Using a Compass to Detect Islands in the Sky

Question

In a setting that consists of skies with hovering islands, no 'lower ground', and no unified/core magnetosphere (but no EM space hazards to worry about either), how would the utility of compasses change?

I'm thinking that if the islands (the bigger ones of which would be up to Greenland sized, maybe some coming close to Australia-sized) have the usual metal ores, the compasses would (hopefully) be useful for detecting the direction to the nearest island.

How useful would compasses be for such a purpose, in terms of scaling of their sensitivity based on island size and distance to the ship (e.g. past which thresholds of mass or distance they would no longer reliably point to an island)?

Answer 1

Compasses are showing the magnetic field of a (our) planet, which has its origin in the core made from iron (supposedly).

It does not show a "way" towards the next largest "mass" (be it island or land) and is mostly unaffected by ores or metals in the ground - unless those are also affected by some kind of electricity or magnetic field which then might influence the compass.

Essentially you have three options:

1. Your world has some kind of magic to it that allows compasses to work like you described them.

2. You need some kind of central EM-field / magnetosphere which provides the field lines influencing the compass to show a "global/universal" north/south direction.

3. You need to rig each of your islands to have their own magnetic field by "some reason". This way a compass could show a north/south direction per island. Regions with overlapping magnetic fields could cause the compass to tune to the field lines of the neighboring island (given this one is stronger there). Knowing "island sailors" might be able to interpret the strange behavior of a compass to navigate correctly between the islands.

Answer 2

You can follow the field lines

Firstly, for a compass to work, you need a magnetic field. You cannot have one without the other. So let's give your "world" an unconventional one:

Suppose that before your floating islands were apart, they were together in a floating pangea. Still an island in the sky, but just bigger. Additionally, the core of the island is ferromagnetic. Not an active dynamo, more like a cold permanent magnet. As a magnetic dipole, there will be a north and a south. We'll say north and south are aligned longways on your continent. If you look to the horizon forwards, thats north, the horizon backwards is south.

A neat property of magnetic dipoles is that if you split them in half, you get two magnetic dipoles. This means that we can split your pangea into some continents and expect each of them to become a magnet as well.

Let's assume that the event that broke your super continent apart, left the pieces in magnetic alignment. Or, over time, all magnetic dipoles align to a shared direction. This means that all of the Islands Norths point in the same direction.

Here you could end up with a cool phenomenon. While traveling in between continents, the field lines would be say pointed North, but on the outside they would be pointed south. So your compass would point in different directions depending on your location within the floating continents.

Answer 3

From a practical perspective, the purpose of a compass here on Earth and a compass in your world is identical. What, realistically, is the difference between a compass laying on a beach at sea level, held in my hand at an altitude of 4100 ft, or used in an airplane at 35,000 feet? None. Compasses were (and still are) used on sailing ships. Does it tell them where land is? No. It tells them which direction they're going. Knowing where land is, is an entirely different problem.

This would suggest that in your world you need a compass combined with something that can indicate direction in the third dimension. But we kinda have that, too in the form of an altimeter, a device that indicates how high above sea level one is.

So you might be asking the wrong question. Your floating islands (or a vessel traveling between them) would be perfectly happy using the combination of a compass and altimeter for navigation. But, as was famously said in the movie The Hunt for Red October...

[Kamarov] Give me a stopwatch and a map, and I'll fly the Alps in a plane with no windows.

[Yuri] If the map is accurate enough.

Your real problem is the map or chart helping people navigate between the islands. Landmasses have the most convenient property of (usually) not moving fast enough to matter in a person's lifetime. Your floating islands, unless tethered, don't have that convenience. That means mapping tech that tells a traveler where they are and where they need to go today, because wind currents will have moved the islands by tomorrow.

Fair warning. This particular rabbit hole can go very, very deep. Before asking for help developing the details behind how your civilizations map the locations of the islands, be absolutely sure you actually need it. It's the kind of detail most authors gloss over because as cool as it might be, it's usually a distraction to the story rather than a benefit to it. Although a story-based issue that would be inappropriate to ask about here, what might be far more interesting for your story is the behavior of navigators when interacting with the map or chart. It leaves the reader with a fulfilling sense of completeness in your worldbuilding without actually building that aspect of the world and a bit of mystery that readers love for the imagination and movie directors love as it gives them latitude about representing it on screen.

Answer 4

Magnetic fields are closed

As others have said, a compass is only useful at finding islands if your islands have a magnetic field. That's fine, you can give your big islands magnetized iron or nickel cores. The problem is the orientation of the field. If the axis of the magnet is vertical, then the generated magnetic field at any point on the same plane as the island is also vertical, meaning a compass held in normal orientation would just spin freely. So if all of your islands and ships are at the same altitude, island-specific magnetic fields don't help.

This does, however give some possible story points. There are two modifications you could make.

"Surface" to take a reading

The compass is useless at the same altitude as the islands. But, if you go above or below the origin plane, the magnetic field gets some lateral component pointing toward the island. So either the ships must always travel at different altitudes, or maybe they periodically go up or down by some mechanism to take readings. Maybe some poor shmuck gets lofted up in a balloon or lowered in a basket to take the reading.

Pulsars

Rather than having the magnetic field axis vertical, make it horizontal. If the islands are stationary, your compass only points toward the island if you are on the axis. If you are 90 degrees to the axis, your compass points perpendicular to the island. But, if you also give each of the islands some spin, the compass needle would jump toward the island as the field axis goes past you, then settle into nearly perpendicular the rest of the time, with a very obvious pattern. If each of the islands' spin with a different frequency, you could identify which island(s) you are near based on your compass' frequency response, just like we use the unique frequencies of pulsars to identify them.

Moreover, if the islands are close enough together or the fields strong enough that you are within range of multiple, you can use the direction of any two islands to triangulate your absolute position.

Answer 5

The answer is a resounding "it depends," like many things in worldbuilding, but you'd have to do some work to make it work like you want.

The biggest challenge would be the sensitivity of the compass. Compasses would need to be substantially more sensitive to be effective. The Earth's magnetic field at the surface is in the range of 25,000 nT to 65,000 nT. It's hard to say how strong your island's perturbations are, because they are fantasy, but real life local perturbations are on the order of tens to hundreds of nT. So the signals they provide are substantially weaker. This means your compass will have to do a better job of isolating the signal from other sources of noise. For example, if you are thinking of a traditional liquid-filled compass like might be used by a hiker, the eddies in the liquid will be more important than they are on Earth.