Glossary
Birth time
Rising and setting lines (AC/DC) move quickly — roughly a degree of chart every four minutes — so they need a reasonably exact birth time. Meridian lines (MC/IC) are steadier and survive an approximate time well.
When a birth time is unknown, the honest move is to compute the meridian lines and leave the horizon lines off, rather than faking everything from a noon default. Birth certificates, hospital records, and family memory (in that order) are the usual sources for pinning a time down.