Glossary term: Lunar Phase

Also known as moon phase
Redirected from Waning Gibbous

Description: The changing shape of the visible portion of the Moon illuminated by the Sun is known as a lunar phase. Half of the Moon is always, apart from during lunar eclipses, illuminated by the Sun. On Earth we see different parts of the Moon illuminated as it moves in its orbit around us. The lunar month starts and ends at the same phase. For example, if we start from a new moon, due to the revolution of the Moon around Earth (taking approximately a month, 29.5 days) the size of the illuminated part of the Moon gradually increases (waxing phase) becoming a crescent. The half moon occurs at 90 degrees from the start point. The illuminated portion of the Moon continues to increase, becoming gibbous (convex-shaped, or bulging-shaped). Full moon occurs at 180 degrees. After this point, the shape gradually starts decreasing (waning phase), resulting in a gibbous moon, the half moon phase at 270 degrees from the start, the crescent moon, and ending as a new moon at 360 degrees. Even though the Moon shows half phase at 90 and 270 degrees, the completely opposite sides are illuminated.

Related Terms:

See this term in other languages

Term and definition status: This term and its definition have been approved by a research astronomer and a teacher

The OAE Multilingual Glossary is a project of the IAU Office of Astronomy for Education (OAE) in collaboration with the IAU Office of Astronomy Outreach (OAO). The terms and definitions were chosen, written and reviewed by a collective effort from the OAE, the OAE Centers and Nodes, the OAE National Astronomy Education Coordinators (NAECs) and other volunteers. You can find a full list of credits here. All glossary terms and their definitions are released under a Creative Commons CC BY-4.0 license and should be credited to "IAU OAE".

Related Media

The nearly full Moon with craters, light highlands and dark plains

Full moon

Caption: The image shows the nearly full Moon observed with a small telescope and a DSLR camera.
Credit: Luc Viatour credit link

License: CC-BY-SA-3.0 Creative Commons Attribution-ShareAlike 3.0 Unported icons