Weight and density are the BIGGEST factors in the principle of "what goes up must come down" - What keeps the bodies in the firmament afloat? Instead of giving gravity all the credit, think of electromagnetism and God's handiwork.
Psalm 104:1-9 KJV - Bless the Lord, O my soul. O Lord my God, thou art very great; thou art clothed with honour and majesty.2 Who coverest thyself with light as with a garment: who stretchest out the heavens like a curtain:
3 Who layeth the beams of his chambers in the waters: who maketh the clouds his chariot: who walketh upon the wings of the wind:
4 Who maketh his angels spirits; his ministers a flaming fire:
5 Who laid the foundations of the earth, that it should not be removed for ever.
6 Thou coveredst it with the deep as with a garment: the waters stood above the mountains.
7 At thy rebuke they fled; at the voice of thy thunder they hasted away.
8 They go up by the mountains; they go down by the valleys unto the place which thou hast founded for them.
9 Thou hast set a bound that they may not pass over; that they turn not again to cover the earth.
Psalm 104:19 KJV - He appointed the moon for seasons: the sun knoweth his going down.
Genesis 1:14-16 KJV - And God said, Let there be lights in the firmament of the heaven to divide the day from the night; and let them be for signs, and for seasons, and for days, and years:15And let them be for lights in the firmament of the heaven to give light upon the earth: and it was so.
16 And God made two great lights; the greater light to rule the day, and the lesser light to rule the night: he made the stars also.