Eclipse (source code)
= Eclipse
{wiki}
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>.<Why Do Eclipses Happen? by NASA>[Geometry: the inclined lunar orbit, eclipse seasons and the distinction between solar and lunar alignments]{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.
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