The Greek ekleipō means to fail, give out, or cease — used of failing resources, fading light, and the end of life. Jesus uses it in Luke 22:32 praying that Peter's 'faith may not fail (ekleipō).' Luke 23:45 uses it of the sun's light during the crucifixion — 'the sun was eclipsed' (ekleipō). The astronomical term 'eclipse' comes directly from this Greek root.
Two uses of ekleipō bracket the crucifixion in Luke's Gospel. Before: Jesus prays that Peter's faith will not ekleipō — fail utterly — under the coming trial. During: the sun itself ekleipō-s, as if creation mourns. The prayer and the sign interpret each other: just as the darkness was not permanent (resurrection came), so Peter's failure would not be final. The intercessory prayer of Christ sustains the faith of His people through their darkest hours. What ekleipō-s in human experience, Christ's prayer prevents from failing ultimately.