Astronomy is a science that lends itself to the recording and study of observations: the rigorous notings of the motions of the stars, planets, and the moon are left on thousands of clay tablets created by scribes. Even today, astronomical periods identified by Mesopotamian scientists are still widely used in Western calendars: the solar year, the lunar month, the seven-day week. Using these data they developed arithmetical methods to compute the changing length of daylight in the course of the year and to predict the appearances and disappearances of the Moon and planets and eclipses of the Sun and Moon. Only a few astronomers' names are known, such as that of Kidinnu, a Chaldean astronomer and mathematician who was contemporary with the Greek astronomers. Kiddinu's value for the solar year is in use for today's calendars. Astronomy and astrology were considered to be the same thing, as evidenced by the practice of this science in Babylonia by priests. Indeed, rather than following the modern trend towards rational science, moving away from superstition and belief, the Mesopotamian astronomy conversely became more astrology-based later in the civilisation - studying the stars in terms of horoscopes and omens, which might explain the popularity of the clay tablets. Hipparchus was to use this data to calculate the precession of the Earth's axis. Fifteen hundred years after Kiddinu, Al-Batani, born in what is now Turkey, would use the collected data and improve Hipparchus' value for the precession of the Earth's axis. Al-Batani's value, 54.5 arc-seconds per year, compares well to the current value of 49.8 arc-seconds per year (26,000 years for Earth's axis to round the circle of nutation).