Earth's shadow (or Earth shadow) is the

shadow A shadow is a dark area on a surface where light from a light source is blocked by an object. In contrast, shade occupies the three-dimensional volume behind an object with light in front of it. The cross-section of a shadow is a two-dimensio ...

that

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

itself casts through its atmosphere and into outer space, toward the antisolar point. During the

twilight Twilight is daylight illumination produced by diffuse sky radiation when the Sun is below the horizon as sunlight from the upper atmosphere is scattered in a way that illuminates both the Earth's lower atmosphere and also the Earth's surf ...

period (both early

dusk Dusk occurs at the darkest stage of twilight, or at the very end of astronomical twilight after sunset and just before nightfall.''The Random House College Dictionary'', "dusk". At predusk, during early to intermediate stages of twilight, enoug ...

and late

dawn Dawn is the time that marks the beginning of twilight before sunrise. It is recognized by the diffuse sky radiation, appearance of indirect sunlight being Rayleigh scattering, scattered in Earth's atmosphere, when the centre of the Sun's disc ha ...

), the shadow's visible fringe – sometimes called the dark segment or twilight wedge – appears as a dark and diffuse band just above the

horizon The horizon is the apparent curve that separates the surface of a celestial body from its sky when viewed from the perspective of an observer on or near the surface of the relevant body. This curve divides all viewing directions based on whethe ...

, most distinct when the

sky The sky is an unobstructed view upward from the planetary surface, surface of the Earth. It includes the atmosphere of Earth, atmosphere and outer space. It may also be considered a place between the ground and outer space, thus distinct from ...

is clear. Since the angular diameters of the Sun and the Moon as viewed from Earth's surface are almost the same, the ratio of the length of Earth's shadow to the distance between Earth and the Moon will be almost equal to the ratio of the diameters of Earth and the Moon. Since Earth's diameter is 3.7 times the Moon's, the length of the planet's umbra is correspondingly 3.7 times the average distance from the Moon to Earth: about . The diameter of Earth's shadow at

lunar distance The instantaneous Earth–Moon distance, or distance to the Moon, is the distance from the center of Earth to the center of the Moon. In contrast, the Lunar distance (LD or \Delta_), or Earth–Moon characteristic distance, is a unit of measure ...

is about , or 2.6 lunar diameters, which allows observation of total

lunar eclipse A lunar eclipse is an astronomical event that occurs when the Moon moves into the Earth's shadow, causing the Moon to be darkened. Such an alignment occurs during an eclipse season, approximately every six months, during the full moon phase, ...

s from Earth.

Appearance

Earth's shadow cast onto the atmosphere can be viewed during the "civil" stage of

, assuming the sky is clear and the

is relatively unobstructed. The shadow's fringe appears as a dark bluish to purplish band that stretches over 180° of the horizon opposite the

Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

, i.e. in the eastern sky at

and in the western sky at

. Before

sunrise Sunrise (or sunup) is the moment when the upper rim of the Sun appears on the horizon in the morning, at the start of the Sun path. The term can also refer to the entire process of the solar disk crossing the horizon. Terminology Although the S ...

, Earth's shadow appears to recede as the Sun rises; after

sunset Sunset (or sundown) is the disappearance of the Sun at the end of the Sun path, below the horizon of the Earth (or any other astronomical object in the Solar System) due to its Earth's rotation, rotation. As viewed from everywhere on Earth, it ...

, the shadow appears to rise as the Sun sets. Earth's shadow is best seen when the horizon is low, such as over the sea, and when the sky conditions are clear. In addition, the higher the observer's

elevation The elevation of a geographic location (geography), ''location'' is its height above or below a fixed reference point, most commonly a reference geoid, a mathematical model of the Earth's sea level as an equipotential gravitational equipotenti ...

is to view the horizon, the sharper the shadow appears.

Belt of Venus

A related phenomenon in the same part of the sky is the Belt of Venus, or anti-twilight arch, a pinkish band visible above the bluish shade of Earth's shadow, named after the

planet A planet is a large, Hydrostatic equilibrium, rounded Astronomical object, astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself. The Solar System has eight planets b ...

Venus Venus is the second planet from the Sun. It is often called Earth's "twin" or "sister" planet for having almost the same size and mass, and the closest orbit to Earth's. While both are rocky planets, Venus has an atmosphere much thicker ...

which, when visible, is typically located in this region of the sky. No defined line divides the Earth's shadow and the Belt of Venus; one colored band blends into the other in the sky. The Belt of Venus is quite a different phenomenon from the

afterglow An afterglow in meteorology consists of several atmospheric optical phenomena, with a general definition as a broad arch of whitish or pinkish sunlight in the twilight sky, consisting of the bright segment and the purple light. Purple light mai ...

, which appears in the geometrically opposite part of the sky.

Color

When the Sun is near the horizon around

, the sunlight appears reddish. This is because the light rays are penetrating an especially thick layer of the atmosphere, which works as a filter,

scattering In physics, scattering is a wide range of physical processes where moving particles or radiation of some form, such as light or sound, are forced to deviate from a straight trajectory by localized non-uniformities (including particles and radiat ...

all but the longer (redder) wavelengths. From the observer's perspective, the red sunlight directly illuminates small particles in the lower atmosphere in the sky opposite of the Sun. The red light is

backscatter In physics, backscatter (or backscattering) is the reflection of waves, particles, or signals back to the direction from which they came. It is usually a diffuse reflection due to scattering, as opposed to specular reflection as from a mirror, ...

ed to the observer, which is the reason why the Belt of Venus appears pink. The lower the setting Sun descends, the less defined the boundary between Earth's shadow and the Belt of Venus appears. This is because the setting Sun now illuminates a thinner part of the upper atmosphere. There the red light is not scattered because fewer particles are present, and the eye only sees the "normal" (usual) blue sky, which is due to

Rayleigh scattering Rayleigh scattering ( ) is the scattering or deflection of light, or other electromagnetic radiation, by particles with a size much smaller than the wavelength of the radiation. For light frequencies well below the resonance frequency of the scat ...

from air molecules. Eventually, both Earth's shadow and the Belt of Venus dissolve into the darkness of the

night sky The night sky is the nighttime appearance of celestial objects like stars, planets, and the Moon, which are visible in a clear sky between sunset and sunrise, when the Sun is below the horizon. Natural light sources in a night sky include moonlig ...

Color of lunar eclipses

Earth's shadow is as curved as the planet is, and its umbra extends into outer space. (The

antumbra The umbra, penumbra and antumbra are three distinct parts of a shadow, created by any light source after impinging on an Opacity (optics), opaque object of lesser size. In cases of equal or smaller impinging objects, only an umbra and penumba ...

, however, extends indefinitely.) When the

, Earth, and the

Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...

are aligned perfectly (or nearly so), with Earth between the Sun and the Moon, Earth's shadow falls onto the lunar surface facing the night side of the planet, such that the shadow gradually darkens the full Moon, causing a

. Even during a total lunar eclipse, a small amount of sunlight however still reaches the Moon. This indirect sunlight has been

refracted In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenome ...

as it passed through

Earth's atmosphere The atmosphere of Earth is composed of a layer of gas mixture that surrounds the Earth's planetary surface (both lands and oceans), known collectively as air, with variable quantities of suspended aerosols and particulates (which create weathe ...

. The air molecules and

particulates Particulate matter (PM) or particulates are microscopic particles of solid or liquid matter suspension (chemistry), suspended in the atmosphere of Earth, air. An ''aerosol'' is a mixture of particulates and air, as opposed to the particulate ...

in Earth's atmosphere scatter the shorter wavelengths of this sunlight; thus, the longer wavelengths of reddish light reaches the Moon, in the same way that light at

appears reddish. This weak red illumination gives the eclipsed Moon a dimly reddish or

copper Copper is a chemical element; it has symbol Cu (from Latin ) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orang ...

color.

References

{{Reflist, 2, refs= {{cite web, title=Earth's shadow, publisher = www.atoptics.co.uk , author =Les Cowley, date = 16 September 2023 , url = http://www.atoptics.co.uk/atoptics/earshad.htm {{cite book, title=Color and light in nature, author= David K. Lynch, William Charles Livingston, url = https://books.google.com/books?id=4Abp5FdhskAC&pg=PA38, pages = 38, 39, publisher=Cambridge University Press, edition=2nd , date=2001, isbn=978-0521775045 {{cite web, title=What causes layers in the sunrise and sunset?, publisher = earthsky.org , url = http://www.webexhibits.org/causesofcolor/14E.html

External links