Umbra, penumbra and antumbra

Umbra, penumbra and antumbra of Earth and images that could be seen at some points in these areas (Note: the relative size and distance of the bodies shown are not to scale.) “… The Earth’s shadow has two distinct parts,… the UMBRA is the part of the shadow where all direct sunlight is blocked by the Earth; the PENUMBRA of the shadow is where the Earth only blocks some of the sunlight.”

Umbra (A) and penumbra (B)

Theumbra,penumbraandantumbraare three distinct parts of ashadow,created by any light source after impinging on anopaqueobject. Assuming nodiffraction,for acollimated beam(such as a point source) of light, only the umbra is cast.

These names are most often used for the shadows cast bycelestial bodies,though they are sometimes used to describe levels, such as insunspots.

Umbra

"Umbra" redirects here. For other uses, seeUmbra (disambiguation).

Umbra, penumbra, and antumbra formed through windows and shutters

Theumbra(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 totaloccultation.The umbra of a round body occluding a round light source forms aright circular cone.When viewed from the cone'sapex,the two bodiesappear the same size.

Thedistancefrom theMoonto the apex of its umbra is roughly equal to that between the Moon andEarth:384,402 km (238,856 mi). SinceEarth's diameteris 3.7 times the Moon's, its umbra extends correspondingly farther: roughly 1.4 million km (870,000 mi).

Penumbra

"Penumbra" redirects here. For other uses, seePenumbra (disambiguation).

Thepenumbra(from the Latinpaene"almost, nearly" andumbra"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 apartial eclipse. An alternative definition is that the penumbra is the region wheresome or allof the light source is obscured (i.e., the umbra is a subset of the penumbra). For example,NASA'sNavigation and Ancillary Information Facilitydefines that a body in the umbra is also within the penumbra.

Scale diagram ofEarth's shadow,showing how the umbral cone extends beyond theorbit of the MoonThe Moon is indicated by the yellow dot.)

Earth's umbra, as seen during a partial lunar eclipse

Antumbra

Transit of Mercuryin front of the Sun, an extreme version of an annular eclipse. Mercury is visible as a black dot below and to the left of the center. The dark area above the center of the solar disk is asunspot.

Theantumbra(from the Latinante"before" andumbra"shadow" ) is the region from which the occluding body appears entirely within the disc of the light source. An observer in this region experiences anannular 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 shadow