Mel is a Capricorn (a born executive, an ambitious and hard worker), with Cancer rising (a family man, someone to whom home and hearth are essential to life), so that's a challenge already. It's kind of a schizo way to have to live. He is literally torn between the two halves of himself: the ambitious, successful, money-making, A-list actor and Hollywood Power Exec, and the family-loving guy with an emotional need to create children, either of his body or mind. Mel has lost both of the things that are most important to him in the past couple of years--his family and his career.
In the human body, Capricorn rules the knees, and sooner or later, every Capricorn must bend his. I'm just going to bluntly tell you because it's true, that Capricorns think they run everything, and that no one can do anything as well as they can do it themselves. They have a tendency to take over everything in their path. Sadly, for them, unless they come to realize that they do not run the universe, that something much greater than they runs the universe, then the day they will be driven to their knees is somewhere in their future. For Mel, that future is now.
In the human body, Capricorn rules the knees, and sooner or later, every Capricorn must bend his. I'm just going to bluntly tell you because it's true, that Capricorns think they run everything, and that no one can do anything as well as they can do it themselves. They have a tendency to take over everything in their path. Sadly, for them, unless they come to realize that they do not run the universe, that something much greater than they runs the universe, then the day they will be driven to their knees is somewhere in their future. For Mel, that future is now.
The moon is the ruler of his chart because it's associated with Cancer, the sign on his Ascendant, affectionately referred to as the rising sign.
This place on the chart often indicates "you, in this place, at this time," when the moon traverses it like a second hand on a clock.
Mel's moon in Libra, the sign of relationships, makes it dignified, or very strong. He needs another person to balance him, to bounce things off, to be with. The moon represents his emotional needs, and how he feels about women, particularly his mother, his wife, and his lover. He was born to be married and to be a father. His deepest and innermost self requires it. Do you need a refresher?
The moon occupies his 4th house, directing his emotional needs and attention to his home / family/ domestic matters / issues with mother, women and children / foundations and early childhood memories / and his ultimate ending.
FUN FACT: The moon is often prominent in the charts of writers and alcoholics. 
Use this link to skip to A Pisces Midheaven / Part 4 of the Astrology of Mel Gibson
