- (Of the moon) Having the observable illuminated part greater than a semicircle and less than a circle.
- Convex or protuberant.
“The gibbous bulges of the glass sculpture gave it an otherworldly quality.”
“During the gibbous phases, the portion of the moon that is visible is lit by the sun.”
“In the lunar cycle, the gibbous phases occur on either side of the full moon.”
Latin, mid-15th century
Why this word?
While “gibbous” describes the shape of the moon visible during certain portions of the lunar cycle (greater than a crescent, but less than a full moon), the other adjectives that usually accompany the descriptor are “waxing” and “waning.” During the lunar month, the visible moon progresses from the new moon through the waxing crescent to the first quarter, at which point the moon appears half-illuminated. The waxing gibbous phase occurs as more of the moon is visible each day until it reaches the full moon. After the full moon comes the waning gibbous phase, which slowly turns the disc back into a half-moon on its journey through the waning crescent and back to the new moon. This lunar cycle repeats every 28 days.