This is something that is simple yet revolutionary. I have never seen it pointed out and it is about time that it was. It involves expressing direction while traveling in space.
How do we express directions in space? On earth we expect direction with the cardinal directions of north, south, east and, west. To some extent we can extend this system into space. Since the moon is tidally locked to earth, meaning that the same side of the moon always faces earth and the moon doesn't rotate other than it's revolution around the earth, the moon's poles match those of earth and so it is clear which is the north, and which is the south, lunar pole.
Since all of the planets in our Solar System orbit the sun in roughly the same plane we can also apply this to most of the other planets. The north pole of the planet is the one closest to earth's north pole.
The magnetic poles of planets tend to be fairly close to the geographic poles, because the magnetism results from the spin of the planet. So we can also define the north and south poles of the planet by magnetism. But this is complicated in the case of Uranus because the planet has apparently been knocked on it's side by an impact and it's south magnetic pole is actually closer in geographic direction to earth's north pole.
But what about outside the Solar System? Many "exoplanets" have been discovered in orbit around other stars although, at this point, we cannot tell much about them. How can we express directions on those planets, since their orbital planes may be nowhere near the same as in our Solar System?
I have a simple solution that would enable us to express directions on any planet or star in the universe. Virtually everything in the universe rotates, planets, moons, asteroids, stars and, galaxies. The earth rotates eastward. Why don't we just define east as the direction of rotation? Then, when we are facing east, west is behind us, north is to our left and south is to our right.
This means, of course, that the cardinal directions of north, south, east and, west will be relative. Since the rotation of Venus is opposite to that of the earth it's north and south poles must be the reverse of earth's. But this would be a simple yet revolutionary way to be able to express what we could call "local direction" on any astronomical body.
When dealing with planets or stars we only have to deal with directions in two dimensions, since direction on the surface of a sphere can be expressed in two dimensions. That is why the cardinal directions of north, south, east and, west are sufficient, two opposite directions for each of the two dimensions.
But what about directions within our galaxy? We live in a barred spiral galaxy that is rotating. But we have no real easy way to express directions in the galaxy. How would you describe which direction one star is from another in our galaxy? There is no easy way to do it. We cannot use the orbital plane of our Solar System as an effective reference point because it is not the same as the rotational plane of our galaxy, there is a difference of about 60 degrees.
But if we use this principle of definition of direction by rotation it becomes simple, except that we need two additional directions because we are now dealing with three dimensions, rather than two. Let's call the two additional directions "top" and "bottom".
Our barred spiral galaxy is rotating, that gives us a starting point. Suppose we are looking at our galaxy from outside, from the same plane in space as the galaxy is aligned, as if we were looking down at earth's equator. Consider the galaxy as rotating "eastward" but instead of calling what would be north on earth, let's call it the "top" of the galaxy with the "bottom" being in the opposite direction.
So if we were looking at our galaxy from outside, along the rotational plane of the galaxy which is congruent to looking down at earth from above the equator, and the galaxy was rotating from our left to our right, what would be north on earth will be defined as the "top" of the galaxy and what would be south on earth will be defined as the "bottom" of the galaxy.
But to define direction within the galaxy we still need another reference point. We could just define east within the galaxy as the galaxy's direction of rotation, as we would on planets and stars. The trouble with that within the galaxy is that, if we are near the center of the galaxy, east will be one direction on one side of the center and the opposite direction on the other side of the center. So doing it that way probably wouldn't work very well.
So to express directions within the galaxy we need some kind of reference point outside it.
It seems that every galaxy is part of some larger galactic group. The group that our galaxy is in is known as the Local Group. Our galaxy is not the largest galaxy in the Local Group. The Andromeda Galaxy has a double nucleus, making it seem as if it is actually two galaxies that merged together.
Every galactic group has a common gravitational center. Within the galaxy why don't we define "north" as the closest line to the line between the center of the galaxy and the common gravitational center of the galactic group? North may not be the same line because remember that we defined east as the direction of the galaxy's rotation and the line between the center of the galaxy and the gravitational center of the galactic group may not be in the same geometric plane.
Remember that, within a galaxy, "north" is not the same as "top" and "south" is not the same as "bottom". We are dealing with a three-dimensional space, unlike the two-dimensional surface of the earth, so we need six cardinal directions, rather than four.
So when we have "north" within the galaxy defined as the direction along the axis of rotation that is closest to the line between the center of the galaxy and the gravitational center of the local galactic group. Again the reason the two lines may not be the same is that the line between the center of the galaxy and the gravitational center of the local galactic group may not coincide with the rotational plane of the galaxy.
Once we have "north" defined within the galaxy, "south" is naturally in the opposite direction. When we are facing directly "north" "east" is to our right and "west" is to our left. Remember that "east" within the galaxy is not the same thing as the direction of rotation of the galaxy, as it is with the earth and planets and stars. The direction of rotation of the galaxy defines it's top and bottom but not the remaining four directions of "north", "south", "east" and, "west".
With modern astronomy and space exploration we definitely need an effective way to express directions in space. At present we usually try to impose the cardinal directions of our earth on other planets and it is not very efficient.
Here is another posting along these lines, about navigation on earth:
http://markmeeksideas.blogspot.com/2020/11/the-keypad-system-of-navigation.html?m=0
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