Meridian transit
= Meridian transit
A <meridian transit> is a direction's crossing of the observer's <celestial meridian>. Upper <meridian transit> occurs at <hour angle> $0$ and gives the greatest geometric <altitude> in the fixed-direction model; lower <meridian transit> occurs at <hour angle> $12^{\rm h}$ and gives the least. Either crossing can lie below the <astronomical horizon>. https://aa.usno.navy.mil/faq/asa_glossary[US Naval Observatory: Upper and lower culmination]{ref}.