   The second book of On the Revolutions elaborated the concepts in the first book; book 3 dealt with the precession of the equinoxes and solar theory; book 4 dealt with the moon’s motions; book 5 dealt with the planetary longitude and book 6 with   latitude.[9]   Copernicus depended very much on Ptolemy’s observations, and there was little new in his mathematics. He was most successful in his work on planetary longitude, which, as Swerdlow and Neugebauer (77) commented, was “Copernicus’s most admirable, and most demanding, accomplishment…It was above all the decision to derive new elements for the planets that delayed for nearly half a lifetime Copernicus’s continuation of his work – nearly twenty years devoted to observation and then several more to the most tedious kind of computation – and the result was recognized by his contemporaries as the equal of Ptolemy’s accomplishment, which was surely the highest praise for an astronomer.” Surprisingly, given that the elimination of the equant was so important in the Commentariolus, Copernicus did not mention it in book 1, but he sought to replace it with an epicyclet throughout On the Revolutions. Nevertheless, he did write in book 5 when describing the motion of Mercury: