As the modern individual is likely well aware, the earth rotates in two very fundamental ways which affect our daily lives: it rotates every day and orbits the sun every year, forming these important means by which we keep track of time. However, one of the most interesting and confusing ways that the earth rotates in space is in its precession. Precession is caused by tilt of the earth’s axis. Since the earth’s axis does not sit straight up and down but at a tilt of between 22 and 24 degrees, the gravitational pull of the sun and moon in particular causes earth to wobble like a spinning top. The wobble causes the orientation of the earth’s axis in space to change. While we may not notice this change because it takes place over a period of about 26,000 years and it does not affect our calendar, if one could observe the stars across the millennia, they would will see the difference.
This means that traditions which involve the observation of starts, astrology chief among them, will be aware of this change. This is the reason why, in the Western tradition of astrology, called tropical astrology, one’s sun sign does not actually correlate with the constellation in which the sun appeared at the time of birth. For example, I was born in early February, thus according to tropical astrology, I am an Aquarian; however, at the time of my birth in the late 20th century, the sun was actually in Capricorn, not Aquarius. Other forms of astrology, such as Vedic sidereal astrology of India, takes precession into account, so in that tradition, I would be a Capricornian. On the one hand, sidereal astrology will always match the position of the sun in the constellations, but individuals born in early February (midwinter) in 1000 AD would be Aquarians while those born in in early February in 2000 AD would be Capricornians; on the other hand, tropical astrology will always keep those born in early February as Aquarians, whether in 1000 AD or 10000 AD, but the sun will actually appear in the constellation Aquarius only about 1/12 of the precessional cycle.
People often ask which astrology is more accurate, tropical or sidereal. In the modern age of science, there is much weight placed on empirical, scientific accuracy, so tropical (Western) astrology is often accused of being inaccurate (because of precession) and sidereal tolerated as at least in line with astronomy. This is an oversimplification. The truth is that tropical astrology’s zodiac is more focused on the seasons in which one was born as well as the time of birth relative to the solstices and equinoxes and is less focused on the stars themselves. Since the seasons, solstices, and equinoxes are based entirely on astronomy, both of these traditions are as astronomically valid as the other. It should also be noted that the main differences affect the zodiac and have little to do astrology focused on planetary aspects, their transits, or houses. Thus, those who work primarily off of these elements will not be as affected by axial precession, since they are more based on the positions of the earth, sun, moon, and planets relative to each other and do not as heavily involve the sun’s position relative to the stars. In the end then, precession alters the zodiac and draws the lines on separation between tropical and sidereal astrology, with tropical focused on the consistency of the seasons and sidereal focused on the where the sun appears in the constellations. The more weight you place on the zodiac, the more difference there is, but if you focus on aspects, houses, and transits, things don’t change so much.