waning

Waning is the act or process of decreasing in size, amount, or intensity. In ordinary use, it describes something that is fading or diminishing. In astronomy, waning refers specifically to the Moon’s phases when the visible illuminated portion is shrinking, as the Moon moves from a full moon toward a new moon. The term can also describe diminishing trends in other contexts, such as interest, power, or daylight.

In lunar terms, waning begins after the full moon and continues until the next new moon. The sequence includes waning gibbous, the last quarter (often called the third quarter), and waning crescent. The opposite phase is waxing, when the illuminated portion grows. The Moon’s orbital cycle is about 29.5 days, and during the waning portion the visible lighted area decreases from night to night.

Waning is used metaphorically to describe trends or conditions that are decreasing in strength, prominence, or significance. Common phrases include waning interest, waning influence, and waning daylight as seasons change. The word often conveys a gradual rather than abrupt decline.

Etymology traces waning to Old English wanian, meaning to wane, fade, or decay. The noun form wane appears in early English texts and has since become a standard term in both scientific and everyday language to denote diminishing magnitude or presence. Related concepts include waxing, the corresponding increase.