Nabonassar founded a kingdom in Babylon

Babylon

Babylon was an Akkadian city-state of ancient Mesopotamia, the remains of which are found in present-day Al Hillah, Babil Province, Iraq, about 85 kilometers south of Baghdad...

 in 747 BC. This is now considered as the start of the Neo-Babylonian Dynasty

Neo-Babylonian Empire

The Neo-Babylonian Empire or Second Babylonian Empire was a period of Mesopotamian history which began in 626 BC and ended in 539 BC. During the preceding three centuries, Babylonia had been ruled by their fellow Akkadian speakers and northern neighbours, Assyria. Throughout that time Babylonia...

. At the time the Assyrian Empire

Assyria

Assyria was a Semitic Akkadian kingdom, extant as a nation state from the mid–23rd century BC to 608 BC centred on the Upper Tigris river, in northern Mesopotamia , that came to rule regional empires a number of times through history. It was named for its original capital, the ancient city of Assur...

 was in disarray through civil war and the ascendancy of other kingdoms such as Urartu

Urartu

Urartu , corresponding to Ararat or Kingdom of Van was an Iron Age kingdom centered around Lake Van in the Armenian Highland....

. An army commander involved in the civil war, who adopted the name Tiglath-pileser III

Tiglath-Pileser III

Tiglath-Pileser III was a prominent king of Assyria in the eighth century BC and is widely regarded as the founder of the Neo-Assyrian Empire. Tiglath-Pileser III seized the Assyrian throne during a civil war and killed the royal family...

 with his accession, won control of Assyria the following year 746 BC. Shortly thereafter he retook Babylon under the suzerainty of Assyria, and Nabonassar continued to rule as a vassal king for 14 years, until 734 BC.

The first of a series of tablets collectively called the Babylonian Chronicle record events beginning in the reign of Nabonassar.

The Greek astronomer Claudius Ptolemaeus

Ptolemy

Claudius Ptolemy , was a Roman citizen of Egypt who wrote in Greek. He was a mathematician, astronomer, geographer, astrologer, and poet of a single epigram in the Greek Anthology. He lived in Egypt under Roman rule, and is believed to have been born in the town of Ptolemais Hermiou in the...

 started an era

Era

An era is a commonly used word for long period of time. When used in science, for example geology, eras denote clearly defined periods of time of arbitrary but well defined length, such as for example the Mesozoic era from 252 Ma–66 Ma, delimited by a start event and an end event. When used in...

, i.e. a start point for chronological

Chronology

Chronology is the science of arranging events in their order of occurrence in time, such as the use of a timeline or sequence of events. It is also "the determination of the actual temporal sequence of past events".Chronology is part of periodization...

 calculations, in the first year of his reign, on New Year's Day in the Egyptian calendar

Egyptian calendar

The ancient civil Egyptian calendar had a year that was 360 days long and was divided into 12 months of 30 days each, plus five extra days at the end of the year. The months were divided into three weeks of ten days each...

: Wednesday 26 February 747 BC in the proleptic Julian calendar

Proleptic Julian calendar

The proleptic Julian calendar is produced by extending the Julian calendar to dates preceding AD 4 when its quadrennial leap year stabilized. The leap years actually observed between its official implementation in 45 BC and AD 4 were erratic, see the Julian calendar article for details.A calendar...

. On this day the Nabonassar era (AN - Anno Nabonassari) began. The starting was used by Ptolemy because it was the earliest reign that included an astronomical observation he used, and was used by later astronomers, but not by the Babylonians themselves.