‘The sun will no more be your light by day,
Nor for brightness will the moon give light to you,
But Yahweh will be to you an everlasting light,
And your God your glory.
Your sun shall no more go down,
Nor will your moon withdraw itself,
For Yahweh will be your everlasting light,
And the days of your mourning will be ended.'
If we had any doubts previously that this Zion is finally not earthly but heavenly it is made clear here. They will have no need of sun or moon (Revelation 21:23) for Yahweh will their everlasting light, and their God will be their glory (Revelation 22:5). He will be their sun and their moon, thus the lights of sun and moon will no more go down or in some way hide themselves. There will always be light, for Yahweh Himself will be their everlasting light, and all their days of mourning will be over. The new earth that Isaiah envisages is nothing like any previous earth.
(Such a world of continual everlasting light will be fine for those who have been raised with spiritual bodies, but would hardly suit those who have to sleep and are unable to do so because of the radiance of the light. This description has to be idealistic, not realistic).
In the old world the sun and moon ruled day and night and determined times and seasons. But now God has replaced both sun and moon. All is now determined by Him. There will be everlasting light from Him, all that is needed for the heart of man. And there will be no more sorrows and no more tears (Revelation 20:4). The picture is again one of perfection. Instead of depending on creaturely supply, all we need will come directly from the Source of all things.
‘For Yahweh will be your everlasting light.' In this is summed up Yahweh's blessing on His people. They will dwell in ‘Zion' and His light on them will be everlasting. No more cloud to hide Him, no more sin and darkness, no more dependency on the vagaries of earthly weather. The light will be permanent and complete (see Revelation 21:23-24).