The poem sets up a small, strange scene. A captain, in the middle of the day, wants a woman to teach "the mathematics of evening." The title, "Consultant," suggests she is an expert brought in for her knowledge, though the poem never says exactly who she is or what ship or place the captain commands. We have to infer much of this from the images.
What she is asked to teach is odd and beautiful: the red land, the running water, the way light and shadow shift as the day ends. Evening is treated like a subject with rules, something that can be calculated. Then the last image breaks that order. A moth rises "in clear daylight," too early, as if it has misread the hour. It shows that nature doesn't always follow the numbers.
The mood is quiet, dreamy, and a little uneasy. The speaker seems to watch from a distance, noticing how precise knowledge and the unpredictable world sit side by side.