while precession is a very slow process which makes only small changes in the azimuth of a star rising in the East. Dating artifacts Among the artifacts that can most readily be dated by astronomical techniques are depictions of the positions of the celestial bodies at a particular time. Since the motions of the celestial bodies are all at different periods, it takes many centuries for all the planets plus the Sun and the Moon, to reach the same positions in the signs of the Zodiac. For a configuration accurate to ±15° (that is, within a single sign) the positions of these seven bodies will only return to the same configuration once in about 3700 years. A particular case involved a medieval illuminated manuscript which portrayed the position of these seven celestial bodies on 18 March 816; corresponding to the period when the manuscript was written. This calculation demonstrated that this illustration was not a copy of an earlier classical depiction of the position of the stars. The rapidly moving Moon is the most sensitive indicator for the exact time; if one can estimate the indicated position of the Moon to within a degree, the time of the diagram can be computed to within an hour. A striking example of this method was an astrological portrait of Sir Christopher Hatton (1540–1591), which depicted the positions of the seven classical planets in the zodiac and noted the computed positions of the planets to the nearest minute of arc. Here the largest source of error in the date was the uncertainty of 16th-century astronomical calculations. The resulting time was about noon of 12 December 1581. Dating structures by their orientation A more controversial archaeoastronomical approach has been used to date structures that are believed to have been oriented on astronomical principles by measuring their orientation and computing the date in the past when a single specified celestial body, whether the Sun or a selected star, rises or sets at the measured azimuth. The astronomer Norman Lockyer applied this method to Stonehenge by measuring the orientation of the Stonehenge avenue and comparing it to the position of solstitial sunrise, which changes slowly due to the changing obliquity of the ecliptic. The archaeologist F. C. Penrose applied a similar method to ancient Greek Temples, attempting to establish their dates by relating their orientation to the appearance of stars on the horizon, the position of