Why is ante meridiem called ante meridian?

Answer 1

It isn't. Although meridiem and meridian both derive from the Latin word meridies, meaning midday, there is no such term as ante meridian. The term meridian usually refers to an imagineary line of longitude upon the earth, whereupon the time is midday (12 noon) when the line passes directly under the sun due to the earth's rotation. Meridiem, on the other hand, refers to midday itself, such that ante meridiem means before midday (before noon) and post meridiem means after midday (after noon, and hence afternoon), thus dividing the 24 hour day into two equal 12 hour periods starting at midnight (ante meridiem) and midday (post meridiem). The terms are abbreviated to a.m. and p.m. respectively, and are used whenever referring specifically to the 12 hour clock. Thus 2 a.m. literally means the second hour in the 12 hour period that precedes midday.