Okay, here's something for all you astronomers, geomancers, and DM/campaign-designers out there to consider. I was looking at our real-world slow, intermittent discovery of planets and moons and was reminded: even though spelljamming societies have the advantage of being able to leave the atmosphere to explore, there have to be a lot of planets that go undiscovered until somebody almost literally flies into them. Before you put a planet "on the map" so to speak you might want to consider some of the factors that go into the probability of discovery.
Somebody once said that space is big. Really big. You may think it's a long walk down to the corner pub but that's peanuts to space. [Okay, it was Douglas Adams, author of Hitchhikers Guide to The Galaxy.] The point I'm trying to make is that even with the ability to fly around space at a million miles a day you won't see everything just because you're closer to it. For example, a planet like Pluto is very small and very far away. Unless a spelljamming ship is within, say, a million miles the chances of discovery are extremely unlikely. That means that you can be well within the same space on the planetary display chart and not see it!
So what do you want to consider in order for a planet to have been discovered?
1) The use of an Arcane Planet Locator as described in the original set. It will not find planets smaller than size B, that is anything smaller than 10 miles. Although this leaves a lot of room for discovering small worlds, I have come to decide that this little gizmo takes all the fun out of exploring. In my campaign it now only tracks those planets that the owner knows of and "programs" into the device.
2) Inner worlds might be easier to discover than outer worlds due to having the ability to observe against the backdrop of the sun of a significant size. The inner portions of a sphere are also more likely to be heavily traveled making it more probably for somebody to simply stumble upon a small planet. Inner worlds also reflect much more of a suns light than outer worlds just because they are closer to it so it's easy for some passing spelljammer to spot a crescent reflection.
3) Size of a planet makes a great deal of difference. A 10,000-mile diameter world is far more likely to be seen than a 100-mile diameter world.
4) The amount of spelljammer traffic in the sphere also factors in. More ships mean more likelihood of stumbling into the unexpected. But remember: space is big - the chances for 1000 spelljammers to discover a world are higher than for 10 spelljammers but not THAT much higher.
5) Time. The amount of time that a sphere has been regularly visited by spelljamming traffic may be the biggest factor and this means that you need to have some concept of history in place in your campaign. If spelljamming has only been around for 100 years or so there will be a LOT of undiscovered worlds even in high-traffic areas. If spelljamming has been around for 1000 years then more worlds will have been discovered, but there may also be worlds that are actually LOST again. New colonies can die out or be destroyed by war or indigenous wildlife and those newly discovered worlds can be easily forgotten. If a world is discovered but not widely documented on charts its existence or its accurate location can be mistaken.
Not every discoverer of a new world is going to want to go blabbing it to the rest of the universe. Being the only one who knows about a planet can be HIGHLY valuable! Unless the discoverer can find no profit to be made or feels he'll never be able to keep the crew quiet about it it's in his best interests not to reveal the secret. Many discoverers are thus likely to die before revealing the discovery (perhaps even dying as a result of exploring their discovery alone). Even if a new planet IS reported it may go unnoticed. This is likely if the initial exploration turns up nothing of great value and it may not be discovered again unless somebody happens upon a dusty ships' log in some forgotten library.
6) Astronomy. Modern astronomy is not just looking through telescopes. I don't know the exact numbers but I'd be willing to wager that well over half the moons and planets of our solar system were discovered by mathematics being applied to data on orbits than by direct observation. The mathematical anomalies in a planets orbit will suggest a moons existence and then it would be verified by observation. That is, by examining a planets orbit the gravity from a small moon or another planet can be deduced. A Spelljammer campaign is not only unlikely to have advanced mathematics like calculus which is necessary for this sort of application but gravity would not make it's presence known in that way. Spelljammer gravity has a very finite, and limited range so it can't be used to explain the orbits of planets.
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