How does the imagery in each of these poems by Sappho contribute to the mood of each poem: "You Are the Herdsmen of Evening" and "Tonight I've Watched"?
Each poem is short and so has a single mood (feeling within the poem). The mood of "You are the herdsman" is playful and warm. In "Tonight I've watched" the mood is solemn and sad. In the first one ("You are the herdsman"), the metaphor and imagery compare the Evening Star of Greek mythology--who is the son of Atlas and called Hesperus--to and describe it as a shepherd. In the second ("Tonight I watched"), the poetic persona's metaphor and imagery compares her lonely state to the going down of celestial bodies whose going is like youth going.
In the first, the imagery of the shepherd of everything adds a playfulness to the mood as the shepherd is pictured gathering up "whatever" the "Dawn's light dispersed" into the day.
You are the herdsman of evening
Hesperus, you herd
Dawn's light dispersed
That what was dispersed out into the image of the sun-lit day included sheep, goats and children adds warmth to the mood, a warmth of feeling heightened by the image of children now returning to their mothers' arms.
You herd sheep--herd
home to their mothers
In the second, the imagery of the persona's youth and happiness going down like the moon and Pleiades adds solemnity to the mood through the comparison of a state of being to celestial bodies, which are high and revered. The imagery of youth being "half-gone" like the night adds sadness that is accentuated by the final image of her alone in her bed.
Tonight I've watched
The moon and then
The night is now
half-gone; youth goes; I am
in bed alone