The Canon of Kings was a dated list of kings used by ancient astronomers as a convenient means to date astronomical phenomena, such as eclipses. The Canon was preserved by the astronomer Claudius Ptolemy, and is thus known sometimes as Ptolemy's Canon. It is one of the most important bases for our knowledge of ancient chronology.
The Canon derives originally from Babylonian sources. Thus, it lists Kings of Babylon from 747 BC until the conquest of Babylon by Achaemenid Persians in 539 BC, and then Persian kings from 538 to 332 BC. At this point, the Canon was continued by Greek astronomers in Alexandria, and lists the Macedonian kings from 331 to 305 BC, the Ptolemies from 304 BC to 30 BC, and the Roman and Byzantine Emperors, although they are not kings; in some manuscripts the list is continued down to the Fall of Constantinople in 1453.
The Canon only increments by whole years, specifically the ancient Egyptian year of 365 days. This has two consequences. The first is that the dates for when monarchs began and ended their reigns are simplified to the beginning and the ending of the ancient Egyptian year, which moves one day every four years against the Julian calendar. The second is that this list of monarchs is oversimplified. Monarchs who reigned for less than one year are not listed, and only one monarch is listed in any year with multiple monarchs. Usually, the overlapping year is assigned to the monarch who died in that year, but not always. Note that the two periods in the Babylonian section where no king is listed the first represents two pretenders whose legitimacy the compiler did not recognize, and the second extends from the year Babylon was sacked by Sennacherib, King of Assyria to the restoration of Esarhaddon.
The Canon is generally considered by historians to be accurate, and forms part of the backbone of the commonly accepted chronology from 747 BC forward that all other datings are synchronized to. It is not, however, the ultimate source for this chronology; most of the names and lengths of reigns can be independently verified from archaeological material (coinage, annals, inscriptions in stone etc.) and extant works of history from the historical ages concerned.