Lunar phase

A lunar phase is the changing apparent shape of the Moon’s sunlit portion as seen from Earth. It results from the Moon’s changing position relative to Earth and the Sun.

Synodic month: The average time between successive identical lunar phases, about 29.53 days. It measures the complete cycle from one lunar phase back to the same phase.
Earth–Moon system: The gravitationally bound system consisting chiefly of Earth and its natural satellite, the Moon. Lunar phases arise from the changing geometry within this system and its orientation to the Sun.
Waxing crescent: The phase after new moon when the visible sunlit portion is less than half and increasing. It shows the illuminated portion growing as the Moon moves away from new moon.
Lunar calendar: A calendar whose months follow the phases of the Moon. Its months commonly begin near new moon or the first visible crescent.
Earthshine: Sunlight reflected from Earth that faintly illuminates the Moon’s night side. It can reveal the dark portion of a crescent without changing the phase geometry.
Solar illumination: The lighting of an object by sunlight. Sunlight always illuminates roughly half the Moon; viewing geometry determines how much is visible.
Ecliptic: The apparent annual path of the Sun across the sky, corresponding approximately to Earth’s orbital plane. The Moon’s orbit is tilted to the ecliptic, so phase cycles do not usually produce eclipses.
First quarter: The phase when approximately half of the Moon’s visible disk is illuminated and the visible portion is increasing. It is about one quarter of the way through the phase cycle, despite its half-lit appearance.
Islamic calendar: A lunar calendar of twelve months used to determine Islamic dates and observances. Month beginnings are traditionally tied to sighting the new crescent.
Lunar day: The interval between successive appearances of the Sun at the same location on the Moon, about 29.5 Earth days. Its length resembles the phase cycle, but it describes local solar time on the Moon.

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