Original sentence: It is also worthwhile to remember that, before the invention of the telescope, the classical planets were seven in number: Mercury through Saturn, minus the Earth, plus the Sun and Moon (in certain cultures this is the reason for a seven day week)
Edited sentence: It is also worthwhile to remember that, before the invention of the telescope, the classical planets were seven in number: Mercury through Saturn, minus the Earth, plus the Sun and Moon (this is the reason why a week has seven days)