I don't follow. How does this difference allow Hermetic magic to directly affect the moon, sun, or stars? Seeing their positions in the sky and having them affect Hermetic magic is a basic premise of the durations Diameter, Sun, Moon, and Year. But those don't break this limit.
Chris
It's simple. The paragraph summarizes the Ptolemaic model as base of Ars Magica's cosmology: "The lunar sphere is the innermost of the celestial spheres, which carry the stars and planets on their revolutions around the earth, ...".
Cheers
What is there to prevent him from creating the ritual with the small change that his access and control of access is only temporary? AFAIK, nothing what so ever. This is not a change in parameters. This is the kind of minimal modifications usually considered to be part of the design process.
To that, I would add that i think a lot of covenants would be willing to also pay for the additional level of protection from a higher Aegis. Ie., the profit per Aegis would likely be more than just 2 Vis. So the pay would probably at minimum be based on the actual Aegis level. And it would be those covenants unable to cast a high level one themselves that would use the service(or those rich/lazy enough to not want to do it themself).
I expect the cost for a level 60 would somewhere in the 10 to 25 Vis range depending on overall supply and order situation in general.
And about Penetration, why the assumption that any use of WC would have to be with apprentices? I would expect the Mercurians to have some semblance of, if not cohesion, at least communication, which means this service could also be handled by a small group (2-5) of Mercurian magi, as long as one of them has the AC needed to get to the client covenants, it should be very workable.
Haven´t read the entire thread so hopefully not doubling up on something already said.
If nobody specializes in rego vim it is quite normal for you to have a not-that-great aegis. As DW points outwillingness to pay comes to the fore there 
If you expect trouble that is. We have had a level 25 aergis during whole sagas, even ones where supernatural enemies knocked on our door regularly, and it was OK. Big stuff you take out the artillery. Stuff at Might 25 or less (pests, according to ArM5 RAW) is unable to pester you. Enough for us. But if you want extra protection it would be nice, yup.
Sorry, my mistake. But vagrancy isn't that either. It's not having a House which is what I confused with not having a covenant. (It's in HoH:TL at page 49.) The article also adds that you can be charged with vagrancy for not residing in a Tribunal or not complying with a Tribunal's residency requirements. There may be tribubals who say you have to have a covenant. I'm almost sure there are but my memory is clearly not up to the task of retaining Hermetic Law clearly.
Still, I wouldn't trust the casting of my covenant's Aegis to any outsider. Pure paranoia on my part.
To me, this sounds like an appeal for a troupe decision.
When deciding, the troupe should also consider that such 'minimal modifications' on a spell requiring a major breakthrough to be invented with "different parameters" (ArM5 p. 161) could be manifold: if the caster's rights can be made to expire after some time that way, those of everybody else participating at the ritual can as well. Hence allowing such 'minimal modifications' will not increase the level of trust for an outside Aegis contractor, but rather reduce it considerably, as long as his customers are not able to check the features of the cast Aegis independently.
Cheers
Oh, I thought you were saying magic could now break the lunar limit. But I still don't follow. Why is it the Firmamento could not rotate at a slightly different angular rate than Sol? I thought this was part of the system. I get that the Ptolemaic model has problems, which is why it will be challenged and then replaced in a few hundred years. I'm just not sure why different rotation rate between the different spheres would do that. I thought the main thing with the shift to the Copernican model was dealing with epicycles and retrograde motion.
No such assumption. There were calculations showing how it might done with apprentices if a magus didn't want to share the profits. I could certainly imagine a group of Mercurians who cast WC for each other for a small share of the pay, which might only amount to 1 p.v.f. after a few castings.
Have to agree with One Shot here. Whoa! How'd that happen? 
Each troupe should agree what "different parameters" means? I have been tempted to start a thread with voting on that to get an idea where opinions lie. For example, with my pen-and-paper (can I really call it that if we all use laptops now?) troupe we felt the normal size of Boundary was too small for many covenants, so we figured a extra magnitudes for size were OK. So our decision was that Boundary could not be changed but that size could be just like strength. In the last saga that didn't last too long we were working on a huge boundary because the strong part of the Aura rotated seasonally and we didn't want to cast 4 Aegis rituals a year, both for Vis and time issues.
Chris
Chris
Problems with the geocentric and ptolemaic models were known since their definitions, and in consequence many variations have been proposed. For some early examples look up K. F. Johansen, H. Rosenmeier, A History of Ancient Philosophy: From the Beginnings to Augustine (1998) - or for something easier accessible en.wikipedia.org/wiki/Geocentric_model .
The standard ptolemaic model at the base of Ars Magica cosmology is presented in A&A p.16ff. It has an unmoving earth at the center of the fixed star sphere, which moves from east to west around the axis through the earth's poles, and in itself is unchangeable. It also provides the base rotation for the spheres in which moon, sun and the planets move - which we understand as the rotation of the earth. The sun performs a very simple movement with constant speed along the ecliptic, whose axis is inclined by 23.4 degrees against the axis through the earth poles.
This model does not leave room for a difference between the sidereal year and solar year. I try to give the explanation in utterly simple terms now: since the center of the earth is also the center of the fixed star sphere, both do not move against each other - so the movement of the sun observed/measured with respect to the fixed star sphere is the same as the movement of the sun observed/measured with respect to the center of the earth. To account for any difference in observation between sidereal and solar year in the context of the standard ptolemaic model, the stars on the fixed star sphere would have to move against each other on that sphere.
Cheers
This does not work out mathematically. Let's simplify it. You have two concentric, coplanar circles. Mark their common center "E." Mark a point on the outer circle "F." Mark a point on the inner circle "S." E is the center of the outer circle and both do not move against each other. Now let F (and all other points you care to mark) move around the outer circle at a constant angular speed. Let S move around its circle at a constant angular speed very slightly different than F's angular speed. Then when S completes exactly one circle F will not be in exactly the same place. (The same is true if we adjust S so that it does not remain coplanar, as long as its angular velocity in that plane does not match the angular velocity of F in the plane.) This will account for the difference between a sidereal year and a solar year. Thus the conditions that "the center of the earth is also the center of the fixed star sphere, [and] both do not move against each other" do not force a sidereal year and a solar year to be equal.
Chris
No, this has nothing to do with the difference between a sidereal and a solar year at all. In the standard ptolemaic model it just means, that years (the time intervals of the sun circling the earth along the ecliptic) are not aligned to days (the time intervals of the fixed star sphere rotating over the earth).
Perhaps you look up the definitions of sidereal year and solar year some time. Take en.wikipedia.org/wiki/Tropical_year :
Or - still better - go to your teacher. I am rather tired of explaining the obvious on a forum not made for astronomy of the ancients, and others may be as well. 
The difference between sidereal and tropical / solar year is caused by precession of the rotational axis of the earth. It was known to ancient astronomers before Ptolemy, and to Ptolemy, as verry slow movement of the ecliptic against the fixed stars. See en.wikipedia.org/wiki/Axial_prec ... tronomy%29 for an overview. It is also treated in Ptolemy's Almagest - but is not reflected in the ptolemaic model described in ArM p.16ff, and cannot be shown on an armillary sphere either. The reason is, that for the standard ptolemaic model at the base of Ars Magica or an armillary sphere the rotational axis of the fixed star sphere does not 'wobble' against the ecliptic.
As usual with medieval Aristotelian physics, we are saddled here with an approximation - not even accounting for the full knowledge of ancient scientists - to be taken as the complete truth. How Aristotle himself and his followers accounted for lack of precision in their theories is still an object of research, though. See plato.stanford.edu/entries/arist ... thematics/ :
Of course this is a boon for many, and in particular for those few who do not even follow up on astronomical definitions before discussing astronomy. 
Cheers
As fascinating as the whole discusion about sidereal vs. tropical year is, and we can open a similar can of worms for duration Moon, shouldn't we propose to David Chart following errata:
Moon: The spell lasts until the first sunrise after both the full and new (first visible crescent) moon have set.
Year: The spell lasts until the first sunrise after the fourth equinox or solstice following its casting.
Best Regards,
adumbratus
Edit: inserted definition of new moon.
+1
Your rephrased definition of duration Year: would provide clarity, and help non-native speakers understand it at once.
And I can appreciate the symmetry in duration Moon:. I would suggest trial as a house rule for a month or three before proposing it as a general rules change / errata.
Cheers
Let's get this straight indeed!
David Chart did post on the Berklist way back that by RAW the Aegis has to penetrate.
More precisely, he posted something like this. Aegis initially was meant to require no penetration, as that would make it "closest" to the Aegis of previous editions. However, no one remembered to add the explicit exception to the RAW. Since in playtest the Aegis worked well enough even with penetration required, it stayed that way and now, by RAW, it needs to penetrate like every other Hermetic spell, even with non-Hermetic origins, for which no explicit exception is made.
I'd be also much in favour of this wording.
Ideally, this would be accompanied by a similar clarification for Sun duration, something like:
Sun: The spell lasts until the end of the next sunrise or sunset. A sunrise ends when the sun is completely above the horizon. A sunset ends when the sun has completely disappeared below the horizon.
While I realize it's not exactly what the OP was about, I think it's sufficiently close that I think it fits this thread.
What about a Muto Vim specialist with Faerie Magic who offered to cast, in conjunction with a covenant's Aegis, a Muto Vim spell that changes the duration from Year to Until:(Condition)? A fitting Condition would be that a specifically designated gem, the Hearthstone, be broken or carried out of the Aegis' Boundary.
The spell would use the Significant change guideline, and thus would affect any Aegis up to (Level of the Spell - 15) at Range:Voice and Duration:Sun. Thus, it could boost an Aegis up to level 35 without being itself a Ritual. But even for a more powerful Aegis, it would be worth the cost: cast the Aegis and the Muto Vim spell once, and the Aegis lasts until the Condition is broken or the (main) caster of the Aegis dies or enters Twilight -- many years, possibly decades.
How about D: Aura instead?
But no, there is actually a problem with an eternal Aegis:
New members will not have participated in casting it and will be hindered.
This can be handled via tokens and invitations ofcourse, but would you feel that you were truely a member of a covenant where you were not included in the Aegis?
Even worse, should all of the original casters die, no new invitations could be extended.
That would also work, as would D:Hidden. Both require a very specific Inner Mystery, rather than just Faerie magic, however.
Also, Aura has the problem that you have no way of interrupting it if needed (well, you can always bring it down with a PeVi spell if needed).
The idea is not so much to make the Aegis eternal, as to make it last a decade or two. This reduces the actual cost (and risk of botches etc.) by an order of magnitude. As for invitations, it would seem reasonable that, if you join an established covenant, you may remain a "probationary member" for, say, a decade or two, before finally gaining the right to participate in the ritual.
I just re-read Until (Condition) and I like it alot!
Especially since it cannot be dispelled before time (unless that's part of the condition) and lasts potentially as long as needed (I could have sworn Until (Condition) had a max of 1 year, but appearantly not).
Very nice.
That might not be possible, because Muto Vim explicitly only works with Hermetic magic, and Faerie magic is not technically Hermetic.
However, you could do something similar with Rego Vim --- using the "sustain a spell" general guideline.
Uh? Faerie Magic (the Outer Mystery of House Merinita) is Hermetic.