The umbra, penumbra and antumbra are three distinct parts of a shadow, created by any light source after impinging on an

opaque Opacity or opaque may refer to: * Impediments to (especially, visible) light: ** Opacities, absorption coefficients ** Opacity (optics), property or degree of blocking the transmission of light * Metaphors derived from literal optics: ** In lingui ...

object. Assuming no

diffraction Diffraction is defined as the interference or bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. The diffracting object or aperture effectively becomes a s ...

, for a

collimated beam A collimated beam of light or other electromagnetic radiation has parallel rays, and therefore will spread minimally as it propagates. A perfectly collimated light beam, with no divergence, would not disperse with distance. However, diffraction pr ...

(such as a point source) of light, only the umbra is cast. These names are most often used for the shadows cast by

celestial bodies An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists in the observable universe. In astronomy, the terms ''object'' and ''body'' are often us ...

, though they are sometimes used to describe levels, such as in

sunspots Sunspots are phenomena on the Sun's photosphere that appear as temporary spots that are darker than the surrounding areas. They are regions of reduced surface temperature caused by concentrations of magnetic flux that inhibit convection. ...

Umbra

The umbra (Latin for "shadow") is the innermost and darkest part of a shadow, where the light source is completely blocked by the occluding body. An observer within the umbra experiences a total

eclipse An eclipse is an astronomical event that occurs when an astronomical object or spacecraft is temporarily obscured, by passing into the shadow of another body or by having another body pass between it and the viewer. This alignment of three ce ...

. The umbra of a round body occluding a round light source forms a

right circular cone A cone is a three-dimensional geometric shape that tapers smoothly from a flat base (frequently, though not necessarily, circular) to a point called the apex or vertex. A cone is formed by a set of line segments, half-lines, or lines conn ...

. When viewed from the cone's apex, the two bodies appear the same size. The distance from the Moon to the apex of its umbra is roughly equal to that between the Moon and Earth: . Since

Earth's diameter Earth radius (denoted as ''R''🜨 or R_E) is the distance from the center of Earth to a point on or near its surface. Approximating the figure of Earth by an Earth spheroid, the radius ranges from a maximum of nearly (equatorial radius, deno ...

is 3.7 times the Moon's, its umbra extends correspondingly farther: roughly .

Penumbra

The penumbra (from the Latin ''paene'' "almost, nearly") is the region in which only a portion of the light source is obscured by the occluding body. An observer in the penumbra experiences a

partial eclipse An eclipse is an astronomical event that occurs when an astronomical object or spacecraft is temporarily obscured, by passing into the shadow of another body or by having another body pass between it and the viewer. This alignment of three ce ...

. An alternative definition is that the penumbra is the region where ''some or all'' of the light source is obscured (i.e., the umbra is a subset of the penumbra). For example, NASA's Navigation and Ancillary Information Facility defines that a body in the umbra is also within the penumbra.''Event Finding Subsystem Preview''
Navigation and Ancillary Information Facility. Earth umbral cone (partial)

Antumbra

The antumbra (from Latin ''ante'', "before") is the region from which the occluding body appears entirely within the disc of the light source. An observer in this region experiences an annular eclipse, in which a bright ring is visible around the eclipsing body. If the observer moves closer to the light source, the apparent size of the occluding body increases until it causes a full umbra.

References

{{Portal bar, Physics, Astronomy, Stars, Spaceflight, Outer space, Solar System, Science Optical phenomena Shadows

Umbra

Penumbra

Antumbra

See also

References