and the other an annual fluctuation.

Lunar theory

The fact that the position of the Moon can be measured extremely accurately against the backdrop of the fixed stars means that the many irregularities in its motion are relatively easy to detect. Before the advent of the telescope, for example, Ptolemy discovered the evection, and Tycho Brahe, the variation. In the nineteenth century, the ease with which observers could uncover new inequalities posed a considerable challenge for theoreticians. To add to the difficulties, the series by which these irregularities were determined typically converged slowly, and the selection of coordinate system was of crucial importance.

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