New Delhi, Aug. 16 -- Two people can type identical birth details into two different astrology apps and get back two different charts. Same date, same minute, same city. One app puts the Sun in the tenth house, the other in the ninth. Neither app is broken, and neither is lying. They are using different house systems, and that disagreement is the most confusing thing in astrology that almost nobody bothers to explain.

The houses are the part of a birth chart most readers skip on the way to the parts that end up on social media. Sun signs are easy. One date, one answer, no arithmetic. Houses are harder. They need an exact birth time; they shift by roughly one degree every four minutes, and depending on which method your software uses, the...