I totally agree. I never have to think about distinguishing between north and south (or left and right).
But east/west? I used to say that I’m east/west dyslexic.
I first really noticed it when I had to label a bunch of photos of a landfill under construction with captions like “View to west of [whatever]” and realized after labeling a bunch of them that I had inadvertently mislabeled most of the east/west directions. Even after being aware of this, I still kept mislabeling them, and only caught the errors when double-checking.
OK, I have a good one: say you have a planet. Now, the angular momentum of the Solar System defines a plane dividing space into two halves, so it should be pretty clear which is the north pole and which is the south pole (though you have some tricky marginal cases like Uranus). But which direction is east, and which is west? Also, should east or west be positive?
The planet or moon could be rotating and/or orbiting retrograde, for starters. There is planetocentric longitude which increases positively eastward, and I think it has to be assumed here that east is counterclockwise if you look down from above the north pole. That does not seem so bad, except that it seems that “dwarf planets, asteroids, and comets” spin in the right-hand sense about their “positive pole”, which is referred to as the north pole in some documentation but may actually point south [a pole can precess or otherwise move back and forth between north and south, for starters]…
If that were not enough, there are also planetographic coordinates, where the longitude of a point on the equator directly in front of a distant, fixed observer in an inertial reference frame increases with time… except for the Earth, Moon, and Sun, where it is defined the other way around!? As for dwarf planets, asteroids, and comets, the above PDF says there are so many inconsistent standards the only winning move is not to attempt to use planetographic coordinates for those.
East and west (and hence north and south) are defined by the planet’s rotation, not by the solar system’s. In other words, east is the direction in which celestial objects rise, and west is the direction in which they set.
And right hand rule (arbitrarily not left hand rule) applied to the planet’s rotation ought to define planet-centric north. If we now have any thing in our solar system defined differently, we oughta fix that before any more data is accumulated.
Meanwhile the same rule applied to the star ought to define north for the star. And the same rule applied to the bulk of the star’s satellites, ought to define north for the system. I would expect any discrepancy between these two methods to be very very very rare, and a sign of a system that suffered major disruption sometime in its history. Minor differences in alignment, sure. 90+ degrees difference where north for the star is south for the system, pretty much no, never.
Where by “applied to the bulk of the star’s satellites”, you presumably mean their orbital motion, not their own individual rotations.
Or, if you want to make things absolutely unambiguous, just say that it’s based on the total angular momentum of the entire system (which in most cases will be dominated by the orbital motion of the largest planet).
Yes, planet’s orbital motion, not individual rotation, was what I meant. Sorry to be ambiguous.
@DPRK’s mention of total angular momentum is what triggered my thinking. But then I got to thinking that maybe there’s value in allowing for the very special case where the star’s individual rotation & the satellites’ collective orbits = satellites’ total angular momentum are quite obviously rotating opposite one another.
Yup, you might end up with cases where the star’s north is significantly different from the system’s north. I’d expect that it’d be incredibly rare, but hey, it’s a big Universe out there.
You would be correct but for the fact that the north pole is defined as the same direction as the Earth’s north pole, not (for planets and moons, at least) by their own rotation.
That sounds reasonable, but that is not the definition used by the International Astronomical Union:
So, indeed, what about the right-hand rule?
Interestingly enough, most of the angular momentum in our solar system is due to Jupiter. The Sun only accounts for 0.3 percent of it! It is still in the same direction, though. Understanding the possible relationships (could it rotate the other way in some systems?) requires a good model of solar system formation…
Well that’s a stupid definition. Do sol-centric coordinates also apply to extrasolar planets? Or are they based on their own solar system? What if their solar system isn’t neatly planar? And what about rogue planets that don’t orbit any star?
I have not seen any reference to this. Obviously, it was not an issue in 1976; only in relatively recent years have they begun to be able to measure the rotation and axial tilt of exoplanets, let alone map their surface features.
Addressing the future: The WGCCRE began in 1976 and, as part of the IAU, established fundamental principles regarding planetary coordinates and planetary mapping. After 45 years, some obvious questions naturally arise. Are these principles still adequate? Are changes needed? When is any transition or refinement needed in a coordinate system and how should it be done?
The Sun is much bigger, but Jupiter has a much much longer lever arm! As Archimedes supposed said, “If I had a place to stand and a suitable lever I could move the world” (meaning the Earth).
FWIW if you go to a table of exoplanets, you can download orbital elements, including inclination. However, this is measured relative to the line of sight between the Earth and the object, which is useful because of the way it is measured using transit photometry and so forth, but, from a certain point of view, it is an extreme use of geocentric, not even heliocentric coordinates, to describe planets orbiting a completely unrelated star.
To the extent that it matters (which is basically not at all), those inclinations would be heliocentric, not geocentric. For a geocentric inclination, you’d need to specify where the Earth is in its orbit. If you don’t specify that, then you’re implicitly assuming that the Earth is in its average position, and the average position of the Earth is the Sun.
I have a fairly poor sense of direction. Dad has an excellent one. Panama threw us both for a loop. I never knew where I was. Which ocean I was looking at. Was I looking north? East? West? South? Who knows!
As I like maps, I have a pretty clear idea of where I am and how to get to where I’m going, and the cardinal directions thereof. But North is not just one direction, it ranges from NW through to NE, so though I can point in a Northerly or Easterly direction, I may be as much as 45° out of whack, and can get lost in an unfamiliar location quite easily if the info I’m working from is vague enough.
However; East is where the sun rises, West is where it sets. That much seems like plenty enough to orient yourself.
As Archimedes supposed said, “If I had a place to stand and a suitable lever I could move the world” (meaning the Earth).