Salvete Sodales!
I just have a simple question regarding nativity horoscopes. If I know a person's time and place of birth I can cast a nativity horoscope, fine. This really helps in performing magic on that person, fine. But if I managed to cast that horoscope, can I then reuse it for that target for all future spell castings or is this horoscope only valid for a certain time? That's just it. Thanks for answers in advance.
Vale,
Alexios ex Miscellanea
A nativity horoscope never changes - reusable as far as I know.
I'm not 100% certain, I think the story guide could go either way.
If the nativity horoscope includes an analysis of the current astrological influences affecting the target, then I would say you need to do a new one each time. If the nativity horoscope only concerns the astrological influences on the target at the time of his or her birth, then I would say once is enough. I don't remember what astrologists believed at the time.
My Arts and Academe is on loan to a friend but when I have it back I will try to look it up. Still, if someone else wants to look it up, they can. There is a whole section in there on horoscopes and horoscope effects.
Since a person can be born only once you can suppose that this part of the horoscope is already done. If it's necessary to consider the current stars to make the horoscope, then it should go twice as fast as usual. I have no rules to back me, but this seems quite OK to me.
I would suggest that the "current influences" would be covered by a secondary "horoscope for today", which adds to the static nativity horoscope.
I have a related question -- what information is required to draw an "horoscope for today"? Place and time of birth, as with the nativity? So that when you have them you can do both? Or is it the horoscope for the current place and time, regardless of the intended target?
A horoscope is an astrological description of a time. Technically it is independant of people, it just measures the influence of celestial bodies at a certain area at a specific time.
A nativity horoscope is cast using a person's birth time and place (like RAW describes) and it doesn't change afterwards. Technically it is the same for all people that were born on that day (and in roughly the same area).
In a daily horoscope the time used it this day, it can be calculated for a specific area. IMO it doesn't have to be tied to a specific person.
In game terms I would suggest that a nativity horoscope can always be used on that person and a daily horoscope can be used on anyone, but only on that specific day and within a certain area (both determined at the time of casting the horoscope). Both can be combined in a single spell casting, and their bonusses stack.
That's what I was leaning towards. Thanks for the confirmation that I'm not completely insane. 
A daily horoscope deals with the current location and disposition of the target, and will have an expiry - probably something like the rising of the moon. It is independent of the nativity; and you don't need to have a nativity to draw one up. You'll note that the sympathetic bonuses for nativity and daily horoscopes stack.
You could argue that you need to know the sun-sign of the target to draw up a daily horoscope - witness modern horoscopes in newspapers; they are all specific to one of the 12 solar zodiacal signs. However, I'm not sure how modern the notion of sun-signs is. Personally, I wouldn't require this, and make the daily horoscope specific to the location of a person rather than the person himself. Thus the same daily horoscope could be used for all members of a village.
Mark