Tafsir of An-Nazi'at 79:28

{رَفَعَ سَمْكَهَا}: An explanation of its construction, meaning He made the measure of its height from the earth and its extension toward the zenith long and elevated. It is permissible to interpret samk (height/thickness) as density; the meaning then being that He made its density high in the direction of the upper realm. Density is called samk because of the height of the upper surface above the lower surface. When this extension is observed from the top toward the bottom, it is called depth; a parallel to this is the usage of darj (ascent) and darak (descent). It has come in authentic reports that the height of the lowest heaven from the earth is five hundred years, and the height of each heaven from the next, as well as the thickness of each, is likewise. The apparent meaning is that this is measured by standard travel, and that the mentioned number is intended as a specific limit rather than for multiplication. We adhere to the apparent meaning unless there is an impediment against it.

{فَسَوَّاهَا}: Meaning, He made it even in accordance with what wisdom demanded; He did not leave any part of it devoid of what the wisdom regarding it requires. Among this is its adornment with the stars. It is said that its leveling means He made it smooth, such that there is no depression or elevation in its surface. It is also said that He made it simple, with homogeneous parts and shape, such that no part of it is a surface, another an angle, and another a line; this is the view of its true sphericity, and many have held this view. They said—and the Imam narrated this—that since it is established that [the heaven] is created and in need of a willful Agent, what harm to the religion could arise from it being spherical?

It is also said that its leveling means He perfected it with those things that constitute its completion, such as the stars, the finishing elements, the epicycles, and other things explained in the science of astronomy, deriving this from the expression "He leveled (sawwa) his affair," meaning he set it right, or from the saying "the fruit has leveled (istawat)," meaning it has ripened.

You know that this—based upon the unity of the heavens and the celestial spheres—is unknown to the first generation of Muslims, due to it not being reported from the Master of the Night Journey, the Messenger of Allah (may Allah bless him and grant him peace), and the lack of evidence for it. The proofs that the astronomers cite for these matters are well-known in their state [of uncertainty]. For this reason, [the scholars] did not subscribe to what the opponents of these astronomers—the astronomers of today—demand. And Allah, the Exalted, knows the reality of the state.