William Shakespeare (1564-1616)
Shakespeare is the most celebrated writer in the English language. Famous for his plays (tragedies like Hamlet, Macbeth and King Lear; comedies like Much Ado About Nothing; histories like Henry V), he also wrote a sequence of 154 sonnets, published in 1609.
The sonnet sequence
Shakespeare's sonnets were published in 1609 but mostly written in the 1590s. The full sequence tells a loose story about a young man the speaker loves and admires (the "Fair Youth"), a rival poet, and a dark-haired mistress (the "Dark Lady"). Sonnets 1-126 are mostly addressed to the Fair Youth; 127-152 to the Dark Lady. Sonnet 116 comes in the Fair Youth section.
Love and constancy
Sonnet 116 is often read as a defence of constant love in the face of inconstancy - possibly at a point in the sequence where the speaker and the young man have become estranged. The poem is, in effect, an argument: real love does not change, even if people do. It may be trying to persuade as much as describe.
Elizabethan ideas of love
The Elizabethans inherited two main traditions of love poetry: courtly love (the distant, idealised lady of medieval romance) and Petrarchan love (the tortured lover writing sonnets to an unattainable beauty). Shakespeare often plays with - and against - these conventions. Sonnet 116 takes the Petrarchan ideal of eternal devotion and strips it of flattery, turning it into a stark philosophical claim.
Navigation and the North Star
The "ever-fixèd mark" and "the star to every wandering bark" are references to navigation. In the Elizabethan age of exploration (Drake had circumnavigated the globe in 1577-80, the East India Company was founded in 1600), sailors guided their ships by the Pole Star - a fixed point that never moved across the sky. Shakespeare's metaphor takes cutting-edge technology and turns it into a spiritual image.