Moon
Moon Phases: Why the Moon Changes Shape
Understand waxing, waning, quarter phases, and illumination so you can make sense of the Moon at a glance.
Prepared with AI assistance using the primary sources linked below. How Astroboat works
One Moon, changing sunlight
The Moon reflects sunlight. As it travels around Earth, we see different portions of its sunlit half. A crescent is not a partly missing Moon: most of the hemisphere facing us is simply experiencing lunar night. Earth's shadow causes lunar eclipses, not the ordinary monthly phases.
The phase cycle takes about 29.5 days. The Moon takes less time to complete an orbit relative to the stars, but Earth also moves around the Sun, so the Moon must travel a little farther to repeat the same Sun-Earth-Moon arrangement.
Read the phase names
Waxing means the visible illuminated portion is increasing; waning means it is decreasing. Crescent means less than half the disk appears lit, while gibbous means more than half. At first and third quarter, approximately half the visible disk is sunlit; quarter refers to progress through the cycle.
New Moon places the Moon roughly toward the Sun in our sky. Full Moon places it roughly opposite. A phase name describes the lighting geometry, not a guarantee that the Moon is above your horizon.
Use the phase to plan a view
For an evening look, start with a waxing crescent or first-quarter Moon. For faint stars, choose a period when the Moon is below the horizon. A bright Moon is a worthwhile target itself, even when its light washes out nearby faint objects.
Open Astroboat's Moon page, note the illumination and location, and check the rise and set times before heading outside. Compare the view on several nights. Recording the date and a quick sketch makes the cycle much easier to recognize than memorizing eight labels.