If we look at Sacret Geometry(electromagnetic fields) the Season /Month Pillar seems to be the second set of Geometry around a person/place.
So while the Day Master, is in space around the person(etheric body etc) the next set of figures is the Month/Season.
And that happens to be what they can see most clearly, as the fourth(year) set of figures they have to look through the Month to see.
In space, is more or less 30-50 cm around the person in all directions. Again having Cube, Sphere and all other shapes.
In our research we stumbled upon the understanding that all of the figures looked in bazi are coming from the middle Dan Dien(the heart basically).
While all related to TCM and Feng Shui come from the lower one.
And since the Month Pillar/Third Set of figure is the one that they immediately see its suggesting all in there has Images of larger/more. Be that cultural traditions, beliefs, nearby peoples impact, family etc. All carry the idea of more visible, larger and deeply impactful.
Here is metatrons cube on 2 levels(Month and Day lets say).

random image from the web as usual.
So we have the outside 8 spheres as the Month. Then inside 8 as the Day. There are 8 more outside of that in the same way and 8 in the most inner level as well last 16 not visible here.
And it makes sense. What happens in the city around us, in our house etc. is always more impactful then what happens in the other hemisphere.
Since that set of figures it close together to Day set, it impacts more directly the person then the Year may be, depending on what exactly is there.
For some reason the figures doesn't invert depending on hemisphere it seems. Or if they do, there is another change that looks like negates it.
So the light of the Sun that is received during the Day is not the base for the Month Pillar.
Suggesting the Month start and end also may not be depending on the position of the Sun. However, it also may be, as there are other factors coming with it(for example, while the "amount" of light may not matter, the relative direction and part of the movement its received from may be it).
Overall, as most questions like that the answer, in my view, will still be - more research is needed.
So far it seems it doesn't change stuff. As the patterns may be fixed for the whole Earth. Or maybe, because it could be viewed the same way from any direction, and while it may be more accurate with adjustment, it would still be valid without. Will need to know more details on each movement to be able to narrow down when exactly the month changes and why.
Currently we do use 30 degrees of apperent ecliptical longitude of the Sun and the day that ends near it. And if that is it then the part of the sky the Sun is at may be the important part, not the amount of light(what is measured by season is the amount of light each day).
I agree with that as possibility. As it seems early heaven measuring the amount of light. Later heaven - something else.