Here’s the very beginning of one of those magical celestial moments. It’s the time called “the blue hour,” which is a translation of the French characterization “l’heure bleue.” Despite its name, however, the phenomenon sometimes lasts only minutes.
According to the literature (and if I’m reading it correctly), the blue hour can occur when the sun is far enough below the horizon and the ozone layer is in position to help filter and absorb the remaining radiating sunlight, so that our star’s blue wavelengths dominate what we see.
That is, the sun’s red wavelengths at that moment shoot “up” high and escape through space, while its blue light is scattered in our less clear atmosphere and seems to paint everything varying shades of the color. (Image taken in Brooklin, Maine, on January 7, 2025.)