The first line of models the rest of the poem, hinting at its main themes, giving information about the speaker, and previewing the way in which the poem will both adhere to and subvert the Italian sonnet form.
Read alongside the title, the first word, “Earth,” introduces the theme of nature vs. civilization. The title indicates that the setting of the poem is Westminster Bridge, a manmade structure in an urban environment that spans a natural feature (the Thames River). Rather than start with the specific view from the bridge, however, the poem starts with a comprehensive view of the entire planet. Earth, which has the ability to “show,” is personified. This attention to Earth tells the reader that the poem will consider the city’s place within the natural world.
The line also gives the reader information about the speaker. There’s a lot of confidence in the opening declaration. By professing certainty about the highly arguable claim that nothing is “more fair” than the view from the bridge, the speaker emphasizes that this is a poem told from the limited, and possibly unreliable, perspective of one person.
In its imperfect iambic pentameter, the line’s meter imitates the way this person might actually talk. Not every foot in the line is unstressed-stressed, however, as it would be if it were written purely in iambic pentameter:
Earth has | not an | ything | to show | more fair:
The first foot is actually a trochee, a syllable pair that is stressed-unstressed—the reverse of an iamb. This trochee places extra emphasis on the word “Earth,” and allows the speaker to begin on an assertive note with perhaps a hint of a challenge (that is, for someone to reveal anything fairer than the city before the speaker). The line also resembles the real speech of a city-dweller.
The line does end with an iamb, the stress of which highlights the word “fair,” as well as with a colon, which creates an end-stopped line. This clear end-stop forces the reader to pause and consider the boldness of this opening statement, while the use of "fair" marks an instance of both adhering to and breaking with tradition: it was not uncommon in 1802 England to use “fair” to signify beauty, as Wordsworth does here. But to describe smelly, industrial London? That may have come as a surprise!