nowrap">P<sub>s</sub> = pU/(qA)</span>. In the early almanacs, p=235 and q=19 were chosen, so that 235 synodic months were equal to 19 tropical years. Since <span class="nowrap">235=19&times;12+7</span>, 7 leap months had to be inserted in 19 years and the pattern of leap months repeated every 19 years. This particular <i>zh&#257;ng</i> cycle was known as the <i>Metonic cycle</i> in ancient Greece. Other values of p and q were used since the 5th century. For example, p=7421 and q=600 were used in the <i>Xu&