By Chris Deziel, Updated Mar 24, 2022
Dean Pennala/Hemera/Getty Images
As the Moon completes its 27.3‑day orbit around Earth, its illuminated fraction changes daily. When the visible portion grows, the Moon is waxing; when it shrinks, it is waning. Determining the direction is straightforward regardless of your location on Earth.
If you see the Moon near sunset, it is waxing. After a full Moon, the waning phase begins and the Moon becomes invisible at sunset. As the cycle progresses toward new Moon, the Moon rises later each night, ultimately appearing just before sunrise during the waning crescent.
In the Northern Hemisphere, the dark portion of a waxing Moon lies on its left; for a waning Moon it lies on its right. In the Southern Hemisphere the orientation is reversed: the shadow on the right during waxing and on the left during waning.
When cloud cover obscures the Moon, many local newspapers and weather apps include the current phase. This can confirm whether the Moon is waxing or waning.
The most reliable cue is the Moon’s relationship to sunset. Visible at sunset = waxing; invisible after sunset = waning.
For more scientific background, see NASA’s Moon facts page: https://moon.nasa.gov/facts/.
