&equiv; <span class="nowrap">r<sub>2</sub></span> (mod <span class="nowrap">P<sub>L</sub></span>).&nbsp;&nbsp;&nbsp;(1)</p><p>Here <span class="nowrap">P<sub>s</sub></span> is the tropical year in days, <span class="nowrap">P<sub>L</sub></span> is the synodic month in days, and &equiv; denotes congruence. The first equation means that <span class="nowrap">N-R<sub>0</sub></span> is an integer multiples of 60. The second equation means that <span class="nowrap">P<sub>s</sub>N - r<sub>1</sub></span> is an inte