Western Astrology is entirely based on nature and the seasonal cycles that we experience in Europe (it is a Greek system.) When the Sun enters 0 degrees of Aries, we have the spring equinox (yesterday). 0 degrees of cancer is the summer solstice. 0 degrees of Libra is the Autumn equinox. Winter solstice is 0 degrees of Capricorn. It functions independently from the constellations is not in alignment with the stars other than in name.
The signs symbolise the cycle of the seasons. Cardinal signs which are those that I listed above, fall at the start of a new season. Cardinal signs initiate changes and are natural leaders. Fixed signs (taurus, Scorpio, Leo & Aquarius) represent the times of year when we are fully into a season and nothing is changing in nature. These signs are stubborn, loyal and resilient. The mutable signs (Gemini, Virgo, Sagittarius & Pisces) occur right before the equinox or solstice and nature is starting to change. These signs are flexible and go with the flow. We take this formula and apply it to every planet in the birth chart, then we analyse the relationships between the planets. All of these planets represent different life areas. The angles in the chart change every 2 hours and also fall in different signs.
You will not have the same birth chart as anyone else unless they were born on the same day, time and location as you.
We are part of nature and reflect her qualities. The planets do not control anything, they are a reflection of life. Similar to reading omens by observing birds and other animals.
Scholars of astrology recognise that the qualities of the signs are not interpreted in the same way as the constellations of the same names. For example, some stars in Cancer are said the have the qualities of Saturn and Mars (planets of war, conflict, discipline and endings. Vastly different from tropical cancer which is ruled by the moon and said to be the archetype of the nurturing and soft Mother.
Claudius Ptolemy states in his work "Tetrabiblos" "The beginnings of the signs and likewise those of the terms, are to be taken from the equinoctial and tropical points. This rule is not only clearly stated by writers on the subject, but is also especially evident by the demonstration constantly afforded, that their natures and familiarities have no other origin than from the tropics and equinoxes, as has already been plainly shown.
He mentions this issue several times in the Tetrabiblos. It's almost as if he anticipated the buffoonery and misconceptions surrounding Greek, Western astrology.
There are 7 main chakras in the body (not a new age concept but from the Hindu Vedas) and there are 7 classical planets used in astrology which are the objects that we can see clearly in the night sky without telescopes. Each chakra is ruled by a particular planet.
Our bodies are miniatures of the universe, we are the microcosm and this is why astrology is not the cause of events, but a reflection. The stars in the sky are the chakras of the macrocosm. This is why we can read omens and signs in nature because the external reflects the internal. There is no simple explanation as to how astrology works, but this is as close as it gets.
Understanding this concept of course makes it even more absurd that modern astrologers would assign Neptune, Uranus and Pluto as rulers of the signs. This is because they lack the esoteric understanding and education. Astrology was originally an initiatory science of the Priest class.
P.S - The change in rulership of the signs (Scorpio, Pisces and Aquarius) did not happen until the late 1800's and seems to be tied to Madam Blavatsky's theosophy movement. Enough said.