darkening

noun darkening

• penumbra — Astronomy. the partial or imperfect shadow outside the complete shadow of an opaque body, as a planet, where the light from the source of illumination is only partly cut off. Compare umbra (def 3a). the grayish marginal portion of a sunspot. Compare umbra (def 3b).
• obscuration — the act of obscuring.
• eclipse — Astronomy. the obscuration of the light of the moon by the intervention of the earth between it and the sun (lunar eclipse) or the obscuration of the light of the sun by the intervention of the moon between it and a point on the earth (solar eclipse) a similar phenomenon with respect to any other planet and either its satellite or the sun. the partial or complete interception of the light of one component of a binary star by the other.
• occultation — Astronomy. the passage of one celestial body in front of another, thus hiding the other from view: applied especially to the moon's coming between an observer and a star or planet.