Length of Momentary

I've seen this question come up recently as well as witnessing some confusion related to it, mostly around how MuVi works. Just how long can a Momentary spell last? As a basic answer, long enough to complete its effect, but not very long. That's pretty vague, though. In some cases, it certainly may be a very small fraction of a round, noticeably less than a second. But what about the upper end? While people did discuss this quite a few years ago in a thread, that was also prior to some changes to MuVi, so I hope to add some clarity without doing threadomancy.

First we should note that a round is approximately 6 s. Though seemingly always buried, this is stated many times across a number of books.

Now, let's look at some example spells. In the core book we have:

Dust to Dust - It takes two rounds to finish its effect. I've heard comments about the spell lasting one round and then natural decay happens for one round. Anyone who has any experience with things decaying knows that even with only 1% remaining to decay, that 1% will still not decay to dust in only 6 seconds. Even with far less than this, say one eyelash remaining, it will still not decay to dust in only 6 seconds naturally. So we know for sure this isn't natural decay, and the spell lasts about two rounds.

Gift of the Frog's Legs - The leap must be made within 10 seconds, so a little shy of two rounds.

Curse of the Rotted Wood - With smaller things this can take just a fraction of a round. But with the biggest sizes it affects, it takes up to two rounds for the same effect. So we know this can last up to two rounds.

Heat of the Searing Forge - It does damage across three rounds. I've heard comments about that being natural cooling. Anyone who has worked with hot metal knows that the metal will not cool from too hot to handle with that scale of damage to perfectly safe to handle without any harm in 12 seconds. So we know for sure this really isn't natural cooling. Now, how much of the first round remains is still in question, so this lasts a little over two rounds.

Crest of the Earth Wave - It travels at 50 paces/round and can reach 50 paces with a shout, so a full round.

All of MuVi - The default (not fast-casting) to use MuVi on your own spells is to cast it one round and the spell to be affected the next round. The MuVi spell's duration must fully encompass the casting of the second spell, which theoretically could end a little over a round after the MuVi spell has been cast. So just over a round.

So we can see some consistency. A little over a round is OK, so long as it's not much more and the magic is just finishing its relatively immediate effect (the next jump, the next spell, finish decaying, finish cooling down, etc.).

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Round length came up in another thread today or yesterday, and Callen was able to quote three 5ed books all having it pinned down to 6s.

The problem I have with this, is that it is too precise for the narrative style that ArM encourages. For reference, [ArM4:162] has «The amount of time that passes during a round varies between 5 and 20 seconds depending on the activities being performed.» A round is the period it takes to take one action.

The key point is that it does not really matter for the story if the round is 5 seconds or 20, so why care?

And is that not also the case for the question at hand? Why does the duration of a momentary duration matter? Momentary means that the spell does not sustain an effect. It may take time to create the effect, but once the spell is complete, it is over.

Take Conjure the Mystic Tower as an example. It is momentary. But the spell takes 1h45 to cast. Surely there must be some spell power building up gradually, rather than everything happen in the last round of the 1h45. But does it matter to the story?

Logically, the MuVi spells do not last a round. They only take effect when the target spell is cast, and are momentary. When the other spell is cast, the effect. Except that if you use, say Wizards Communion on Conjure the Mystic Tower, the communion lasts, in some sense, for the 1h45 it takes to conjure the tower. But then we do not use round sequencing when ritual spells are cast, so we are still talking about one action.

I think we are down to momentary last only one action, whatever one action is in the current mode of narration. That's simple and consistent. Let's leave seconds for the physicists. They tends to take the magic out of the story.


Yes, an ArM4 reference. Seems like they found the need for a little more specificity when writing ArM5. Perhaps because problems arose from not being specific enough before?

Really? Read the Slow Caster Flaw. Consider fast casting. We have single actions that do not take one round. There are more.

For example, can you use a Momentary MuVi spell on your Formulaic spell if you have the Slow Caster Flaw? There are times when these decisions have specific interactions with rules where it matters.

Why does the MuVi spell not last the round? Its duration must last long enough to encompass the entirety of the casting of the spell to be affected. They finish taking effect when the other spell is cast, rather than waiting until then to take effect.

This is why you cannot use Wizard's Communion for Conjuring the Mystic Tower. Wizard's Communion doesn't last long enough. You need a version of it with a sufficiently long duration to cover that 1 h 45 min. Usually Sun is used, though you could risk Concentration if you want.

Maybe, but if that was their objective, they botched it. There is still enough of that narrative approximation legacy in 5ed, that either you need to ignore such questions as you bring up, or you need to rewrite your own house rules.

That was still the 4ed reference.

That was a typo on my part, I meant to write that it does not last more than one round. Not until the next round, it starts on the next round, when the target spell is cast.

This is not entirely clear in RAW, I think. Unlike most other MuVi spells which target the magical effect which they have to outlast, the communion seems to target the casting.

Delaying a spell like that normally requires a Virtue or a ReVi effect.

No, those MuVi spells do not need to outlast the effect. They need to last through the casting of the other spell. I think it's pretty clear:

Muto Vim Guidelines (p. 159): Replace the final paragraph with the following: "Muto Vim spells work by altering the magical energies that create the spell as it is being cast. The spell is the result of the combination of the base casting and the Muto Vim effect, and has its effect once both the casting and the Muto Vim effect have finished. This means that a Muto Vim spell must have a duration at least as long as the casting of the target spell, but need not last for as long as the spell itself. For normally-cast formulaic spells, a Momentary Duration is sufficient, but if the casting time is longer for any reason, the Muto Vim spell must also have a longer Duration; Sun is sufficient for any practical Ritual."

I think you're using old rules. It was found that there were problems and contradictions, so the MuVi rules were corrected. That's why there are Sun-Duration variants to Wizard's Communion published in later books.

Normally. Here it is an inherent part of the spell guidelines.

[ArM5:159]: Thus, its duration should normally be at
least as long as the spell that is its target.

This is the same as you quote., It has to last as long as the spell, not the casting, is what it says. You are right, I do not know all the corrections which are scattered around the supplements, but then I consider that a futile exercise anyway. If I cannot rely on the core book for core rules, the game is not playable.

No, this is not the same as what I'm quoting. They acknowledged an error and fixed it. You're utterly ignoring that this is an acknowledged error that has been fixed with an edit to the core rules.

You are right. I missed the change in your quote.