The subject of the sentence is, ball.
explain that beauty can fade
In these lines, Shakespeare uses the word fair to talk about beauty. Often during this time period describing a person as fair meant that they were beautiful and unblemished. The lines say that fair declines, or goes down. This means that beauty can fade either by chance or because that is just the way nature intended. Sonnets are love poems. He is not using the sonnet to teach a lesson about nature and he does not define the meaning of fair. Also, in these lines he is saying that beauty changes so the last option is completely wrong.
The plural of 'grotto' is 'grottoes' or 'grottos'.
<span>carefully and scientifically :P</span>
The meaning of the word fair is beautiful (A).