Nabu-rimanni was born in Babylonia. He was the earliest Chaldean astronomer known by name. He was son of Balatu and he started his work during the reigns of Persian kings Darius I The Great (550 BC - 486 BC), (ruled from 522 BC to 486 BC) and his successor Xerxes I The Great (circa 519 BC - 465 BC), (ruled from 486 BC to 465 BC). Nabu-rimanni was contemporary of Cleostratus of Tenedos, Aeschylus, Ferehindus and Herodotus.
From the times of king Nabonassar, (ruled from 747 BC to 732 BC) a lot of Sumerian astronomical written records have been preserved. It is evident from them that Sumerians after long-standing observations of solar and lunar eclipses had shelled a cycle of 18 years and 11 days or 6585 1/3 d which was named the Saros cycle in the 10th century lexicon Suidas and after which the Earth, Sun, and Moon return to nearly the same relative positions and the cycle of lunar and solar eclipses begins to repeat itself.
The Sumerians were not just able to determine astronomical conjunctions of the Sun and the Moon, but they knew for the changes of the lunar movement and the changes of the apparent angular velocity of the Sun and even the planets. They were able to define periods of these irregularities, anomalistic months - consequences of elliptical orbits of celestial bodies. In the early 6th century BC they had determined relative movements of the Sun and Moon, lunar perigee and apogee and their nodes, intersections of lunar orbit with ecliptic, which are responsible for the saros cycle. About 500 BC Nabu-rimanni had corrected these values with his improved observations of eclipses. The movement of the Moon relative to the Sun was slighter for 10" per annum, the movement of lunar perigee was larger for 20" per annum and the movement of its nodes was slighter for 5" per annum. These values were improved afterwards by Kidinnu. Hipparchus later learned for these Kidinnu's values.
Nabu-rimanni devised the so-called System A, a group of ephemerides, or tables, giving the positions of the Moon, Sun, and planets at any given moment. Based on the centuries of observation, these tables were nonetheless somewhat crude and were superseded about a century later by Kidinnu's System B. Nabu-rimanni collected his work in a book in Akkadian Observations of the Moon and Stars.
Nabu-rimanni determined the solar year to be 365 days, 6 hours, 15 minutes and 41 seconds. He used a water clock to measure the days, months and the length of the solar year. He also calculated the length of the synodic month to be 29d 12h 44 m 5.05s or 29.530614d, as compared with the modern value of 29d 12h 44 m 3.49s or 29.530596d with an error of +1.56s. He also had showed how an apparent magnitude of lunar eclipse can be calculated from the deviation of the lunar nodes.\n