In "Astrophil and Stella," Sidney uses images, such as Venus and Mars, Cupid, and the Sisters nine, to capture the celestial theme of his poem. Not only are both Venus and Mars the names of planets, they are also the names of the gods for which we named the planets. The Sisters nine are another name for goddesses we call the Muses, who were responsible for inspiring artists, and Cupid is the name of the god of love. He contrasts these celestial images with terrestrial images, such as nature, king, shadow, and temple. More specifically, Sidney contrasts the celestial theme with the terrestrial theme to show just how much his love for Stella is a struggle for him, even contradicting what he accepts to be a part of nature or what's natural.
We can best see Sidney's struggle in the second stanza of sonnet 5 in which he contrasts the soul and love with the Church. In this stanza, he acknowledges that the eyes were made by God to show a man's soul rather than to obsessively admire female beauty, as we see in his lines, "It is true that our eyes are created to serve / The inner light of the soul." He further acknowledges that man should be governed by his spirituality and religion, rather than be governed by lust, and that those who do not follow the rules of religion are "rebels against Nature, and their efforts harm themselves." He further acknowledges that the romance and passion symbolized by Cupid's arrow is just man's own invention and that mankind foolishly idolizes romance and passion to the point that romance and passion become "false god[s]" and "puts Church and churchmen out of work." He even admits that virtue should be seen as the truest form of beauty and that "earthly beauty," like Stella's, can barely compare to the beauty of virtue. He further acknowledges that mankind has been merely "created to be pilgrims on earth," meaning worshipers of God and not of beauty. But despite understanding all of these things, he "must love Stella." In other words, he is comparing his love for Stella to something base and earthly, while the things of spirituality and religion are the higher, more celestial things. He is further saying that it is the higher, more spiritual, more celestial things that mankind should place the most value in rather than the terrestrial things like beauty, love, and passion; regardless, he is further saying that he is losing his battle between the celestial and the terrestrial because he cannot keep himself from loving Stella.
Hence, the reason why the celestial and terrestrial themes are at odds with each other in Sidney's poem is because he feels his passions for Stella is at odds with his more spiritual, religious side.