On Tuesday, Sept. 29, the Moon sits in a waning gibbous phase with 92 percent of its surface visible, following closely behind the recent Harvest Moon. As the lunar cycle progresses toward the upcoming New Moon over a 29.5-day orbit, observers can easily spot prominent geological features like the Tycho Crater and Mare Imbrium without optical aids.
The Waning Gibbous Phase on September 29
A few days past the September full moon cycle, Earth’s natural satellite begins its gradual descent in visibility. NASA’s Daily Moon Guide tracks the lunar surface at 92 percent illumination for this date.
The changing geometry of the Earth-Moon-Sun system drives this entire sequence. As the Moon completes its roughly 29.5-day orbit, viewers witness the transition from a fully illuminated disk through various gibbous and quarter phases until it reaches the dark New Moon.
Observing Lunar Features With and Without Equipment
Stargazers stepping outside tonight do not necessarily need high-end optics to explore the lunar surface. Unaided eyes can readily pick out massive basaltic plains and impact structures. Prominent maria, including Mare Imbrium and Mare Serenitatis, stand out as dark volcanic plains against the brighter highlands.
Stepping up to binoculars opens up a tighter view of rugged topography. Observers equipped with basic binoculars can resolve the Clavius Crater, the Alphonsus Crater, and the sharp ridges of the Alps Mountains. Meanwhile, amateur astronomers utilizing telescopes gain access to historical exploration sites. A modest telescope reveals the Apollo 11 and Apollo 17 landing locations, alongside the complex terrain of the Fra Mauro Highlands.
The Progression of the Lunar Calendar
Following the waning gibbous and subsequent third quarter phases, the Moon will shrink into a waning crescent before disappearing entirely during the New Moon. The cycle then resets as a waxing crescent builds toward the next Full Moon, which is scheduled to take place on either October 25 or October 26, depending on the observer’s specific timezone.
