In "Ode on a Grecian Urn," why does Keats call the urn a "Sylvan historian?"
"Sylvan" pertains to woods, the forest, or a pastoral landscape. Keats' urn depicts various people and circumstances: a religious ceremony, two lovers, a pastoral piper, townspeople, and the forest itself.
These images "occur" in a woodland (sylvan), rustic scene.
Fair youth, beneath the trees, thou canst not leave
Thy song, nor ever can those trees be bare; (15-16)
These images "occur" in the sense that they happened in the past and are frozen in time on the face of the urn. This is what the speaker is contemplating: the transitory nature of life - immortally frozen in time as a work of art. Therefore, the urn is a historian; it has recorded past images of life in a woodland setting. The urn is a "Sylvan historian," an object that has chronicled and archived an image of rustic life from the ancient past.
Important to the poem's themes of life, time and immortality, the urn has captured a moment when the sylvan scene is full of life: life of the people and of the forest itself:
Ah, happy, happy boughs! that cannot shed
Your leaves, nor ever bid the spring adieu; (21-22)
The leaves, in spring, are blossoming. Life is emerging. As the scene is frozen in time, it will always be in this state of emergence.
In this poem, Keats contemplates a scene painted on a Grecian urn. Greek art was flooding into England at this time, and people were fascinated by it.
Sylvan means wooded. What Keats is sees on the urn is a group of people heading for a pagan festival in a wooded area outside of town. Because the scene takes place in a woodsy, leafy setting, Keats refers to it as sylvan.
The urn records a specific moment from the past, including two lovers just about to kiss. Therefore, because the urn is recording history, Keats calls it "historian."
In the poem, Keats becomes progressively more enthusiastic about the happiness of being a figure painted on a Grecian urn, forever young, forever in love, forever joyfully headed to a festival in a setting where it will forever be a beautiful spring day. As he gazes at the vase, being part of a work of art,—never to aging, growing sick, or dying—seems preferable to him to human mortality.