People often see AM and PM on the digital clock but never tried to decipher “why such abbreviation is used to denote time period?” AM stands for Ante Meridiam whereas PM stands for Post Meridiam. This is because the 24 hours of the day are divided into two periods- AM which means before midday and PM means after midday and each period comprises of 12 hours in number.
Hence, the term AM & PM is associated with the time-period: AM, is an abbreviation of the Latin phrase ante merīdiem meaning “before midday”; PM is an abbreviation of post merīdiem, meaning—“after midday.”
Historical Background
The clock is one of the earliest inventions of mankind. The term “clock” is derived via Dutch, Northern French and Medieval Latin from the Celtic term clagan and clocca which means “bell”. This device was design to know the natural day/night division of a calendar day forms into two cycles- to track the position of the Sun in day and secondly to tracked the position of the Moon and stars in night.
It was Egyptians who divided the day into 24 equal parts by using the base of 12. The entire episode of clock and dividing time period just to reflect the rotation of the earth, that’s why in ancient times sundial is used to measure solar time on light of the knowledge of latitude which was created in Babylon over six thousand years ago.
Ambiguity behind 12 pm or Noon in 12 Hours clock
It is incorrect to say 12 pm or 12 am, since it is not after or before noon. So it is recommended to denote these hours as midnight and noon. Hence, each hour during the day, except midnight and noon, is suffixed by AM or PM to identify whether the hour being referred to is in the morning, afternoon or evening.