In the northern hemisphere, at the winter solstice this is when the Earth becomes more withdrawn and silent, qualities characteristic of saturn. The winter solstice starts at Makara (Capricorn). At the opposite end, when the summer solstice happens, the luminaries are dominating and the Earth becomes more conscious and vibrant, so this is where the luminaries dominate, Cancer and Leo corresponding to the moon. At spring equinox, the sun's presence is increasing and crosses the east directly and this gives the fire element, representing dharma. The influence of the sun becomes northbound, and north's element is represented by water, corresponding to moksha, or self reflection. The point of the sun having it's northmost influence is at the Summer solstice, so there we have the water element. The order of the elements from east to north to west to south is fire, water, air, earth. they correspond to dharma, moksha, kama, and artha. So there is a question of why cancer at the summer solstice, why is it not leo? Well the moon has more of a harmony with the water element there, and the fire element is obviously in tune with the sun. This all unfolds naturally. We always see that the signs are split with a duality, one possessing a feminine side and a masculine side (except where the sun IS masculine and the moon is feminine). Virgo is the female side of mercury, gemini is the male. The signs proceed about in this order, alternating. And you will see how there are always planets in poles, opposing each other. Mercury to Jupiter, and Venus to Mars, and Saturn against the Luminaries. Venus and Mars are opposing each other at points of Rajasic-tamasic activity, where the focus is not on plain illumination or withdrawness, but something in between. Mars and Venus go hand in hand. All of the significations of the planets and the places where they rule are in their natural place according to the sun's movement, and the respected directions of which they rule. It is quite amazing how it all works out, and in this way one must realize the zodiac is tropical, as mentioned in Srimad Bhagavatam. You still use the Vedic techniques, but with tropical zodiac.
Vic DiCara helped me figure this out.