A guide to the poem by Harold Bloom avers that the poem is “a brilliant summation about poetic thinking ... it also expresses the way in which time itself was understood.” By this he seems to mean that for Tennyson, time does not simply express movement in one direction toward a goal, or even movement at all; instead, his poetic language engages with how humans experience nature, space, and time in their limited exposure to the cosmos.
In the first stanza the tears are a paradox: they are “idle,” but they appear to have great importance. This paradox launches the speaker’s investigation of her emotion, seeking to understand whatever “divine despair” seems to be causing her physiological response. The tears come from looking upon the “happy Autumn-fields” and thinking about the lost days, but how is this related to something divine?
The second stanza suggests that the spiritual loss has to do with death. Dear friends come up from the underworld, providing a pleasant and fresh memory like a sunrise (“first beam glittering”). Yet, the memory fades sadly like a sunset (the beam “reddens” and “sinks … below the verge,” and indeed “all we love” sinks that way, in the speaker’s view. This is how the lost days are both sad and fresh.