Jamieson, Fausset and Brown on Revelation 6:12

12. As Re 6:4, 6-8, the sword, famine, and pestilence, answer to Mt 24:6, 7; Re 6:9, 10, as to martyrdoms, answer to Mt 24:9, 10; so this passage, Re 6:12, 17, answers to Mt 24:29, 30, "the sun shall be darkened, and the moon shall not give her light, and the stars shall fall from heaven; ... then shall all the tribes of the earth mourn, and they shall see the Son of man coming"; imagery describing the portents of the immediate coming of the day of the Lord; but not the coming itself until the elect are sealed, and the judgments invoked by the martyrs descend on the earth, the sea, and the trees (Re 7:1-3).

and, lo--So A reads. But B and C omit "lo."

earthquake--Greek, "shaking" of the heavens, the sea, and the dry land; the shaking of these mutable things being the necessary preliminary to the setting up of those things which cannot be shaken. This is one of the catchwords [Wordsworth] connecting the sixth seal with the sixth trumpet (Re 11:13) and the seventh vial (Re 16:17-21); also the seventh seal (Re 8:5).

sackcloth--One kind, made of the "hair" of Cilician goats, was called "cilicium," or Cilician cloth, and was used for tents, &c. Paul, a Cilician, made such tents (Ac 18:3).

moon--A, B, C, and oldest versions read, "the whole moon"; the full moon; not merely the crescent moon.

as blood--(Joe 2:31).