Glossary term: Phase

Description: In astronomy, phase refers to the state of partial illumination of a moon or planet with respect to a specific observer. A distant light source typically illuminates only half of a spherical body's surface; the rest remains dark. Similarly, only about half of a spherical body's surface is within a distant observer's sight. The phase specifies which fraction of the surface within the observer's sight is illuminated. It changes as the relative positions of the object, the observer, and the light source change.

The best-known examples are the phases of the Moon. The relative positions of the Moon, of the Earth as the location of the observer, and of the Sun as the light source, change as the Moon orbits the Earth over the course of about one month. Hence, over that time, an observer on Earth will see different phases of the Moon. The phase where all of the Moon's surface that is in the observer's sight is illuminated is called "full Moon"; when no illuminated surface regions are visible, we have "new Moon". The two half-illuminated phases are called "first" or "second quarter", respectively. Less-than-half illumination makes for a "crescent (Moon)".

Phases are also seen in planets in the Solar System (particularly prominent for Mercury and Venus), and have been inferred for exoplanet systems. For the Moon, even non-illuminated regions are not completely dark: they reflect light reaching them from Earth, in a phenomenon known as Earthshine, first documented by Leonardo da Vinci.

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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

Venus changes from being small and almost full to being large with only a thin crescent illuminated.

A Matter of Perspective

Caption: This exquisite series of images, captured from Santo Domingo, Dominican Republic, between 17 December 2019 and 25 May 2020, shows the phases of Venus as seen from Earth. As Venus and Earth orbit the Sun, we observe different portions of Venus’s sunlit half, similar to the Moon’s phases. The sequence clearly shows Venus as distant, small, bright and gibbous in the lower frames, and ends with Venus reaching the biggest apparent size of all planets (upper frames), very close to the Sun with a small elongation, and appearing as a thin crescent. In the last frame, only 2.8% of the planet’s surface is illuminated.
Credit: Christofer Baez/IAU OAE (CC BY 4.0)

License: CC-BY-4.0 Creative Commons Attribution 4.0 International (CC BY 4.0) icons

At the top Venus is full and appears small. As the images progress it moves to half, crescent and new, appearing larger.

Phases of Venus

Caption: This is a mesmerising series of images of Venus captured from Surgères, Charente-Maritime, France, over a period of six months in 2015. The phases appear similar to the phases we see of our own Moon and occur for similar reasons. Only half of Venus is illuminated by the Sun and, from Earth, we can sometimes only see part of that illuminated half, depending on the relative positions of the Sun, Earth and Venus. Both Mercury and Venus exhibit phases because their orbit is between the Sun and the orbit of Earth. Depending on the position of Venus relative to the Sun and Earth, Venus goes through its phases over a period of time. This sequence of images beautifully showcases the transition from the ‘gibbous’ to the slender crescents. The use of infrared filters helped to capture Venus's dense perpetual cloud cover during daylight in sharp detail, providing a glimpse into the mysterious nature of the planet’s atmosphere.
Credit: Stephane Gonzales/IAU OAE (CC BY 4.0)

License: CC-BY-4.0 Creative Commons Attribution 4.0 International (CC BY 4.0) icons

Mercury appears smaller and fainter than Venus. The bottom right of Venus is an illuminated crescent.

Venus and Mercury Trails

Caption: In this composite image, both Mercury (left) and Venus (right) can be seen heading into the sunset. The phases of each are beautifully captured as they descend. Not all planets or moons in the Solar System show phases as viewed from Earth. This phenomenon occurs because the orbits of Venus and Mercury are positioned between Earth’s orbit and the Sun, sometimes allowing us to see only part of the illuminated portion of each planet. These phases are similar to the phases we see of our own Moon.
Credit: Marcella Giulia Pace (CC BY 4.0)

License: CC-BY-4.0 Creative Commons Attribution 4.0 International (CC BY 4.0) icons