I have been contemplating aerial combat in Ars Magica. Think I may have found an oddity - at different altitudes, the horizon is further away, so sunrise and sunset are at slightly different times.
Which means Sun duration spells and Parma Magica end at different times for someone 1000ft above someone on the ground. So theoretically some mage might be able to take advantage of the time difference, no?
It is best - and most logical for Hermetic magi following Bonisagus' Magic Theory - to use astronomical sunrise and sunset on Earth in ArM5 for D: Sun.
D: Sun spells cast in a cave where the sun never rises would otherwise never expire.
I totally agree that one has to ignore the surrounding topology, which resolves the cave issue. Easily achievable permanent Sun spells would be ridiculous. The long Sun durations North of the polar circle may be disturbing enough.
Once that is settled, there are at least three remaining issues, where astronomical sunset/rise only resolves the matter of altitude.
If you are worried about altitude, you should also be worried about opponents at sight range, which, across open valleys, could easily be far enough to make one combat round difference in astronomical sunset. Enough to give someone an edge.
If you narrate at that level of accuracy, you need to be very particular about the exact expiry and renewal of Parma Magica.
Secondly, when you cast spells at AC range and Sun duration. Is it the caster's or the target's location that determines the duration? Same issue arises, of course, if the target moves during the day. Does the spell expire at sunset/-rise at the original location or the location at expiry?
Personally, I rely on limited narrative accuracy to make most such questions moot.
Yep. Most magi don't have precision clocks aligned to a TMRE p.51ff Magical Armillary Sphere. Even if one does, the right situation for making it relevant remains an SG prerogative anyway.
there is the fact that mountain tops are illuminated by sunset after valley bottoms no longer see sun. But I hadn't thought about underground sunset before.
Odd
There are more tradeoffs as well, higher altitude will always mean longer days and shorter nights if you follow this principle, but it also means that in the morning your parma magica will drop before the people below you, and if they are at lower altitude and to the west this could be quite significant. A magus in provence conducting a target-sunrise casting with an AC might hope to catch a magus in Thebes sleeping and have hours more parma before needing to be renewed.
It is not the sun what ends Sun durations: otherwise, as you guys pointed out above, underground D: Sun spells wouldn't expire.
So, if it is not the sun, actions that expose (or cover) the sun earlier or later don't need to make any difference.
2nd So, then what can it be?
I'd always figured it out as a some kind of mystical wave cruising from east to west following the turning of the world. So a flying magus would have his spells ended at the same time than another magus right below him. I particular all inhabitants of a tower would see their D: Sun spells ending at the same time no matter how high the tower is.
Then I'd say the target gets his sunset
Indeed. Also a magus teleporting from Thebes to Provence can have a D: Sun spell lasting longer. Or drastically ending if he teleports in the morning, where it's still night time in Provence.
Which now that I think of it seems like a sneaky way to pull teleporting spells; maybe you need that mystical wave mumbo jumbo to happen to have a smooth transition between day and night. Maybe if you skip it and suddenly move from day to night then you suffer some odd consequences, maybe even having a dangerous chance to get into Twilight.
To me it is equally logical to use the sunset/-rise just below him as it would be to calculate a hypothetical sunset/-rise for a larger (perfectly spherical) Earth putting the magus on the surface.
Obstruction and altitude are two separate issues. We need to work with an idealised model, without obstruction, lest clever wizards would cheat the celestial bodies. However, there are idealised models to define sunset/-rise at different points in space, also off the surface.
The world is turning? Burn the blasphemer!
What about the fact that the time for sunrise/sunset on a given day can vary a lot depending on how far north you are located? Sunrise in Germany isn't at the same time as sunrise in Italy.
Not to mention what happens if you travel far enough north, past the polecircle.
Whether the sun goes round the earth or th earth spins sunset and sunrise can still be defined in geometric terms of the interaction of th two spheres such the altitude would matter but blocking the sun's rays would not.
I think local conditions in terms of latitude can definitely affect spells. I figured it was intuitively understood that spells with astrological durations (such as diameter, sun and moon) derive their durations by the passage of those bodies through the Heavenly sphere above.
It's the change of their positions that affects when a spell ends, so as above- so below. If you move yourself further north or south, the duration of day changes. The world is spherical and dawn (or sunset) appear depending on the positioning of the sun and earth, so the astrological sympathies can change as well- it's the context of the relationship between your position, earth's position and the sun's position that determines when the spell ends. I don't think it's the mere presence (or absence) of the sun's light, or hermetic magic could end or extend such spells in that way.
Changes in altitude feel slightly different, since the earth is still experiencing dawn or dusk at your terrestrial location at the same time. This might extend your spell, but it seems slightly unstable? I'd probably allow it, since your actions become locationally limited then. The overall benefit is, perhaps, several additional minutes of day time?
Perhaps this is an advantage of putting a castle high in the sky, or living on mountains.
Moving to a point with a polar day seems perfectly reasonable, if you want. There's already a faerie god in Lands of the Nile who can make sun duration effects (including the Parma magica) end by causing the sun within his regio to set, since he fixes its location.
Am not sure the Earth is round in Mythic Europe. It might just be flat.
The Earth has been known to be round since the Greek heyday, so a flat Earth Mythic Europe sounds like overplaying a faerie conspiracy.
But you know, YSMV.
The Earth is known to be round indeed. Eclipses were known to happen at different times depending of the longitude of the place were you were. Eclipses were also already thought about as the Earth and the Moon casting shadows over each other, which also suggested that they were spherical. Even Eratosthenes measured the size of the Earth a thousand and a half years ago (and just because of that he is one of my favourite classical greeks). So well, maybe some peasants here and there could think that the Earth is flat but anyone with a at least 1 in Artes Liberales knows it is round.
That's just because the Earth is tilted. During the summer places in the north will have earlier sunsets and sunrises, and if you travel north far enough a day can last half a year. There are many circunstancial problems traveling far north to make that advantage of Sun spells useful, anyway.
Would it keep the inquisitors at bay if I replace "turning" with "tumbling"? 
Hmm. Earth's precession was observed in antiquity, but interpreted as a movement of the celestial sphere.
An important treatise on trepidation, perhaps written by Thabit ibn Qurra, has come to us as De motu octavae sphaerae.
Asking a Church inquisitor about trepidation might rather lead to an explanation of the kind of clutter (latin: trepidatio) manifest divine truth leads hardened heretics into. 
That the Earth is tilted was not known back then though. Nor that the Earth traveled around the sun.
They used a geo-centric model where the Sun traveled around the Earth in a tilted orbit.
That model gives close to the same results as our helio-centric model though.
Of course it does. It is an arbitrary choice of co-ordinate system.
I am not sure what you mean by not knowing that the Earth is tilted. Obviously, if we define Earth as our point of reference, it is untilted by definition. It would be the Sun's trajectory that would be tilted. Surely they must have noticed that the Sun takes different trajectories depending on the time of year, which is the practical and objective feature.