Our Bonisagus has discovered that her Magical Animal Companion has a power that lasts for what she thought was a Diameter duration. But careful timing shows it is a couple of Rounds different from the standard Diameter duration.
It is in fact the length of time it takes for a Full Moon to cross the sky by the amount of its width.
But none of us players know if a Full Moon diameter lasts more or less than a Sun Diameter duration. Can anyone help?
Using a popular search engine with the query" how long does it take the moon to move its diameter across the sky" gives me the answer that the moon is approximately half a degree. The moon takes 27.3 days to complete one orbit of the earth with respect to the stars (a sidereal month) so it takes 54.5 minutes for the moon to move its diameter against the stars.
However, that's sidereal motion against the stars. Looking at a different answer, if you're just after the moon moving a diameter through the sky as we rotate, the moon rises every 24 hours 50 minutes. The sun and full moon are virtually identical in diameter (hence why eclipses are possible and so spectacular). If the moon takes 24 hours 50minutes between moonrises and the sun takes less from sunrise to sunrise then the time for a lunar diameter will be slightly longer than a solar diameter.
The mean diameter of the moon is very close to the sun's (as seen from earth) but the actual value varies from day to day and year to year. There is about a 14% difference in the visible diameter from the smallest (a micromoon) to the largest (a supermoon). So arguably a lunar diameter is the same as regular solar diameter but with a variance of 14% (~3 rounds).
Thankyou Darkwing,
I thought the Moon went across the sky slower than the Sun, but looking at eclipse videos, it kind of looked like the Moon caught up and overtook the Sun.
See based on this and the paucity of quality astronomic measurement tools of the period, there might not be a difference in the eyes of the common man, and by extension, the two durations would be the same.
Or it could be something for a Bonisagus to research 
You could take it either way. The variance (14% larger and 30% brighter at the perigee) is definitely noticeable to the naked eye but it's also hard to be sure of that without a way to measure luminosity or photographs. Ditto for getting measurements accurate enough for 14% to be verifiable.
Personally I'd say that the variance does exist in Mythic Europe. Even if the common man doesn't know about it the knowledge of the lunar anomaly exists, from Hipparchus and Ptolemy etc, so astronomers would be aware of the concept. They just wouldn't have a way to properly measure/calculate how it will vary (but magi might be able to...).
Here's an idea: a magical photometer. Enchant a greater item, give it an intellego ignem effect for detecting the brightness of light. Use this as a linked trigger to some other effect, say one that generates a musical note, with the trigger being "if the light is brighter than x", with x being a value we expect to be somewhere in the range of the moon between brightest and dimmest. build an open roofed movable structure placed so that the moon will pass directly over it (so it is illuminated only by moonlight).
By doing this over a multitude of nights we can prove if the moon varies in brightness based on whether it activates or not (as sometimes it will be too dim and nothing happens, sometimes it should be bright enough and it activates). Optionally a lens could be used to focus the moonlight if we have accidentally made the item not sensitive enough.
Although that raises questions of whether moonlight is part of the lunar sphere, in which case hermetic magic can't effect it...
Once the light has left the moon, the limits of the lunar sphere shouldn't be a factor I think - at least insofar as Intellego magics go. Though if the rules for Dies Irae are in use, moonlight might potentially fall under Aether rather than Ignem which could prove an obstacle...
not sure than taking into account modern astronomic measurement is the good way to evaluate situation inside mythic europa.
Still, starting from here, as said before, moon doesn't always has the same size, depending on actual distance from earth (aphelion and perihelion, each happening once per lunar month)
same goes between sun and earth (each happening once per year, and not intuitively for northern hemisphere inhabitant, the sun is smallest in jully)
rotation from moon around earth continuously slow down the visual move of moon inside the sky by about... 3.5% (24h50min versus 24h)
moon size change by about 6% up and down starting from mean size
sun size change by about 1.75% up and down starting from mean size
So there's not one definite answer when comparing moon diameter and sun based diameter, if not than "on average" lunar diameter should be a bit longer