Exegesis
"Complexion" in line six, can have two meanings: 1) The outward appearance of the face as compared with the sun ("the eye of heaven") in the previous line, or 2) the older sense of the word in relation to The four humours. In the time of Shakespeare, "complexion" carried both outward and inward meanings, as did the word "temperate" (externally, a weather condition; internally, a balance of humours). The second meaning of "complexion" would communicate that the beloved's inner, cheerful, and temperate disposition is sometimes blotted out like the sun on a cloudy day. The first meaning is more obvious, meaning of a negative change in his outward appearance.
The word, "untrimmed" in line eight, can be taken two ways: First, in the sense of loss of decoration and frills, and second, in the sense of untrimmed sails on a ship. In the first interpretation, the poem reads that beautiful things naturally lose their fanciness over time. In the second, it reads that nature is a ship with sails not adjusted to wind changes in order to correct course. This, in combination with the words "nature's changing course", creates an oxymoron: the unchanging change of nature, or the fact that the only thing that does not change is change. This line in the poem creates a shift from the mutability of the first eight lines, into the eternity of the last six. Both change and eternity are then acknowledged and challenged by the final line.
"Ow'st" in line ten can also carry two meanings equally common at the time: "ownest" and "owest". Many readers interpret it as "ownest", as do many Shakespearean glosses ("owe" in Shakespeare's day, was sometimes used as a synonym for "own"). However, "owest" delivers an interesting view on the text. It conveys the idea that beauty is something borrowed from nature—that it must be paid back as time progresses. In this interpretation, "fair" can be a pun on "fare", or the fare required by nature for life's journey. Other scholars have pointed out that this borrowing and lending theme within the poem is true of both nature and humanity. Summer, for example, is said to have a "lease" with "all too short a date." This monetary theme is common in many of Shakespeare's sonnets, as it was an everyday theme in his budding capitalistic society.
Read more about this topic: Sonnet 18