As helpless as baby kittens are, when a stranger approaches they hiss and spit and kick up a fuss, and make a fine show of bravery. Human beings are rather frail creatures too, with great pretensions. Behind all their hissing and spitting they possess a sheer gumption which is peculiarly human and is most noticeable in moments of stress and harassment. In the horoscope it is shown by the critical degrees.
    The critical degrees are points spaced 12°51' apart, beginning at 0° Aries:  0°, 12°51', and 25°43' of Cardinal signs; 8°34' and 21°26' of Fixed signs; and 4°17' and 17°09' of Mutable signs. This is a division of the zodiac by 28 (since 360/28 = 12°51'), which is symbolically the moon's orbital period. The critical degrees are "lunar" in the sense that this area of life is never stable - there is always some question or problem about it. Any natal planet or horoscope angle that lies within an orb of one degree of any of these points is considered to be critical. Where you find a critical planet or angle in the chart, you find a good deal of hissing and spitting in the life.
Critical degrees can be considered to symbolize one's abstract purpose in life. Only the pursuit of an abstract purpose can sufficiently ennoble and elevate our lives above humdrum routine and endless suffering; give us the courage to overcome when everything seems out to crush our spirit.  This is what planets conjunct critical degrees show: one's abstract purpose in life.  An absence of critical degrees implies the lack of an abstract purpose (beyond getting through the daily grind & indulging oneself evenings and weekends).
"Abstract purpose" (also arts, crafts, invention) doesn't necessarily mean something good - it can be seeking enlightenment, or getting to heaven; or it can mean upholding one's social superiority by extirpating "inferior races"; or saving the environment; or promulgating a proletarian revolution. The point is that having a dominant purpose in life beyond grubbing out a living is the only way to get on top of life - to face the waves and breakers of life dead on instead of being bobbed up and down and thrashed around.  Planets conjunct critical degrees put the area of life symbolized by that planet on hold (in a material sense) so that the spiritual can flower.  So, it is possible to get a clue to your abstract purpose by looking at which planets are critical (and why things are so difficult in these areas from a material point of view). 
In a sentence, the meaning of the critical degrees (whether conjunct a planet in the natal horoscope; or a critical degree line on the earth in an astrolocality map) is: good for spiritual things; and bad for material things.
Critical degrees show a cosmic restraint:  natives will experience obstruction and frustration here (depending on the planet involved) until they learn to surrender their own will in this particular area of life.  By denying satisfaction in external circumstances, natives are forced back upon their own resources: they have to find satisfaction within themselves. These degrees are neither benefic nor malefic: they deny external satisfaction, but increase self-reliance. There is an obstinate struggle here against hampering and isolating circumstances - usually self-created - and also a great reserve of fortitude and strength.
    This strength is especially apparent when a planet or angle comes to conjunction with a critical degree by secondary (day-for-a-year) progression.  Except for the progressed moon, whose effect is much weaker, there is usually a preceding month or months of personal limitation and frustration. Then, within a few weeks of the time after the progression is exact, a crisis occurs.  The situation calls for a transcendence - moving up to a new level of grasp and control. The real significance of these critical moments in life is often properly understood only in retrospect over a period of months after the event.
  Generally, your critical planets and angles reveal not only where you fume, but also where your greatest potentials lie. They are your true points of touch with the depths of your own humanity.

    When there is a Preponderance of planets conjunct critical degrees (four or more) in your horoscope, then you possess a deep sense of accountability to the world at large; yet life keeps you on a very short leash.  You soon learn that even slight deviations from the straight and narrow path can have catastrophic consequences.  You feel compelled to do more than is necessary because you have an acute need to justify to yourself the fact of your existence on this earth. You tend to distrust being too happy - or at least feel vaguely guilty about the idea - which gives you an air of world-weariness. You bear life's burdens with downright doggedness, which on the negative side reveals itself in a singular pigheadedness.  At your best, you possess dignity and a real sense of self-worth which in time can lead you to true wisdom.

    If there is an absence of planets conjunct critical degrees in your horoscope, then you possess great flexibility and adaptability, together with an uninhibited opportunism.  You tend to be rather happy-go-lucky; you get to do pretty much as you please in life because you let nothing interfere with what you really want to do.  At times you may use other people for your own ends and discard them when you have no further need of them.  You have a gift for transplanting yourself body and soul whenever your involvements have reached a point of frustration.  You have little sense of remorse or guilt, and can get away with things that most people can't because life doesn't hold you accountable. An absence of critical degrees provides a relative freedom from compulsion, which can be a great opportunity to practice self-limitation on a voluntary basis.  At your best you are honorable, dispassionate, and just.  At your worst, like Tom and Daisy Buchanan, you tend to leave your messes for other people to clean up.

    When your natal Sun is critical you have a strong will and determination, but very little fulfillment in life.  You feel misunderstood and unappreciated, out of step with the world around you. You have high ideals and a good heart, combined with a fierce pride and unwillingness to compromise.  You are out-of-step with the world around you, but have an unwavering faith in your own worth.   This is not so much a pride before people (though there can be a touchy, snobbish, or overweening tendency) as it is a pride before God.  It's difficult for you to reconcile the demands that the world makes upon you with what your own inner voice is telling you to do.  You tend to find real life too real, too harsh, which lends a softness and sweetness to your nature and gives you a grumpy or brooding air.  At your worst you are given to self-righteousness of self-pity.  Your independent disposition and unwavering faith in your own worth is trimmed by a long period of servitude in which you must bend yourself to the demands of other people or the press of circumstances until the nascent humility, which is your most endearing virtue, truly flowers. 
    When your Progressed Sun arrives at a critical degree your life comes to some dead end or point of futility. There may be arguments or disagreements with associates, or just an awareness of nonfulfillment with where your life seems to be taking you.  Abruptly some new door opens. You make the decision to change direction in life. Filled with inspiration and a new sense of purpose, you leave the uncongenial environment to go off on your own.  It's as if you can feel the hand of God guiding and directing you. In the succeeding months you may reflect upon your own limitations; you come to appreciate what it is that is really of importance to you in the working out of your destiny.

....

(excerpted from The Great Wheel by Bob Makransky)


The Great Wheel can be sampled and purchased at: http://www.smashwords.com/books/view/306020;
Kindle Edition available at: http://www.amazon.com/The-Great-Wheel-Introduction-ebook/dp/B00CD958PS/