Eclipse - Umbra, Penumbra and Antumbra

Umbra, Penumbra and Antumbra

The region of the Moon's shadow in a solar eclipse is divided into three parts:

  • The umbra, within which the Moon completely covers the Sun (more precisely, its photosphere).
  • The antumbra, extending beyond the tip of the umbra, within which the Moon is completely in front of the Sun but too small to completely cover it.
  • The penumbra, within which the Moon is only partially in front of the Sun.

During a lunar eclipse only the umbra and penumbra are applicable. This is because Earth's apparent diameter from the viewpoint of the Moon is nearly 4 times that of the Sun.

The first contact occurs when the Moon's disc first starts to impinge on the Sun's; second contact is when the Moon's disc moves completely within the Sun's; third contact when it starts to move out of the Sun's; and fourth or last contact when it finally leaves the Sun's disc entirely.

The same terms may be used analogously in describing other eclipses, e.g., the antumbra of Deimos crossing Mars, or Phobos entering Mars's penumbra.

A total eclipse occurs when the observer is within the umbra, an annular eclipse when the observer is within the antumbra, and a partial eclipse when the observer is within the penumbra.

For spherical bodies, when the occulting object is smaller than the star, the length (L) of the umbra's cone-shaped shadow is given by:

where Rs is the radius of the star, Ro is the occulting object's radius, and r is the distance from the star to the occulting object. For Earth, on average L is equal to 1.384×106 km, which is much larger than the Moon's semimajor axis of 3.844×105 km. Hence the umbral cone of the Earth can completely envelop the Moon during a lunar eclipse. If the occulting object has an atmosphere, however, some of the luminosity of the star can be refracted into the volume of the umbra. This occurs, for example, during an eclipse of the Moon by the Earth—producing a faint, ruddy illumination of the Moon even at totality.

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