On the evening of March 13, observers worldwide will see the Moon illuminated as a slender waning crescent. This visible phase is a standard stage in the monthly lunar cycle, occurring as the Moon’s orbit positions it between Earth and the Sun. The event is significant for astronomers, photographers, and skywatchers, providing a predictable celestial marker and an opportunity to view a darkened lunar surface illuminated by earthshine.
The Lunar Cycle’s Current Phase
The crescent shape visible on March 13 indicates the Moon is approaching the New moon phase. During this waning crescent stage, the illuminated portion of the Moon visible from Earth decreases each night. This change occurs because sunlight falls on a diminishing area of the lunar hemisphere that faces our planet.
The Moon’s phases are caused by its changing position relative to Earth and the Sun. As the Moon orbits Earth, different portions of its sunlit side become visible. The waning crescent phase directly precedes the New Moon, when the Moon’s sunlit side faces completely away from Earth.
Scientific Explanation of the Phenomenon
The apparent slimming of the Moon is not due to a change in the Moon itself, but a change in viewer perspective from Earth. The term “waning” specifically refers to the decreasing visibility of the illuminated area. The cycle from New Moon to Full Moon and back takes approximately 29.5 days, a period known as a synodic month.
During a waning crescent, only a small sliver on the Moon’s left side (in the Northern Hemisphere) is bright. The rest of the Moon’s face is dimly visible due to earthshine, which is sunlight reflected from Earth onto the lunar surface. This phase is best viewed in the eastern sky before sunrise.
Observation and Cultural Relevance
The progression of moon phases has been tracked by human civilizations for millennia for calendrical, agricultural, and religious purposes. The predictable nature of the cycle allows for accurate long-term forecasting of lunar appearances. Modern astronomy uses these phases for various research purposes, including studies of the lunar surface’s temperature changes.
For casual observers, a thin crescent Moon presents a striking visual, especially when it appears near planets or bright stars in the dawn or dusk sky. The visibility and timing of the crescent’s appearance can vary slightly based on the observer’s exact geographic location.
Next Steps in the Lunar Cycle
Following the crescent phase on March 13, the Moon will continue to wane until it reaches the New Moon phase. During New Moon, the Moon will be positioned between Earth and the Sun, with its dark side facing Earth, making it virtually invisible in the night sky. After the New Moon, the Moon will enter the waxing crescent phase, beginning a new cycle of increasing visibility. Astronomers and almanacs publish precise timetables for these phases, which are calculated years in advance based on the Moon’s orbital mechanics.
