The word noon is derived from Latin nona hora, the ninth canonical hour of the day, in reference to the Western Christian liturgical term none, one of the seven fixed prayer times in traditional Christian denominations. The Roman and Western European medieval monastic day began at 6:00 a.m. (06:00) at the equinox by modern timekeeping, so the ninth hour started at what is now 3:00 p.m. (15:00) at the equinox. In English, the meaning of the word shifted to midday and the time gradually moved back to 12:00 local time - that is, not taking into account the modern invention of time zones. The change began in the 12th century and was fixed by the 14th century. Solar noon is the time when the Sun appears to contact the local celestial meridian. This is when the Sun reaches its apparent highest point in the sky, at 12-noon apparent solar time, and can be observed using a sundial. The local or clock time of solar noon depends on the longitude and date.