Umbra

An umbra is the region from which an opaque intervening object blocks the whole visible disk of an extended source of light. In the lunar umbra, an observer on Earth can see a total solar eclipse; the surrounding penumbra gives only partial coverage.^[ref]

For a spherical source of radius $R_s$ larger than an opaque body's radius $R_o$, separated by centre distance $d$, the common tangent rays meet on the axis behind the body atThis finite cone explains why a smaller occulting body need not produce totality at every distance. Beyond its tip the source can surround the occulting disk, producing an annular alignment. The umbra excludes direct source rays; it need not be perfectly black when scattered or refracted light enters the region.