Earth phase (appearance of the Sun‑lit Earth from the Moon and space)
The Earth phase is the changing shape of the Sun‑illuminated portion of Earth as seen from the Moon or spacecraft. It cycles with the lunar month, produces earthshine and is complementary to lunar phases.
Overview
The Earth phase describes the portion of Earth's disk that is illuminated by the Sun as seen from an external vantage point such as the Moon or a spacecraft. From the lunar surface an observer sees the Earth go through a sequence of phases—full, gibbous, quarter, crescent and new—that change gradually as the relative positions of Sun, Earth and Moon shift. These phases are the counterpart of the Moon's phases seen from Earth: when the Moon appears full to observers on Earth, an observer on the Moon sees Earth near new, and vice versa. Sunlit portion of Earth
Image gallery
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Geometry and cycle
The pattern of Earth phases is governed by simple geometry: the fraction of Earth illuminated depends on the angle formed by Sun–Earth–observer. From the Moon the complete cycle takes roughly one synodic month (about 29.5 days) because it follows the same alignment changes that produce lunar phases. The illuminated fraction varies smoothly as the Moon orbits Earth and as the Earth–Moon system moves around the Sun. View from the Moon illustrates this complementary relationship.
Appearance and brightness
Because the Earth is larger and has a higher average reflectivity than the Moon, it appears considerably brighter and larger from the lunar surface. The Earth's apparent diameter from the Moon is several times that of the Moon when seen from Earth, and cloud patterns, oceans and continents produce striking changes in brightness and color across the disk. A familiar consequence of bright Earth phases is "earthshine": sunlight reflected from Earth that faintly illuminates the dark part of the Moon and is visible to observers on Earth as a soft glow on a crescent Moon.
Historical and observational notes
Apollo astronauts famously photographed an "Earthrise" and the fully illuminated blue marble of Earth, showing phases and weather systems. Ground-based observers and astronomers also study Earthshine to infer global cloud cover and surface properties; spectral analyses of reflected Earth light have been used as an analogue when developing techniques to detect and characterize Earth-like exoplanets. Spacecraft at different vantage points record Earth phases at a range of scales and distances. Orbital positions determine what part of the Earth is sunlit and visible.
Uses, examples and notable distinctions
Understanding Earth phases is useful for remote sensing, for planning lunar operations (illumination affects thermal and power conditions) and for public outreach: the complementary nature of lunar and Earth phases is a clear pedagogical example of celestial geometry. The term "Terra phase" or "phase of the Earth" is sometimes used interchangeably, but all describe the same phenomenon of changing illumination as seen from an external viewpoint. For additional context and diagrams, see further resources.
- Key point: Earth phases from the Moon are opposite to lunar phases seen from Earth.
- Key point: Earthshine links the two phenomena and provides scientific information about Earth's reflectivity.
Questions and answers
Q: What is the Earth phase?
A: The Earth phase, also known as the Terra phase, Terrestrial phase or phase of the Earth, is the shape of the directly sunlit portion of the Earth as viewed from the Moon (or elsewhere extraterrestrially).
Q: How often does it change?
A: The Earth phases gradually and cyclically change over a period of about a month.
Q: What causes it to change?
A: The changes in the Earth phases are caused by shifts in orbital positions of both the Moon around the Earth and of the Earth around the Sun.
Q: From where can we view this phenomenon?
A: We can view this phenomenon from either from the Moon or from other extraterrestrial locations.
Q: Is there any other name for this phenomenon?
A: Yes, it is also referred to as Terra phase, Terrestrial phase or phase of the Earth.
Q: How long does it take for one cycle to complete?
A: One cycle takes approximately one month to complete.
Related articles
Author
AlegsaOnline.com Earth phase (appearance of the Sun‑lit Earth from the Moon and space) Leandro Alegsa
URL: https://en.alegsaonline.com/art/29567
Sources
- livescience.com : "If You're On the Moon, Does the Earth Appear to Go Through Phases?"
- futurism.com : "The Phases of Earth"
- earthsky.org : "Changing Earth phases, seen from the moon"
- skyandtelescope.com : "Observing Earth from the Moon"
- scienceline.ucsb.edu : "If you are on the moon, does the Earth have phases similar to moon phases? Would they be the same or reversed?"
- universetoday.com : "What Does Earth Look Like From the Moon?"
- snopes.com : "Is This a Real Photo of the Earth from the Moon? - Genuine photographs showing the Earth from the moon exist, many from NASA"
- hq.nasa.gov : "Return to Orbit"