waning

If the moon was full, and now it's getting slimmer each night, it's a waning moon: it's getting gradually smaller.

Waning can be used for anything that diminishes, like the brightness of the sunlight in the late afternoon, your interest in cartoons, or your influence over your teenage daughter's fashion choices. It comes from the Old English verb wǫnian, which means "to lessen."

Definitions of waning
1

n a gradual decrease in magnitude or extent

“the waning of his enthusiasm was obvious”
“the waxing and waning of the moon”
Antonyms:
waxing
a gradual increase in magnitude or extent
Type of:
decrease, drop-off, lessening
a change downward

adj (of the Moon) pertaining to the period during which the visible surface of the moon decreases

“after full moon comes the waning moon”
Antonyms:
waxing
(of the moon) pertaining to the period during which the visible surface of the moon increases

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