Exegesis:

hōsper gar hē astrapē astraptousa… ‘for just as the lightning when it flashes….’ The comparison explains why the Son of man will not appear ‘here’ or ‘there’: his coming will be as sudden and as universally visible as the shining of the lightning, cf. Plummer and Lagrange.

astraptō (also 24.4, but with figurative meaning) ‘to flash,’ of the same root as astrapē and used here for that reason in order to stress the idea of very bright lightning. astraptousa is best taken by itself and not in connexion with the subsequent phrase that goes with lampei, see below.

ek tēs hupo ton ouranon eis tēn hup’ ouranon lampei ‘(the lightning) shines from one region under the sky to another under the sky,’ with chōran ‘region’ understood. ouranos means here ‘the sky that overarches the earth.’ Hence hupo ton ouranon virtually means ‘on earth,’ here, ‘on a part of the earth.’

houtōs estai ho huios tou anthrōpou [en tē hēmera autou] ‘so the Son of man will be in his day.’ en tē hēmera autou refers to the day of his glorious appearance in the future.

Translation:

The clause order may better be reversed, e.g. ‘the Son of man … will be like lightning that…’; similarly in vv. 26 and/or 28-30.

As the lightning flashes and lights up, or, ‘as the lightning, when it flashes, lights-up/shines.’ The combination of two rather similar verbs such as ‘to flash’ and ‘to light up’ may make necessary some adaptation, cf. “like the lightning-flash that lights up” (New English Bible), ‘as lightning shines flashingly’ (Leyden), or, with some simplification, ‘as the lightning that shines on, or, flashes over, or, lights up.’ Batak Toba uses here its specific term for ‘flash-of-lightning,’ cf. on 10.18.

The sky from one side to the other, preferably, ‘from one region/side/point under the sky to the other,’ “from one horizon to the other” (Translator’s New Testament, similarly Leyden, Brouwer), ‘over all(-that-is)-under-the-sky from end to end’ (Balinese, where ‘all-under-the-sky’ is a common expression for ‘the whole earth’), or simply, ‘across the sky’ (Navajo).

Quoted with permission from Reiling, J. and Swellengrebel, J.L. A Handbook on the Gospel of Luke. (UBS Handbook Series). New York: UBS, 1971. For this and other handbooks for translators see here . Make sure to also consult the Handbook on the Gospel of Mark for parallel or similar verses.