In the simplest cosmologies Heaven/Sky and Earth make 2 Worlds.
Almost all cultures have a notion of 3 worlds. In the simplest versions, this becomes a simple distinction between Upper, Middle and Under-World. Many cultures transform the upper world into a Heaven, and frequently into a series of heavens. The number varies: e.g. 7 Heavens (Mithras); 13 Heavens (Maya); 33 Heavens (India).
Among the words for heaven in Sanskrit:
Brahman Growth, Expansion
Amrita NectarAsvins Twins (Physicians)
Divya cf. Divine
1 3 5 7 9 added together leads to 25 = Heaven = Abstract = Masculine