“Why the vesain tal u’mutar begins at a different date than it did in the nineteenth century.”
ICOT: That’s a calendar issue. The secular year has a leap year every 4 years, with the exception of every 100 years (i.e. 1800, 1900) which is not a leap year, with the exception of every 400 years (1600, 2000, 2400) which is a leap year. So every time a leap year is skipped on the 100th year, the date vesain tal u’mutar begins changes by a day.
I guess you might consider calendar issues a “scientific” matter in a certain sense.