"And we set up the heaven as a roof well-protected; yet still from our signs they are turning away."
This post is regarding the phrase "roof" which is referred to the atmosphere.
Dr. El-Fandy in his book, says:
The earth’s roof is its atmosphere. It is composed of a mixture of gases, which are preserved and not allowed to escape into space due to the relatively large gravity of the earth. This condition could not be realized on the moon, which lost its roof and thus became roofless, due to its relatively small gravitational force. In this way the moon soon lost the water of its oceans and seas. It evaporated into the vast space.
The earth's atmosphere or roof provides vital services to all living creatures on earth:
1) It contains oxygen necessary for the life of both the animals and the plant kingdoms, as well as Man.
2) It carries water vapour from the earth surface up to the upper layers of the atmosphere where clouds form and give rain. This process involves the source of fresh water on earth.
3) It contains the carbon dioxide gas, which is absorbed by plants receiving sunlight by day to form sugar and starch.
4) Sound waves propagate through the air, thus we can hear the voice of each other. This is not the case on the moon.
5) Daylight is due to the scattering and diffuse reflection of sunrays in the thin air layer extending from the earth's surface up to 200 kilometers and facing the sun. This layer may be referred to as the (skin-layer). The rest of the atmosphere and space are dark and dim. As the earth rotates the skin-layer keeps facing the sun and appears as though it is flayed from the body of the atmosphere.
Thus we can see that the scattering of sunlight is the reason behind daytime on earth. If we ascend outside the atmosphere of the earth, we find that the universal space is dark in spite of the fact that the sun is there. This means that the light is snatched from the night.