Английский Этимологический словарь - noon

- 1140, non "midday, 12 o'clock p.m., midday meal," from O.E. non "the canonical hour of nones, 3 o'clock p.m.," from L. nona hora "ninth hour" of daylight, by Roman reckoning about 3 p.m., from nona, fem. sing. of nonus "ninth." Meaning shifted from 3 p.m. to noon during 12c. when time of Church prayers shifted from ninth hour to sixth hour, or perhaps because the customary time of the midday meal shifted, or both.