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Where should I put the prime meridian?
October 19, 2013 5:48 PM   Subscribe

So I've just sent a probe to map a new Earth-like planet, and I'd like to establish a latitude-longitude standard for the planet. The poles are pretty easy to spot since the planet is rotating, so I plop down the equator halfway between them and boom, I've got my latitude lines laid out. But now it's time to lay out the longitude lines and I need to find somewhere to put the prime meridian. How do I decide where to put it?

I'd like to think of a reasonably standard way of deciding where E 0° should be, and it should also be easily communicated, so that other people using probes can sync their maps and measurements up with mine. Any thoughts on how one would go about this?
posted by ErWenn to Science & Nature (22 answers total) 11 users marked this as a favorite
I'd recommend the subsolar point when the planet is exactly at aphelion. (This is assuming that it's not tidally locked to its star or a satellite.) You're still going to have to define this for some particular epoch (some year), but it's a not-completely-arbitrary thing that would be somewhat stable on decade timescales, unless there are nearby / larger perturbers making the orbit precess.
posted by BrashTech at 5:54 PM on October 19, 2013 [4 favorites]

The main problem is that Earth's prime meridian is basically just any old place -- it was set by the British, and they just picked a point that was useful for them.

The smart thing to do with an exoplanet would be to first determine whether there was sentient life, and what their prime meridian is. Or, if they don't have one, at least pick something that rationally makes sense in light of the existence of sentient life on the planet.

If there is no life, or if we cannot determine the existence of life without setting a prime meridian, I don't know, probably pick something geologically or geographically convenient, I guess? I can see an alien culture assigning the longitude of the far eastern corner of Brazil as the prime meridian, because it's like a nice big arrow pointing to THIS SPOT RIGHT HERE. The western coast of Senegal where Dakar is also seems nice and logical, if one were looking at the Earth from space.
posted by Sara C. at 5:56 PM on October 19, 2013 [1 favorite]

The subsolar point at the time of the vernal equinox would also be a good point, if the planet has significant obliquity.
posted by BrashTech at 5:58 PM on October 19, 2013 [3 favorites]

For sufficiently Earth-like planets, you could use the meridian which passes through the geomagnetic poles, taking advantage of the fact that geographic and geomagnetic North almost certainly won't line up perfectly. That will drift too, though.
posted by Now there are two. There are two _______. at 6:06 PM on October 19, 2013 [1 favorite]

Earth's prime meridian is arbitrary, as is Mars' which is the Airy-0 crater

There is a wiki list of other bodies prime meridians which may give some inspiration.
posted by chrispy108 at 6:13 PM on October 19, 2013

On Earth the international dateline has to wander all over the place to avoid cutting inhabitable parts into two different days.

It would be nice to choose the prime meridian (180° away) so as to minimize this.
posted by jamjam at 6:19 PM on October 19, 2013 [4 favorites]

I don't know of a standard, but here's my idea:

If there are "land" and "sea" masses, where the entities to be studied are primarily on the "land" masses, then I think it would make sense to find the largest gap between "land" masses -- i.e. an area with comparatively little to interest those studying the planet, along all latitudes -- and place the prime meridian toward one end of that gap (the end with the least rotational distance between the prime meridian and the first area of interesting "land").

Of course, most planets our actual science has discovered are far enough away that we don't know what's interesting and what isn't, let alone whether there's actual water.

In the absence of that, and of a civilization on the planet in question, I'd guess that the prime meridian would be just to the "before" side of the first or most prominent or important landmark discernable by the discoverer. "Before" here means turning to the sun immediately previous to the landmark.

This means imposing someone's subjective idea of "important" or "prominent", so maybe have some criteria for that.
posted by amtho at 6:21 PM on October 19, 2013

Actually, another way to do this would be to forget about choosing a point on the surface, and just worry about synchronizing clocks between your probes — either by having them send out time codes by radio, or by using some pulsar in the vicinity as a clock pulse and telling each probe to watch it.

If everyone has their watches synchronized, everyone can see the sun (or some other astronomical body that you've agreed will count as the sun), and you know the period of the planet's rotation, then you can use that to keep track of longitude on the surface.
posted by Now there are two. There are two _______. at 6:24 PM on October 19, 2013

I should point out that this is for the purpose of mapping and describing points on the surface to other explorers, not for establishing time zones. Assume the local life isn't sufficiently intelligent to be making maps or creating landmarks.

I like the idea of using subsolar points at a particular time in the revolution. Anyone know how far the subsolar point at a particular time in the year drifts each year, say, for Earth? Also, how hard is it to measure this value and how accurately can it be measured?

Geographical points could work, but if there are tides of any sort (or other water-level fluctuations), then the prime meridian wouldn't be very precise.

I'd thought of the magnetic poles, but since they're so close to the geographic poles, you'd need to be very accurate. Being off by 500 feet on Earth at magnetic north might put you off by a mile and a half at the equator. Also, it apparently moves almost 40 miles a year.
posted by ErWenn at 7:01 PM on October 19, 2013

There's a more fundamental problem; which way is west? It's not so obvious when planetary bodies don't all spin the same direction, or have the same orientation of the pole vs. the solar system. Sky & Telescope had a recent article on this problem and IIRC there's not even a proper consensus on the coordinate system for either Mars or our Moon; I forget which.

To your follow up; most planetary bodies don't have strong magnetic poles.
posted by Nelson at 7:06 PM on October 19, 2013 [3 favorites]

The first lander on the surface is the prime meridian.

Have the probe send down a rover or a shelter or a remote lab. Not only does the prime meridian thus have historical significance, it also lies on the most studied part of the planet, where the most about it is known, and thus has significance to explorers that follow the probe.

Distance that explorers move away from the prime meridian translates directly to distance traveled into the unknown.
posted by anonymisc at 9:05 PM on October 19, 2013 [7 favorites]

Dava Sobel's book Longitude is an excellent discussion of longitude, prime meridians, and why this question doesn't have a good answer.
posted by Hatashran at 9:12 PM on October 19, 2013 [4 favorites]

hmm...good question...all of the points discussed tend to move around...almost all planets wobble a bit, even tidally locked using points at aphelion or equinox or whatever will probably not stay accurate for long, and, as you said, drawing a line between geographic and magnetic poles will drift even faster, if it even has a magnetic field.

I suggest highest point on the surface and draw a line through the poles...Mt. Everest on Earth, Olympus Mons on Mars, etc...this will probably be the most stable point on any non-molten or non-gaseous surface...and there's really no point in mapping a molten or gaseous surface, like for example, Jupiter. For those kinds of bodies you might as well use the subsolar point when the planet is at aphelion since any precessing or drift will be drowned out by the planet's own turbulence anyway.
posted by sexyrobot at 11:26 PM on October 19, 2013 [2 favorites]

Assuming it doesn't have a native intelligent lifeform of its own, in which case *their* form of planet-mapping should take precedence, for your newly-discovered planet I'd to place the meridian at the landing site of the original probe.

As others say, Earth's own prime meridian is an arbitrary spot, chosen for convienience. If not an equally-arbitrary choice like the primary landing, then second-choice would be a straight line through an arbitrary spot in the seas.
posted by easily confused at 2:37 AM on October 20, 2013 [1 favorite]

As it happens, there are folks who have thought about this a bit, and they're called astronomers. The International Astronomical Union actually has a standard, which runs as follows [from this paper]:

...the sub-planetary intersection of the satellite's equator and the plane containing the center of the satellite, the planet and the Sun at the first heliocentric conjunction of the satellite after 1950.0.

This technique has some limitations which have been addressed by emendations since adopted in the 1970s, and others which are somewhat in dispute. No system is perfect.

Such a procedure could clearly be adapted for any other stellar-centric system, although might be difficult to apply in the case of the recently-discovered rogue planet PSO J318.5-22^.
posted by dhartung at 4:58 AM on October 20, 2013 [6 favorites]

Anyone know how far the subsolar point at a particular time in the year drifts each year, say, for Earth?

Since an Earth year is about 365.25 days (give or take several minutes depending on both which type of year and which year you are measuring), the subsolar point on earth at a given time of year will be ~90° of longitude away from the subsolar point at the same time next year.

If you're going to use BrashTech's method of the subsolar point at aphelion, or something like that, you'd have to specify the year, say maybe the first aphelion after your probe reaches the planet.
posted by DevilsAdvocate at 8:13 AM on October 20, 2013 [1 favorite]

The method cited by dhartung's applies only to satellites (i.e. bodies orbiting planets), because you need three bodies to make a conjunction. If your planet has a satellite, you could do something analogous by using the sub-satellite point at the first full moon after your arbitrary time point (the arrival of the probe, B1950 or J2000, or whatever you like.)

DevilsAdovcate makes a good point about the earth's rotation being pretty much decoupled from the solar year, so the subsolar point at equinox or perihelion or aphelion is not at all consistent year-to-year, even without gravitational perturbations causing the orbit or axis to precess.

However, if you know the orbit and spin state of the planet, and you work out the perturbations by the other objects (which requires knowing their orbits and masses, and maybe the distribution of mass with objects that are massive, near, and/or very aspherical), you can observe the current system and extrapolate back to the exact location of the point at epoch in a straightforward manner—by which I mean, you do lots and lots of math, and then do even more math if you need greater precision, but everyone will agree on the result, which is what really matters for establishing a coordinate system.
posted by BrashTech at 8:56 AM on October 20, 2013

Thanks to all the great responses.

The position of the lander is a very good idea as it's likely to stay put and be easy to find (radio beacon or something) and label very accurately, although I was imagining observation from space, instead of from the ground. I think that if you do have a landing craft, this is clearly the best answer, unless the first landing was close to one of the poles.

Absent a physical marker that we dropped to the surface, the question still remains.

Obviously, all this is kind of moot if the visible surface changes a lot (a la a gas giant). Any map would only be useful as long as there are surface features we can detect.

Fortunately, the hypothetical I mentioned suggests an Earth-like planet, by which I meant that we could assume the presence of magnetic poles, liquid water, relatively stable landmasses, relatively stable tectonics, and a mostly transparent atmosphere in a non-tidal-locked orbit with small, but noticeable eccentricity.

To me, the most natural suggestions so far are the highest point on the landmasses; the suborbital point at aphelion, perihelion, or one of the equinoxes; and the magnetic poles. All of these move to some degree or another (even the highest point on the surface will move slowly due to tectonics and infrequently but erratically due to falling rocks or whatnot), so you'd want to fix a point in time to measure them from (the first aphelion/perihelion after the probe arrives or after we get a good picture is a natural suggestion for a reference epoch). They all seem equally natural to me, so the question becomes which is most practical.

The magnetic poles require extreme accuracy in measurements (I don't know how easy it is to measure them very accurately), which is probably a point against them. But even worse is the problem that they move somewhat erratically (compared to orbital mechanics), so it'd be hard to project backwards in time to figure out where it was at the reference epoch. We'd probably need to get good maps right away so that we could keep referring to that point.

Subsolar points can be projected backwards pretty easily if we've got enough data on the orbit, which is probably the case if we managed to put a probe in orbit. But it's still a calculation. Does anyone know how accurately we could pin this down and what would be involved in getting that accurate measurement?

I think I'm leaning towards the highest point on the surface. If the highest point is close to the poles, we could pick the highest point along the equator. We'd only need to refer to careful maps and/or information about conditions at the moment of the reference epoch if we needed very accurate measurements. Since it moves so slowly, two different probes that had no communication at all (not counting initial programming before the planet was even discovered) would probably end up using coordinate systems very closely to each other.
posted by ErWenn at 9:12 AM on October 20, 2013

Oh, and if you're measuring the first aphelion and perihelion after arrival, I think that the differences between sideral, tropical, and anomalistic years would be rendered moot, if I'm understanding them correctly.
posted by ErWenn at 9:13 AM on October 20, 2013

Here's the article I was thinking about: not Sky & Telescope at all, but rather Wired Science. I think it's a quite complete and readable answer to your question.

It references a 2009 IAU paper and gives a readable summary. It's complicated, but 0° is defined either by an arbitrary landmark (for planets) or based on tidal rotation locking (for moons). Also goes into detail on the Mars confusion, where half the community defines longitude increasing to the east and half to the west. Smaller bodies are more complicated and do not have consistent geodesy. Pluto being reclassified screwed things up, for instance.
posted by Nelson at 10:47 AM on October 20, 2013

It ought to be stressed that longitude lines have nothing to do with time zones. E0 was a good idea, but its utility as a navigational tool led us into errors of another sort. We just sort of drifted into the fallacious notion that it would be good for the sun to rise at the same time everywhere, without taking into account that, whether we like it or not, it's the same time everywhere all the time anyway, and trying to make the sun rise at six o'clock everywhere would lead to such idiotic notions as Daylight Savings Time, and concurrent Wednesdays and Thursdays in neighboring towns.

Anyhow, for star-travelers, longitude likely would be arbitrary. If I were writing a story, I would use it as an element in the plot plot line: It could turn into a point of contention. If the residents are Earthling colonists, it could be a sore point between them and the federation. Conflict and resolution follows.

Or, non-human sentient residents, having been on friendly terms with Earth visitors, could discover one of our charts (their ambassador was visiting the Earth mother-ship in orbit), and having had E0 explained, and tells us that we've just committed some unpardonable religious /moral transgression. Conflict and resolution follows.

Or, the mapping unit drops a probe on some open ground, and future visitors use a transponder to coordinate their maps to E0. Your mappers can use one probe at E0, and another at E180, or other appropriate places, so long as the probe identifies its longitude in its transmission. In this instance, the transponder is analogous to Geodetic Survey markers (benchmark devices) planted by surveyors. Except that this marker beeps.

But, gas giants like Jupiter present a whole other set of problems.
posted by mule98J at 11:52 AM on October 20, 2013

Alternatively, if you're sending probes---note: plural---to this planet, is it out of the question to consider putting a constellation of GPS-like satellites and terrain mappers in orbit?

Problem solved in the sense that the choice is arbitrary but self-consistent from then on (to the few cm scale with present technology). You're also immune to changes in planetary geology, magnetic field, and freak disasters that affect just one or a few of your probes or important markers. Plus, you get planetary navigation, communication, and stable timing infrastructure, which is pretty handy. Truck-drivers charging their rigs down unpaved narrow alleyways into rivers notwithstanding.
posted by fatllama at 5:20 PM on October 20, 2013

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