6:12I watched as he opened the sixth seal. There was a great earthquake. The sun turned black like sackcloth made of goat hair, the whole moon turned blood red,
I watched as he opened the sixth seal.
These three things along with the next two verses appear to be all one event. It may be that a great impact from outer space will cause the earth (all of it) to shake so violently that volcanoes erupt everywhere spewing ash throughout the atmosphere. This would darken the sun and at night make the moon appear blood red.
When God moves to "talk" to people, these types of natural phenomena appear (Exodus 19:16-25). Also note that repentance and consecration are required before entering the presence of God.
The Old Testament prophets also saw, in visions and dreams, similar events. Jesus refers to the signs to take place in the sun, moon, and stars. Here are a few references: