The standard work on astronomy, as has already been noted, was that called 'The Observations of Bel,' compiled originally for the library of Sargon I at Accad. Additions were made to it from time to time, the chief object of the work being to notice the events which happened after each celestial phenomenon. Thus the occurrences which at different periods followed a solar eclipse on a particular day were all duly introduced into the text and piled, as it were, one upon the other. The table of contents prefixed to the work showed that it treated of various matters—eclipses of the sun and moon, the conjunction of the sun and moon, the phases of Venus and Mars, the position of the pole-star, the changes of the weather, the appearance of comets, or, as they are called, 'stars with a tail behind and a corona in front,' and the like. The immense collection of records of eclipses indicates the length of time during which observations of the heavens had been carried on. As it is generally stated whether a solar eclipse had happened 'according to calculation' or 'contrary to calculation,' it is clear that the Babylonians were acquainted at an early date with the periodicity of eclipses of the sun. The beginning of the year was determined by the position of the star Dilgan (α Aurigæ) in relation to the new moon at the vernal equinox, and the night was originally divided into three watches. Subsequently the _kasbu_ or 'double hour' was introduced to mark time, twelve _kasbu_ being equivalent to a night and day. Time itself was measured by a clepsydra or water-clock, as well as by a gnomon or dial. The dial set up by Ahaz at Jerusalem (2 Kings xx. 11) was doubtless one of the fruits of his intercourse with the Assyrians. The Zodiacal signs had been marked out and named at that remote period when the sun was still in Taurus at the beginning of spring, and the equator had been divided into sixty degrees. The year was correspondingly divided into twelve months, each of thirty days, intercalary months being counted in by the priests when necessary. The British Museum possesses fragments of a planisphere from Nineveh, representing the sky at the time of the vernal equinox, the constellation of Tammuz or Orion being specially noticeable upon it. Another tablet contains a table of lunar longitudes.