A discussion came up with another group of players today; while our troupe had always assumed spell durations to work in one way, and we had never even thought another interpretation was possible, well, some clarifications are apparently needed.
Suppose one afternoon a magus casts a MuCo spell, with Target: Circle and Duration: Sun, that turns people into frogs.
Would only the people that are in the Circle at the time of the casting be affected, or would anyone who entered the Circle before the next sunset?
What if the Target was Room?
In each case, would the frogs turn back into humans as soon as they left the Room, or would they remain frogs until the next sunset no matter where they are?
People inside the circle turn to frogs. They remain frogs until sunset even if they leave the circle. We play that people entering the circle later on do not turn to frogs, but I can see that the opposite can be defended as well.
I've always thought that when a spell affects an area, it affects everything in that area, including things that enter the area, until the things leave that area, up to the duration of the spell. Because the Target is everything in the area, anything that enters the area during the duration of the spell becomes part of the target, and anything that leaves the area is no longer part of the Target and is thus no longer affected. Whereas spells that target specific individuals and groups continue to affect them until the spell expires, regardless of where the targets go.
I thought that was part of the point of the differentiation between Group/Room.
I think I agree with Xavi's answer. If the target were Group, then a member of the targeted group wouldn't stop being a frog just by walking away from the other victims, nor would a newcomer suddenly become a frog later that day when arriving to investigate the incident. By analogy, only targets within the Circle or Room at the time of casting should be affected. (I see ETC disagrees though.)
(On the other hand, in ArM5 there is that bit about PeTe spells with greater than Momentary Duration continually destroying dirt that tries to fill the magical hole....)
Again if the Circle still unbroken or drawn, any other that come in it is transformed by the duration.
The question is clear on Circle and Ring duration, but some difficult to another durations.
Keep in mind that some Targets can be used without Arcane Conections, Room by example.
I don't necessarily disagree, that's just how I've always seen it played and had it explained to me (I've never been a SG for a stock Ars Magica game). I could be wrong. Despite having played this game for a long time (better than 15 years), it continually surprises me in many ways.
Ah, but the Target: Group description specifically mentions that Group "membership" is determined at the time of the casting. None of the other "area" Target description does. That can be seen as an indication that Group is the exception to the rule.
And also check out, say, the Mentem spells Poisoning the Will and The Shrouded Glen. Both are Target: Boundary, and both clearly affect people who enter the targeted boundary after the spell is cast (the description of the first spell says so explicitly - even saying that people who leave the boundary after the spell is cast on them stop being affected - and the second spell makes no sense if not). So maybe Circle or Room (or Structure) is analogous to that...?
I think it ends up to "whatevwer the spell is supposed to do". It happenms where there are contradicitons like this; the same parameters can be used for several things. So, these parameters can be used to design a spell that turns everybody stepping into the circle into a frog for the duration, or to only affect those within the circle at the time of casting. Never played the first option myself.
I also think this to be the "best" interpretation.
So, anything within the Circle is affected until end of Duration, anything that enters, is affected the same way, anything that leaves the Circle is no longer affected because THEY were not the Target of the spell, the Circle was and if they´re no longer within the Circle, they´re not affected.
I disagree for the simple reason of the problem with how you could then state that a if you cast a Circle/Ring effect, then the affected can leave the Circle and retain the effect as long as the Circle is whole.
You just created an almost indestructible(cant dispel something that you dont know where it is), semipermanent and completely portable(the spell will still be in effect even if the affected person is literally on the other side of earth) spellcasting method.
Well, if you do a Sun/Circle you certainly can leave the Circle. But since Ring duration "lasts ... until ... moves outside ...", there is no such problem.
With the restrictive interpretation, most magi can ceremonial spont lamps, ovens and fountains. With the lax interpretation, think of all the silly useful spells that become accessible : shower, depilatory zone, dishwasher, cloth dryer, animal tranquillizer, fast-growth plant nursery. Magic as technology.
Or let's warp to the future as seen from the 60s... Step in this circle every morning : it will clean, shave and comb you. Step in this other circle : your clothes will be cleaned, mended and ironed afresh. Those rings can be done in hour-long ceremonials accessible to non-specialists.
Well sorry for that, the problem is still there, just pick Moon Duration instead. Or worse still, do a Permanent Ritual for Boosting Characteristics... NOW what you get is a Circle that, as long as its intact will permanently raise the characteristics of anyone who goes into the Circle and then out again.
One Circle for each characteristic based on +1 up to 5 and anyone can run into and out of the circles as many times as needed to max all stats. Great idea.
And the above ("lasts ... until ... moves outside ...") is yet another reason to apply the same on Circle.
Which is extremely boring. Circle and Ring spells are already too easy to exploit.
Eh, in what way would the above examples not be possible regardless which interpretation is used?
If I understand correctly, your issue is with the 2 extra magnitudes of Circle vs Group. Because all you say is already RAW-kosher for Individual and RAW-acceptable for "within Circle at the time of casting".
Oh hell, a Diameter Ritual Circle of CrMe70 designed for you. You have 2 minutes to run in/out until you get +5 Intelligence. 14 pawns of vis instead of 45, and anyone else can dip into the circle at the same time. Perfect proof this is silly.
Yeah, you need this if you want Circle to activate whenever someone steps inside. I prefer to have Circle "at the time of casting" and Ring "until moves outside" because of the symmetry.
The restrictive interpretation requires you to be inside the Circle at the time of casting, you'd have to cast it every morning. The lax interpretation allows you to do it once and step inside every morning.
If you want a really restrictive interpretation, you can rule that a valid target leaving or entering the circle constitute a breach of the circle and therefore ends the spell entirely. By valid target, I mean that if the spell uses the Corpus technique, animals or water crossing it won't break it (unless they physically break the actual circle, of course), but people will. Which certainly stops most silliness, but also makes such spells less powerful. After all, I don't remember 'breaking the circle' ever been clearly defined, and you can find examples in literature (fantasy, obviously) of a circle been broken by someone stepping inside.
Personally, I do interpret the Circle guideline to mean that only things inside the circle at the time the spell is cast can be affected. If Ring duration is also used, it works until the target exits the circle. Otherwise it works for the normal duration, even if the target does leave the circle, as long as the circle remains unbroken - whatever that may mean, exactly. That also means that aside from wards with their specific rules, circles spells cannot directly be used as traps unless also using a Watching Ward spell. But you could use a circle/ring spell to create an illusory floor over a pit