North of Boston, Nahant is a small peninsula, reaching out into Massachusetts Bay. Offshore to the northeast is Egg Rock: landmark, lightcatcher. One October evening, at sunset, rose rose in the eastern sky and a blue line appeared along the horizon, gradually expanding into a band.

The color of the sky to the east at sunset, to the west at sunrise is paler than the setting or rising sun, but the succession of colors is just as compelling.

The blue band is the shadow of the earth, cast by the setting sun on the atmosphere. Here, as earth's shadow ascended during slack water at tide's turn, shallow waves moved toward shore and lines of blue and pink separated from the mauve, reflecting different parts of the sky.

The earth's shadow can be seen on most clear days a few moments after sunset, before sunrise. Colors vary from day to day, the lilac here is rare. More common are the colors in Twilight i and ii.