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The umbra, penumbra and antumbra are three distinct parts of a
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. The umbra of a round body occluding a round light source forms a
''Event Finding Subsystem Preview''
Navigation and Ancillary Information Facility.
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.
The umbra, penumbra and antumbra are three distinct parts of a shadow
A shadow is a dark area where light from a light source is blocked by an opaque object. It occupies all of the three-dimensional volume behind an object with light in front of it. The cross section of a shadow is a two- dimensional silhouett ...
, 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, 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, diffractio ...
(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, 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. 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 co ...
. When viewed from the cone's apex, the two bodies appear the same size.
The distance
Distance is a numerical or occasionally qualitative measurement of how far apart objects or points are. In physics or everyday usage, distance may refer to a physical length or an estimation based on other criteria (e.g. "two counties over"). ...
from the Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
to the apex of its umbra is roughly equal to that between the Moon and Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's sur ...
: . Since Earth's diameter 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 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
The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research.
NASA was established in 1958, succeedin ...
's Navigation and Ancillary Information Facility defines that a body in the umbra is also within the penumbra.Navigation and Ancillary Information Facility.
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.
See also
* Antisolar point * Earth's shadowReferences
{{Portal bar, Physics, Astronomy, Stars, Spaceflight, Outer space, Solar System, Science Optical phenomena Shadows