Revelation 8:1

And when he opened

(κα οταν ηνοιξεν). Here modal αν is used with οτε (used about the opening of the preceding six seals), but οταν is not here rendered more indefinite, as is sometimes true (Mark 3:11; Revelation 4:9), but here and possibly (can be repetition) in Mark 11:19 it is a particular instance, not a general rule (Robertson, Grammar, p. 973).There followed a silence

(εγενετο σιγη). Second aorist middle of γινομα. "There came silence." Dramatic effect by this profound stillness with no elder or angel speaking, no chorus of praise nor cry of adoration, no thunder from the throne (Swete), but a temporary cessation in the revelations. See Revelation 10:4.About the space of half an hour

(ως ημιωρον). Late and rare word (ημ, half, ωρα, hour), here only in N.T. Accusative of extent of time.