Product Code Database
The umbra, penumbra and antumbra are three distinct parts of a shadow, created by any light source after impinging on an opaque object of lesser size. In cases of equal or smaller impinging objects, only an umbra and penumba are generated. Assuming no diffraction, for a collimated beam (such as a point source) of light, only the umbra is cast.
These phenomena are generally observed within solar systems, as the size of the stars within the system are larger than the orbiting satellites, hence these terms are most often used for the shadows cast by celestial body, though they are sometimes used to describe levels of darkness, such as in sunspots.
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 is 3.7 times the Moon's, its umbra extends correspondingly farther: roughly .
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.
|
|
