The first four lines of the "Prothalamion" establish the poem's mood and some of its central concerns. The speaker begins by describing the weather, using a series of adjectives like "calm," "trembling," and "gentle" to suggest that the poem takes place on an unusually beautiful and serene day. As he does so, he invokes a number of gods from Greek myth: Zephyrus and Titan. Zephyrus embodies the west wind—which the Greeks considered to be the gentlest and mildest of winds. He is associated with morning, suggesting that this poem takes place during the early morning. He is also associated with spring—and thus with fertility, pregnancy, and rebirth.
The presence of Zephyrus early in the poem suggests a few important things about the "Prothalamion." First, the poem is interested in using Greek mythology for its own purposes. Though it was written by an English poet (and set in England), it draws on myths from an ancient and distant culture in its depictions of natural harmony and beauty. Second, the poem is deeply concerned with fertility, reproduction, and the rituals that surround those acts—specifically marriage.
The second Greek figure, Titan, is less specific than Zephyrus: the Titans were a race of Gods, including Gaia (Earth) and Chronos (Time). In this instance, "Titan's beams" refers to the sun, whose beams "glister"—shine with unbroken brilliance—on the day. In the Renaissance, leading Protestant intellectuals often interpreted Biblical images of the sun as metaphors about temptation. In these early lines, there is a sense that the beautiful weather the speaker describes is threatened. In line 3, for instance, the speaker notes that Zephyrus "delay[s]," the unpleasant heat of Titan's beams—and, perhaps, the sin and temptation they symbolize. One wonders how long this delay will last: presumably, not forever. As the speaker paints a picture of a beautiful, unspoiled day, he also suggests that it will inevitably fall into sin and decline.
The meter of these opening lines is strongly iambic; they are rhymed abba, like the first quatrain of a Petrarchan sonnet. However, there are disturbances in the meter: for instance, the first line opens with a trochaic substitution:
CALM was | the day, | and through | the trem | bling air
The trochee in the first foot suggests, again, that the calmness and beauty of the day is something temporary and unusual: just as the line resolves into an iambic rhythm after its first foot, so too the calm of the day will disappear in time.