It has been pointed out that Circle Target is ambiguous as written.
A circle is drawn while casting the spell.
Does the spell affect, for its whole Duration, the things that are in the circle when the spell is cast? (Interpretation A)
Or does the spell affect any thing (of an appropriate type) that enters the circle during the Duration? (Interpretation B)
Ring Duration is clear: it affects things that are within the ring at the time of casting, because the Duration lasts until they leave. That would suggest the first interpretation for Circle. On the other hand, wards might suggest the second, because creatures that turn up later are still warded.
I do not have a clear memory of what was intended here, which suggests that we may not have been as consistent about this as we should have been. Thus, I would like people to point me to examples of published spells that are broken on one or other of the interpretations. ("Broken" may be subjective — at this point, if you think it is broken, it qualifies as an example.)
"Be Rid the Tell Tale Smoke" (MoH 28) has already been offered as example that is broken under A, and it does seem to be.
This problem should be cleared up in the errata, but I'm not sure which fix will do the least damage. (Wards can be handwaved as actually affecting the area, not the creatures, whichever way we go, so they are not decisive.)
There is an illustrative joke about that: an agricultural engineer, a physicist and a mathematician are given 10 meters of chain-link fence and are told to compete among them to see which one can fence the largest area. The agricultural engineer makes a square of 2.5 meters per side, saying "ok here it goes, this is a 6.25 square meters area". The physicist come up with a circular fence, bragging "I win, I win, see, a circle with a perimeter of 10 meters have a radius of about 1.6 meters and an area of 7.96 squared meters, and as the circle is the most optimal shape in terms of area versus perimeter I am the winner!, there isn't even any need to see what the mathematician have come up with...". So everybody turns to the mathematician, who is rolling up the fence as hard as he can around his little finger. The physicist ask him: "now what the hell are you doing!?". The mathematicians grins back at him, shakes his trapped fingers and says "see, my little finger is on the outside of the circle, so..."
TL/DR; over a sphere, the inside of a circle can be seen as the outside of a circle. Given that since Herodotus it is known that the earth is pretty much a sphere, I think both interpretations are actually not regarding outside or inside, but if the spell is triggered while not crossed or while crossed.
The problem of inside & outside flip of course is that you get a way to have a spell affect pretty much the whole world in there, so probably something about limiting the maximum size of a circle (to less than a boundary maybe?) except for wards would be fine.
I can't speak for the original designers' intent, but in our games we mostly follow interpretation A, except for wards which prevent the potential targets from crossing the ward. One way to resolve this is that the warded against target was on its side of the circle at casting and cannot cross it.
Overall, I do agree that a Circle should probably be rewritten if there is a rule update.
The issue can be extended to any (ArM5 p.112f) Target.
Is a spell's Target always completely determined at casting time - so that later changes to it do not affect or cancel the spell?
(A1) Does e. g. a man affected by a D: Sun T: Room Curse of Circe still remain a pig when he leaves the room? (A2) Does a man entering that same room after the T: Room Curse of Circe was cast (but within its D: Sun duration) remain a man?
(B1) Does the weather vane on a castle affected by Illusion of the Misplaced Castle remain in the illusion after it got dismounted on the real castle? (B2) Does a weather vane mounted anew onto a castle after that was affected by Illusion of the Misplaced Castle (but within its D: Sun duration) appear on the illusion?
Are there any differences in the properties of a spell's Target, which depend on Technique or Form of the spell?
I am not nearly confident enough in my understanding of the relationship between Ignem, Imagenem, light, and visual species to say if A Time for Slumber (MoH 14) is kosher. It claims to "dim the blinding light of the desert sun", and I have no idea if according to the medieval paradigm that requires it to dim the interior of the circle, or continually dim the light cast by the sun as it enters the circle. Similarly, A Place of Recuperation (same page).
Circle of Undisturbed Rest (ReIm5, MoH30) "prevents iconic and echoic species from leaving the circle"; according to ArM579, "All things constantly give off species", so this is clearly broken under A. It could probably be validly formulated as a ward, if a Rego Imaginem guideline for warding against species existed. If one does, I was unable to find it.
(Circle of Undisturbed Rest is also broken in general because it claims it's based on a "Base 1" guideline, and no Base 1 Rego Imaginem guidelines exist. It also attempts to affect species directly, rather than the process by which they are produced, making it outside the normal bounds of Hermetic Imaginem. It should almost certainly be based off the PeIm5 guideline, destroying its target's ability to affect any four senses, making it PeIm20.)
Circle of Eternal Spring (CrHe4, MoH38) and Grow the Bountiful Feast (CrHe4, MoH75, the exact same spell by a different name) aren't blatantly nonfunctional in either case, but are dramatically more or less useful depending on whether it requires recasting around every bed after every planting season.
Banishment of Warmth (PeIg15, MoH132) explicitly states that it functions according to interpretation A, which might be notable if any of us trusted MoH to set valid precedent.
I don't think it would be an awful reading to allow for either option but that it must be determined at time of invention. Some canon spells work on one assumption and some on the other and both are common in fictional uses of magic.
"Choose at time of invention" would certainly avoid breaking any published spells.
I would need to think about precise wording, though.
A) Every valid target that is within the Circle at the time of casting is affected. The spell continues to affect these targets for its full Duration, even if they leave the circle, and even if the circle is broken. (Note that, if the Duration is Ring, the spell ends completely if any target leaves the circle.)
B) The spell affects any valid target that enters the circle for its duration. Targets that leave the circle are no longer affected by the spell, and if the circle is broken, the spell ends. Note that targets that are destroyed by the spell cannot leave the circle, and damage done by a Perdo spell with a longer duration is not automatically repaired when the spell ends. If the spell has Ring duration, the spell ends completely if any target leaves the circle.
When you say "if the Duration is Ring, the spell ends completely if any target leaves the circle" do you mean if 5 people are inside a Ring spell and then only 1 of them leaves, the spell ends on all 5 of them, or just that one guy?
If a ring/circle spell with the virtue from Societates p.113 uses a room as the ring/circle to make the air fresh (like Chamber of Spring Breezes) does that just not work because if any of the air leaves the room the spell ends? Or a ring/circle on a barrel to turn all the water inside to steam just stops as soon as any steam blows out of the ring/circle?
Otherwise they read as the two versions clearly to me, but I'm a circle guy so I know what the difference is supposed to be so I'm not really able to say if its perfectly clear to everyone.
Only slight tangent, defining a circle in 3D space would be nice. Does it count a sphere? A plane? If I put a circle around a tree, do I need to worry about subterranean roots, or high branches? How about a circular mirror?
And what counts as breaking the circle? Drawing a line in chalk across a silver inlaid circle? Laying an iron bar across it? Or someone stepping across a line?
I guess you're right that there is no particular need to mention the virtue as you can just make any ring do the things I said. The fresh air example was meant to be a MuAu version that makes the air in the ring fresh, sorry for any confusion caused by my like of clarity.
The core question still remains. Does one member/part of a thing leaving the ring end the spell entirely, or just on that member/part?
For D:Ring, yes, explicitly, if any target or part of the target leaves the circle, the spell ends entirely on all of them. For T:Circle, it ends if the circle is physically broken, or if the duration ends (so Ring/Circle spells end entirely if any target leaves the circle, because that ends the duration, but e.g. Sun/Circle spells might not).