A.T. Robertson Commentary Revelation 6:12

"And I saw when he opened the sixth seal, and there was a great earthquake; and the sun became black as sackcloth of hair, and the whole moon became as blood;" — Revelation 6:12 (ASV)

There was a great earthquake (σεισμος μεγας εγενετο). "There came a great earthquake." Jesus spoke of earthquakes in his great eschatological discourse . In Mt 24:29 the powers of the heavens will be shaken. Σεισμος is from σειω, to shake, and occurs also in Re 8:5; 11:13,19; 16:18. The reference is not a local earthquake like those so common in Asia Minor.

As sackcloth of hair (ως σακκος τριχινος). Σακκος (Attic σακος), Latin saccus, English

sack, originally a bag for holding things (Genesis 42:25,35), then coarse garment of hair (τριχινος, old word from θριξ, here only in N.T.) clinging to one like a sack, of mourners, suppliants, prophets leading austere lives (Matthew 3:4; Matthew 11:21; Luke 10:13). Here the hair is that of the black goat (Isaiah 50:3). Cf. Joel 2:10; Ezekiel 32:7f.; Isaiah 13:10; Mr 13:24f. See Ec 12:2 for eclipses treated as symbols of old age. Apocalyptic pictures all have celestial phenomena following earthquakes.

As blood (ως αιμα). In Ac 2:20 we find Peter interpreting the apocalyptic eschatological language of Joe 2:31 about the sun being turned into darkness and the moon into blood as pointing to the events of the day of Pentecost as also "the great day of the Lord." Peter's interpretation of Joel should make us cautious about too literal an exegesis of these grand symbols.