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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 in the umbra experiences a total eclipse.
The penumbra (from the Latin paene "almost, nearly" and umbra "shadow") 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's Navigation and Ancillary Information Facility defines that a body in the umbra is also within the penumbra.
The antumbra (from Latin ante, 'before') is the region from which the occluding body appears entirely contained 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 he moves closer to the light source, the apparent size of the occluding body increases until it causes a full umbra.
- Event Finding Subsystem Preview Navigation and Ancillary Information Facility.
- Page 55 in: T. Hoell; International Symposium on Special Aspects of Radiotherapy 1998 berli; International Symposium on Spe; Hinkelbein, W.; Wiegel, T. (1999). Controversies in Neuro-Oncology: 3rd International Symposium on Special Aspects of Radiotherapy, Berlin, Germany, April 30-May 2, 1998 (Frontiers of Radiation Therapy and Oncology). S. Karger Publishers (USA). ISBN 3-8055-6834-7.