The Bible speaks of three heavens: the first heaven, the second heaven, and the third heaven. We know that there are three heavens because the apostle Paul said in II Corinthians 12:2,
“I knew a man in Christ above fourteen years ago, (whether in the body, I cannot tell; or whether out of the body, I cannot tell: God knoweth;) such an one caught up to the third heaven.”
If there is a “third heaven,” then there must be a “first heaven” and a “second heaven.” There couldn't be a “third heaven” if there wasn't a “first heaven” and a “second heaven.”
The first heaven is the atmosphere around our earth. It extends from the surface of the earth to about 100 kilometers above the earth (where the Karman line is). This includes the troposphere (11 kilometers), the stratosphere (39 kilometers), the mesosphere (35 kilometers), and the first 15 kilometers of the thermosphere. This is where the birds fly, airplanes fly, the clouds are, weather balloons are, etc.
The second heaven is what we call “outer space” or “space” for short. It begins where the earth's atmosphere ends (at about 100 kilometers above the earth, where the Karman line is) and extends outward. This is where our moon is, the planets are, the stars are, etc.
The third heaven is, obviously, where God is. This is where the apostle Paul was “caught up to.” And even though the third heaven has always existed—just like God has always existed—it has not always been called “the third heaven.” It only began being called “the third heaven” when “God created the heavens (the first heaven and the second heaven) and the earth” (Genesis 1:1) about 4½ billion years ago.
Whenever “Heaven” is referred to, and it is clear from the context that “the third heaven” is being referred to, the word “Heaven” should be capitalized because the word “Heaven” is a proper noun that is the name of a specific place (the third heaven).
But if the Bible says “heaven” or “heavens” and it is clear from the context that it is not specifically talking about “the third heaven,” the words “heaven” or “heavens” are being used generically and should not be capitalized.
The Bible says that Heaven (the third heaven) is north of the earth (Psalm 48:2; 75:6–7; Isaiah 14:13). It is not visible to us because it is in the spiritual realm.
(more to come)