Era
An Era is a (usually lengthy) period of time with common characteristics. For example -- the Biblical era, the Roman era. In Geology, Era refers to four well defined time spans covering the history of the Earth. From oldest to youngest, these are the Proterozoic, Paleozoic, Mesozoic, and Cenozoic eras. In Geology, eras are grouped into still longer periods called Eons. Era is also popularly used to denote the passing of shorter periods, such as the "Big Band" era, or the death of Frank Sinatra being the end of an era. ERA is an acronym for Equal Rights Amendment.
Calendar era
In a calendar, the era is the date from which years in the calendar are counted. For example, in the Gregorian and Julian calendars, dates are measured from the beginning of the Christian era. There are many different calendar era systems. Some are listed below along with the abbreviations.
- CE - meaning Common Era, which is used in the Gregorian calendar to mean the same thing as A.D., but without the religious connotation. Year 1 is the beginning of the Christian era.
- A.D. - for the Latin Anni Domini, meaning in the year of the Lord. Years are counted from the beginning of the Christian Era. A.D. is used in both the Julian and Gregorian calendars. The years are equivalent to years in the Common Era.
- Note: Years are numbered the same in the Julian and Gregorian calendars, but the calendars are not identical. To distinguish betwen them, O.S. and N.S. were often added to the date, especially during the 17th and 18th centuries, when both calendars were in common use. O.S. or Old Style was used for the Julian calendar. N.S. or New Style was used for the Gregorian calendar.
- A.C., B.C. and B.C.E. - for the Latin Ante Christum, the English Before Christ and Before Common Era, respectively. In all cases, years count backward from the year 1 A.D. or, equivalently, 1 CE. Note that there is no year "0". (There is a year 0 if negative numbers are used, so that the year -25 CE is the same as the year 26 BCE)
- A.U.C. - for the Latin Ab Urbe Condita, meaning from the founding of the city. Used in the Roman calendar, which began with the founding of Rome. The beginning of the Christian era was 753 A.U.C., so that the year 2003 C.E. is the same as the year 2756 A.U.C.
- A.M. - for the Latin Anno Mundi, meaning year of the world. This is used in the Hebrew calendar which counts years from the creation of the world, which is assumed to have taken place in the year 3761 B.C.E.
- A.H. - for the Latinized Anno Hegirae, meaning years since the Hijrah, Prophet Muhammad's migration from Makkah to Madinah in September 622 C.E., which is taken to be the beginning of the Muslim era. This is used in the Islamic calendar. (Note that, since the Islamic calendar is a lunar calendar, years are shorter than years in solar calendars.)
- Japanese eras began with the ascension of an emperor. The most recent eras are:
- Chinese era names or Nian Hao