The moon sign is a better tool to tell of your intransigent, receptive actions or reactions, whereas the fire sign is used to predict your proactive tendencies or your general line of thought. Let's go back to the above example. Say you are a Sagittarius but you were born with an Aries moon sign, the sign of the sometimes bullheaded ram. Sounds like a catastrophe doesn't it? It's not so bad when you look closely.
This example tells us many things, since your sun sign sums up your primary traits, your likely actions and thoughts, while the moon sign describes your emotional side, or your likely reactions. Sun equals thought and action, moon equals emotion and reaction.
In the case above, with a sun sign in Sagittarius, the standard astrological profile tells us that this person is outgoing, reckless, loves the outdoors and nature, and is very loyal, caring, highly spur of the moment, carefree, and independent. The moon sign on the other hand is in that of Aries, which tells us that emotionally this person is head strong, fearless, possesses leadership skills, lacks tact and is blunt. This person, then, would tend to be a natural leader, unafraid to say what they see, often to a fault, yet loyal and generally giving of personal space, which they would require themselves. Really to get a total picture of one's personality there is plenty of other info required, such as rising signs, and so forth. It is best to remember when trying to get a complete picture of anything, the more information available, the more complete the picture will be. Secondarily to your sun sign, though, your moon sign is vital.
Your astrology moon sign can be found at a myriad of websites, or by contacting an astrologer. By learning this information and utilizing this valuable tool, you can begin to understand the over all science of astrology, and can learn to read your own traits. You'll find your moon sign, with a little help, is not that hard to read.