I think the meaning is this: 1. Despite the moon being now only a quarter full and already sinking it is still very big/bright. 2. Therefore, when the moon is full, and fully risen, it is even more bright. 3. Even in this "full" state the moon is scarcely (very rarely) brighter than the "strange new star" So, the "strange new star" is very bright!

I'm confused about why it is "they were of a comparable brightness". There is a “scarcely” in second clause, shouldn't it be "they weren't of a comparable brightness"?

"breadth" is the measurement of something from one side to the other. Your piece describes a quarter moon, which would be less in size, and brightness to a full moon. Despite the fact that the quarter moon is much bigger from side to side than the "strange new star" they were of a comparable brightness.

Hi Irene, In all its glory... in all its possibility... in all its scope, wonder, magnificence. In all its potential. I hope this helps, Stephen