Bloodletting was scheduled by the Moon, and the printed almanac that told you when was one of the best-selling books in Europe for two centuries.

The rule was to bleed on a waxing moon and avoid the sign governing the part being bled. Almanacs carried volvelles — paper dials you turned — so a barber could work it out without arithmetic. It is the clearest case of lunar doctrine directly shaping medical practice, and it did real harm, which is worth saying plainly.