Epoch

People use a start date to count years. This date is called an epoch. It is a point in time. Some calendars start at a big event. For example, one starts when a city was built. Another starts when a new leader began. Some calendars follow the moon. Others follow the sun. These dates help us track history. They show us how time moves. It is a way to tell stories of the past.

People use a starting point to count years. This point is called an epoch. An epoch helps us measure time. Most epochs start with a big event. This event can be a change in the world.

Many old calendars use different epochs. The Yoruba calendar starts with the creation of the earth. The Roman calendar started when their city was built. The Islamic calendar starts with the Hijra. This was a move in the year 622. Some calendars follow the moon. Others follow the sun.

Some systems track leaders. In Japan, years start when a new emperor begins his rule. This is called a regnal era. In the past, China used this way too.

Even science uses an epoch. Scientists use a date to study very old things. They use a date from 1950. Computers also use an epoch. Unix time starts on January 1, 1970. Even stories have them. In Star Wars, people count years from a big battle.

An epoch is a special starting point in time. It is the moment a calendar begins to count years. People choose this point to measure time from a specific origin. This origin is often called a reference epoch. Sometimes the epoch is a single, clear event. This is known as an epoch event. Other times, the change happens more slowly over time.

Different cultures use different ways to pick an epoch. Some choose a moment of creation. For example, the Yoruba calendar uses 8042 BC. This was the year the god Obatala created Ile-Ife. The Mesoamerican Long Count Calendar uses 3114 BC. That was the year the fourth world was created. The Hebrew calendar uses 3761 BC as its start. These choices help people organize their history.

Many famous calendars use events from history. The Islamic calendar starts with the Hijra in AD 622. This calendar follows the moon instead of the sun. The official Iranian calendar also uses the Hijra. However, that calendar follows the sun. The Julian and Gregorian calendars use a different start. They use the Incarnation of Jesus. A man named Dionysius Exiguus calculated this in the 6th century.

Some systems use the lives of leaders to count years. This is called a regnal era. In Japan, the years start when an emperor begins his rule. China used a similar system before 1912. Taiwan still uses the Minguo calendar from 1912. This year marks the start after the Xinhai Revolution. North Korea used a system starting in 1912. It used the birth year of Kim Il-Sung.

Even science and computers use epochs to stay organized. Scientists use a date for radiocarbon dating. Their reference point is January 1, 1950. Computers use a system called Unix time. This system starts at midnight on January 1, 1970. Even stories in movies use epochs. In the Star Wars universe, people count from the Battle of Yavin. These points make time easier to understand.

An epoch serves as a fundamental reference point in chronology. It is the specific instant in time chosen as the origin for a calendar era. This starting point allows people to measure the passage of time from a fixed center. Scientists and historians call this the reference epoch. The moment of the epoch is usually decided by following established conventions. Often, the epoch is defined by a specific, clear event known as an epoch event. In cases of more gradual change, a deciding moment is chosen when a specific criterion is finally met.

Many pre-modern eras were founded on religious or mythological events. The Yoruba calendar, known as Kọ́jọ́dá, uses 8042 BC as its epoch. This date marks when the god Obatala created Ile-Ife and the earth. Other systems use the concept of Anno Mundi, which means "Year of the World." The Byzantine calendar used 5509 BC for this purpose. The Hebrew calendar uses 3761 BC as its starting point. Similarly, the Mesoamerican Long Count Calendar uses the creation of the fourth world in 3114 BC. These eras provide a way to connect human time to the origins of the universe.

Ancient civilizations also used specific historical milestones to organize their years. The ancient Greeks used Olympiads to mark four-year periods between Olympic Games, starting in 776 BC. Roman calendars used the term Ab urbe condita, which means "from the foundation of the city." This era began in 753 BC. The Buddhist calendar often uses 544 BC, which is the date of Buddha's parinirvana. In India, the Vikram Samvat is a notable Hindu epoch from 58 BC. This system is still used today as the national calendar for Nepal and Bangladesh.

Religious shifts have also created major calendar changes. The Julian and Gregorian calendars use the Incarnation of Jesus as their epoch. This was calculated in the 6th century by Dionysius Exiguus. Modern research suggests this date is actually about four years after the best estimate for the birth of Jesus. The Islamic calendar uses the Hijra in AD 622 as its epoch. This calendar is purely lunar and consists of 12 lunations. Because of this, its years are ten or eleven days shorter than a solar year. The official Iranian calendar also uses the Hijra, but it is a solar calendar. It marks the start of each year at the northern spring equinox.

Modern political changes have led to the creation of new eras. The French Republican Calendar used the beginning of the Republican Era on September 22, 1792. This was the day the French First Republic was proclaimed. In Thailand, King Chulalongkorn decreed a National Thai Era in 1888. This dated from the founding of Bangkok on April 6, 1782. Later, in 1941, Prime Minister Phibunsongkhram decided to count years since 543 BC. This created the Thai solar calendar using the Thai Buddhist Era. The Minguo calendar in Taiwan dates from January 1, 1912, following the Xinhai Revolution.

Some systems are based on the lives or power of specific leaders. These are called regnal eras. In the official Japanese system, years are numbered from the accession of the current emperor. The year the accession occurs is considered the first year. China used a similar system based on the emperor's accession before 1912. The People's Republic of China adopted the common era calendar in 1949. Canada still uses regnal eras for its Acts of Parliament. For example, it currently refers to the year as 4 Charles III. North Korea used a system starting in 1912 based on the birth of Kim Il-Sung.

Scientific and technological fields require their own precise epochs. Scientists use the "Before Present" system for radiocarbon dating. The reference date for this system is January 1, 1950. Computers rely on Unix time to organize data. The Unix epoch is 00:00:00 UTC on January 1, 1970. Geologists use the Holocene calendar, which sets its epoch at 10,000 BC. This is a rough approximation of the start of the Holocene epoch on the geological time scale. Even fictional universes use these rules. In Star Wars, the Battle of Yavin serves as the epoch. In Avatar: The Last Airbender, the epoch is the genocide of the air nomads.