And THE CITY has NO NEED of the SUN or of the MOON to shine upon it,
Here we see a clear difference between the old Jerusalem (present earth) and the new Jerusalem (CITY). Presently, the earth needs SUNlight for its existence and our survival. The MOON really isn't needed for survival except perhaps for romantics.
Even before the present sun and moon were created (Genesis 1:14-19), there was light (Genesis 1:3-5). Similarly, there may have be a light for the new Jerusalem, but as this verse shows, there is NO NEED for the SUN or the MOON as far as a light source.
for
We know from this verse that the GLORY of GOD (Revelation 21:9-11) is sufficient to illuminate the CITY. Symbolically, the GLORY of GOD is sufficient to illuminate our spiritual man.
When Jesus walked on this earth, He clearly states the He is the LIGHT (John 8:12-20). Not only that, true followers of Jesus will have the LIGHT of life. Paul confirms that believers on this earth when they walk as children of light, expose the darkness (evil) to what is the fruit of the light consisting of all goodness and righteousness and truth (Ephesians 5:1-8-14). When these children of light are translated to the new Jerusalem (CITY), they as the Bride (verse 9) will continue to glow with the Light of Life (John 8:12).
From this verse, one could almost think the CITY is really one massive light source. Yet we know that it has physical dimensions (verse 15).
An aside: It is no accident that Einstein's ratio of energy to mass is proportional to the speed of LIGHT squared. While we can't say that the CITY adheres to "Einstein's physics", it is obvious that there is a relationship between matter and energy even in the new CITY. After all, it is the same GOD who made both the old earth and the new earth (verse 1).