Shakespeare uses astronomy to connect divine attributes to human attributes. Human can observe the stars from Earth, providing a direct link to the celestial world. In his 14th sonnet, Shakespeare says he does not use astronomy to predict the future; he writes of seeing the stars in the eyes of youth. This brings practicality to a subject often veiled by uncertain meaning.
Shakespeare views astronomy with straightforward pragmatism. This is an important concept to remember when writing a sonnet. Sonnets tend to gravitate toward the most ineffable subjects. It is important to provide a solid foundation in daily reality so your audience can connect with the ineffable.
Like all sonneteers, Shakespeare's work in the genre overwhelmingly deals with love. He broadens this topic by imbuing it with necessary dualities such as good and evil. In sonnet 144, Shakespeare introduces theological entities such as the devil, angels and hell. He was a product of a deeply Catholic environment. Theology was a topic widely discussed in his time and its appearance in his sonnets reveal some of the most obvious dichotomies of love. These include sin and virtue; damnation and salvation and the painful subject of infidelity versus steadfast commitment.
Most sonneteers have played with the idea of death and dying. Since love is the central theme in sonnet-writing, death is used as a foreboding device to hint at its potential tragedies and temporal limits. Most sonnets have a bittersweet quality. Sonnets acknowledge the transient nature of human life but they often strive to describe the eternal. "Happy to have thy love, happy to die," Shakespeare writes in sonnet 92. He suggests that humans are limited by mortality, and yet love transcends even death.
Human physiology is an important theme in many sonnets. The recurring theme here is a connection between the intangible feelings of poetry with physical reality. Shakespeare often relates the emotional world to the anatomical. Beyond the obvious use of the heart to symbolize love, Shakespeare uses the feet to represent mobility in life, the hands to represent creative abilities, the skeletal system to represent stability and the loins to represent untamed passion. Shakespeare and other sonneteers like John Donne often use the senses to convey our direct link with the phenomenological world.