penumbra

/pəˈnʌmbrə/

/pɪˈnʌmbrə/

Other forms: penumbras; penumbrae

When you measure your shadow to calculate the angle of the sun in the sky, be sure to measure to the edges of your shadow, to the penumbra, the part of a shadow that is not as dark as the center.

You're likely to come across penumbra most often in astronomy, as with an eclipse, where shadow is a defining feature. In a lunar eclipse, the edge of the earth's shadow — the part that isn't fully dark — is its penumbra. Sun spots also have a penumbra, the outer edge that's not quite as dark as the center. The word comes from the Latin root umbra, which means "shadow." The pen part means "almost," so a penumbra is "almost shadow."

Definitions of penumbra

1. noun

   a fringe region of partial shadow around an umbra
   type of:

   shadow

   a dark shape created by an object blocking a source of light