A space probe is currently orbiting the Vesta asteroid sending back amazing pictures.
Phrases like “Vesta’s southern section is dominated by a giant crater” and “The first close-up pictures of the massive asteroid Vesta reveal a northern hemisphere littered with craters” All very impressive but how is North/South determined?
The Planets go round the Sun on approximately the same plane and are reasonably stable so having North/South the same as our Earth seems logical. but these asteroids sort of do their own thing and I realize a point of reference is necessary but how is the orientation decided.
An interesting question. The phrase that immediately popped into my head is “right-hand-rule”, where if the asteroid rotates such that the surface goes in the direction your right-hand fingers curl, then the thumb points north.
Some Googling indicates that this guess is correct: In 2009 the responsible IAU Working Group decided to define the poles of dwarf planets, minor planets, their satellites, and comets according to the right-hand rule.[1] To avoid confusion with the “north” and “south” definitions relative to the invariable plane, “positive” is the pole toward which the thumb points when the fingers are curled in its direction of rotation (“negative” for the opposite pole). This change was needed because the poles of some asteroids and comets precess rapidly enough for their north and south poles to swap within a few decades using the invariable plane definition.
I’m surprised it took until 2009, though… seems like an obvious choice, though perhaps my familiarity with computer graphics helped.
Computer graphics isn’t the only place the right-hand rule is used:
-In electrical wires, the RHR indicates the orientation of a magnetic field when the thumb points in the direction of current.
-For screws with right-handed threads, fingers curled in the direction of rotation result in the thumb indicating the direction of movement of the screw.
-In kinematics/statics/dynamics, fingers curled in the direction of a torque result in the thumb pointing in the direction of the torque vector.
Bottom line? the RHR is in wide use by engineers of all kinds. Like you, I’m surprised this convention for celestial bodies wasn’t decided until recently. But then, maybe they don’t particularly teach the RHR at astronomy school?..
My guess would be that astronomers had been using that convention for ages, and it just wasn’t until 2009 that someone realized they’d never actually made it official. Possibly there were one or two astronomers who were using a different convention, and it was starting to cause confusion.
Yup–it’s used all over the place. Basically, it’s useful any time you have “stuff” happening along an axis in 3D, and need to pick out one end of the axis as the positive direction. I mentioned graphics only because we use the RHR for objects freely rotating in space–exactly the situation here, so the connection was obvious. Other uses, like magnetic flux, are a little more abstract, so knowledge of only that might not map so easily.
Not possible, at least for a rigid body not subject to external forces. When you hear “spinning on more than one axis”, what really meant is that in some coordinate system, an object is spinning relative to at least two of the X, Y, and Z axes. But there is still only one axis of rotation, and as such there is some coordinate system where the object is spinning (say) only in Z.
I think you’re right. I’ve only skimmed the paper so far, but as best I can tell they take it as assumed that the RHR or LHR is the correct way to define north (or “positive”, as they say), and don’t offer any defense of that. It’s more a case of getting everyone to use the same convention, and setting some rules for objects where the axis changes over time.
Ok I sort of get the get the “Right Hand Rule” but wouldn’t you first need to know where “Up” is?
I remember a program about the Space Station where the had to color code the passages because “go down the passage and turn right” has a different outcome if you are floating “upside-down”
Nope, that’s not necessary–that’s the beauty of it. There is only one axis of rotation, and so your thumb must be aligned in one direction or the other. Only in one direction will the curl of your fingers match the rotation. Of course, this only works if the body is rotating in the first place, but pretty much everything in the universe rotates to some degree.
The space station thing is a slightly different problem. You are correct that simply using left and right would lead to ambiguity. But suppose you have a poster of an astronaut at the junction, with his arms pointing in the two possible directions. You could then say “go down the passage that the astronaut is pointing to with his right hand”, and it wouldn’t matter if you were upside-down or not.
While the RHR applies to dwarf planets only, I wonder if it shouldn’t be applied as a rule of thumb for all planets. The only exception would be the planet Venus.
It does. If you check Wikipedia, it lists Venus’s axial tilt as 177 degrees, which is basically upside down. That way, a person can look at a planet’s axial tilt measured in degrees and determine which way it’s rotating.
AFAIK, the north pole on Venus is pointing in the same direction as Earth and Sun. Re: Post #2 of this thread in terms of the scope of the deciding what bodies are assigned to the RHR definition. It doesn’t say planets.
Astronomers used to use the right-hand rule and no doubt some (most?) still do. But the official north is defined (by the IAU) as that pole which points to either the north celestial hemisphere or north of the ecliptic (I forget which). I also forget when they instigated this rule, but some time in the 80s, I think.
I’ve always thought it was a dumb rule, but they probably have some bureaucratic reason for it.
I’m having trouble getting my head round this. Not too difficult believe me! Dr. Strangelove talks about “only one axis of rotation” - OK I won’t argue with that but taking the example of an astronaut facing you with his arms outstretched, if he flips head over heels his right arm will still be pointing to the right, but if he spins round to face the other way left/right will be reversed, also if he rotates, like the hands of a clock, his head will be at the bottom and again left/right will be reversed.
Since starting this thread I’ve been doing some thinking and could they not determine the North/South by the magnetic field?
I’m assuming that any rock in space with enough mass would have a magnetic field and I guess most space probes have a magnetometer so that would establish North/South, or is that to simplistic?
It’s not mass that leads to a magnetic field, but flowing conductive fluid (this is called the dynamo effect). Something without conductive fluid (like Mars, for instance) can have a very small relic magnetic field trapped in solid iron or iron oxide, but that’s only if it has iron or iron oxide (many space rocks don’t), and only if it once had a flowing fluid-generated field to imprint on it. And even those things that do have a dynamo, it reverses direction every so often: This is on a regular schedule for gaseous bodies (every 11 years for the Sun, for instance), and more sporadic for the Earth since the fluid involved is more viscous. So unless you want to re-define “north” and “south” every few years, it’s not very useful.
Oh, I think I see where the confusion is. You’re still thinking in terms of arms on a body, but the RHR has to do with hands. The distinction is crucial* .
Imagine putting your right hand on a globe, palm down–almost any random spot will do. Put your thumb at a right angle to the fingers. Now, look at a point on the globe and see how it moves when the Earth rotates its normal way (from west to east). Rotate your hand around (keeping your palm on the surface) so that your fingers point in the direction of this movement.
Can you see that there’s only one rotation that works, and that this is the one where your thumb points north? Can you also see that if you use your left hand, you can do the same thing but that your thumb points only south?
This will work no matter where you are on the globe, except directly at the poles where the surface doesn’t move at all.
You could work out a system that involved arms if you wanted, but hands make it more clear.
Somewhat ironically, Earth’s north pole is a magnetic south pole. Of course this is because the side of a compass magnet that pointed north was labeled that way (probably millenia ago)–even though it was actually attracted to magnetic south.
Suppose you’re an astronaut and you get dumped on a random asteroid. You want to find the north pole–how do you do so?
Well, you start by looking up at the sky, where you can see the stars moving around. If you look closely enough, you can also see that there’s one point in the sky that is fixed, and all the other stars move around that point.
Suppose you walk to the point on the asteroid where that point is directly overhead. Look up–is the sky rotating clockwise or counter-clockwise? If it’s going counter-clockwise, you’ve found the north pole. On Earth, the star directly above you would be Polaris.
Look at your hands, and do the standard “thumbs up” gesture. Your thumbs are pointed towards the northern celestial pole. But only on your right hand do the fingers curl with the direction that the planetoid moves.
The opposite is of course true at the south pole. The sky rotates clockwise when you look up, and the planetoid moves in the direction that your left-hand fingers curl.
Suppose you flip your right-hand to do a thumbs-down–your fingers are pointing in the right direction, but your thumb is now pointing north (down) again! So it always works, no matter where you are or how you are flipped.
