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An eclipse occurs when an alignment of astronomical bodies obscures a source of light or places an illuminated body in another body's shadow. During a solar eclipse, the Moon interrupts the view of the Sun; during a lunar eclipse, the Moon enters Earth's shadow.

The monthly sequence of Moon phases is insufficient to produce monthly eclipses. The Moon's orbital inclination usually carries it above or below the required alignment. Eclipses become possible when the relevant phase occurs sufficiently near an orbital node.^[ref]

The finite apparent size of the Sun makes shadow geometry richer than a single dark cone: the umbra excludes the entire solar disk, whereas the penumbra excludes only part. An extended lamp and an opaque ball reproduce this distinction on a screen, allowing the shadow's sharp central region and softer surroundings to be compared without astronomical distances.

A penumbra is a region behind an opaque object from which only part of an extended source of light is visible. It contrasts with the umbra, where the entire source is blocked. During a solar eclipse, an observer in the lunar penumbra sees a partial solar eclipse.^[ref]

An extended lamp, an opaque ball and a screen make the mechanism visible: different points of the lamp cast displaced shadows. Where only some of those shadows overlap, the screen receives reduced irradiance. Enlarging the lamp generally broadens the soft transition, whereas an ideal point source gives a sharp geometric boundary. The penumbra is a consequence of source extent even in geometrical optics; it does not require diffraction.

A solar eclipse occurs when the Moon passes in front of the Sun for an observer on Earth. The same event appears different from different locations: an observer in the umbra sees totality, while an observer in the penumbra sees only part of the solar disk covered.

If the Moon's apparent angular diameter is too small to cover the Sun, the central alignment produces an annular eclipse instead. The comparison is between apparent sizes, which depend on both physical size and Euclidean distance. During totality the normally overwhelming solar disk is hidden, exposing the much fainter solar corona and allowing observations of nearby background stars.^[ref]

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.