

To give directions, you only need to define the three basic directions “up”, “right”, and “forward” and go from there. The third can be derived from the first two, so really you just need two of them. Usually you use some sort of reference point(s), maybe a star or a planet or your own spaceship, whatever.

ELI25 note: a set of *n* directions for an n-dimensional coordinate space is called a *basis* space, and requires *n* orthogonal vectors. Converting from one basis to another is very easy with linear algebra. With as few as three points that aren’t all on the same line (e.g., center of the sun, North Pole of the sun, some other star) you can create a full basis because of the neat property that the cross product of two vectors is always orthogonal to both input vectors.

Hi y’all,

This topic is very complex and certainly need simplifying. That is the goal of the sub. Something that confuses some is that the target audience is ‘lay-person’ and not a literal five-year-old. ^(shout-out to r/ELIactually5, which gets no love)

So as mods we have a really interesting problem. What do lay people understand? What words are known to the average joe on the street. ^(As reddit mods we are, of course, of the upper reaches of the intelligence spectrum)[.](https://youtu.be/wyyr3L3rFvA?t=10)

So we have to assume that x-thousand upvotes (and only a few reports) means it was digestible to most.

Feel free to use the reports or comment in the sub in my footer if you want to discuss the rules. I’ll even link it here in the sticky.