Tangential to this, if you're interested in how the calculation of Easter led to using cathedrals as astronomical instruments, check out The Sun in the Church b

Tangential to this, if you're interested in how the calculation of Easter led to using cathedrals as astronomical instruments, check out The Sun in the Church by J.L. Heilbron. Frankly, it's a slog to read at times, but there's some very fascinating stuff in there, including some impressive engineering feats.