We cannot automatically dismiss a hadith just because the chain of transmission is weak. Historically, scholars would look at clues in the hadith to verify it’s authenticity. If the hadith is narrated in an authentic source (like al-Kafi), the content does not contradict the Qur’an, the content is confirmed by other similar narrations or other guidelines we have in our faith, then the hadith can be accepted even if the chain is weak. As for the Prophet (s) being the sun and Imam Ali (a) being the moon, there is nothing questionable about such content, and other narrations confirm this. The Prophet (s) is the source or revelation and religious teachings. Imam Ali (a) did not bring new revelation or laws. He reflected the Prophet’s (s) teachings. This analogy is beautiful and accurate.