My sun sign is Taurus, the bull. To many people, astrology is just that.

Others are so invested in the divinatory art, however, that they base their relationships, decisions and life goals on the intricate secrets of their birth chart and trusted daily horoscope.

Several systems of astrology are in use today. Picking apart their differences and origins reveals surprising insights into the relationships between astrology and astronomy, and whether astrology has any practical use.

From antiquity, the movement and changes of celestial bodies fascinated humans. Ancient stargazers drew on cosmic phenomena in their myths and applied them to their daily life. The Greeks inherited their astrological system from the Mesopotamians, who learned theirs from the Sumerians. The Chinese, Indian and other cultures have their own versions of the zodiac.

In 2000, Dr. Michael Rappengluek of the University of Munich argued that prehistoric paintings in the famous Lascaux caves in France included a star chart. The nearly 20,000-year-old paintings depict clusters and dots corresponding to the lunar cycle and constellations such as the Pleiades.

Although astrology is not a science, it overlapped with astronomy in its early days. The person who studied stars and celestial phenomena also interpreted the significance of those phenomena on human life and relationships.

The two fields diverged in the 17th through 19th centuries, but astrology stills retains some aspects of astronomy. From that perspective, astrology can sometimes serve as a launching pad to understand scientific ideas about celestial bodies and their movement.

Imagine you’re staring into the night sky, surrounded by stars. If you visualize those stars as being on the inside of an infinite sphere of space surrounding Earth, you’ve just pictured what’s known as the celestial sphere.

The celestial equator is the projection of Earth’s equator into outer space and onto the celestial sphere. Stars appear to move parallel to the celestial equator. Measurements on the celestial sphere are usually expressed in terms of right ascension and declination, analogous to Earth’s longitude and latitude.

From the vantage point of the North Pole, the celestial equator is at the horizon. For each degree closer to the Earth’s equator the observer travels, the celestial equator moves higher by a degree. If you know your latitude, you can calculate the angle between the celestial equator and the horizon as you face south; it’s just 90 degrees minus your latitude.

The latitude of Edmonds is just under 48 degrees, meaning you can see the upper belt of the celestial equator around 42 degrees above the southern horizon. At the Earth’s equator, the celestial equator would appear directly overhead. No matter the position of the observer, the celestial equator crosses the horizon at due East and due West, forming a giant semicircle.

Let’s continue this Earth-centric vision of the cosmos. Every year, the sun appears to loop around the Earth’s sky in a path termed the ecliptic. The Earth’s spin axis is tilted 23.5 degrees with respect to its orbit around the sun, a phenomenon responsible for the seasons. Consequently, the ecliptic plane is also tilted 23.5 degrees with respect to the celestial equator. According to C. Rod Nave, a professor at Georgia State University and originator of the HyperPhysics project, all the planets orbit the sun along roughly the same plane, suggesting that  the solar system was created from “a disk of material out of which formed the sun and the planets.”

In astrology, a “planet” is more about symbolism than a scientific definition. Astrology includes the sun and moon as well as the traditional planets and the recently downsized Pluto (added after its discovery in 1930) in this category. At the moment of someone’s birth, the planets are at certain positions along the zodiac, supposedly reflecting the particular forces at play that will shape the newborn’s personality and destiny.

Most horoscopes in Western countries are based on the tropical zodiac, which is determined by the planetary bodies’ positions during the vernal equinox. The signs change with the seasons, with “cardinal” signs beginning on each equinox or solstice. For example, Aries starts on the spring equinox, Cancer on the summer solstice, Libra on the autumnal equinox and Capricorn on the winter solstice.

What’s the difference between an equinox and a solstice? When the celestial equator and the ecliptic cross, an equinox occurs. During the equinox, night and day are even lengths. The solstice occurs when the sun is farthest away from the celestial equator, leading to the longest night or longest day of the year.

An early astronomer, Hipparchus of Nicea, studied the equinoxes and compared his findings to observations made 169 years earlier. Around 130 BCE, he made a shocking discovery: the sun’s position during those intersections was slightly different from previous equinoxes.

According to Dr. David P. Stern, author and curator of astronomy archives on the Goddard Space Flight Center webpage, “Hipparchus concluded that the intersection marking the equinox slowly crept forward along the ecliptic, and called that motion ‘the precession of the equinoxes.’” A slight wobble in the Earth’s axis causes this phenomenon.

Some astrology systems, such as Jyotish or Vedic astrology, employ the sidereal zodiac, which avoids this gradual shift. It still evenly divides constellations into 30-degree slices, not taking into account their actual size.

Derived from the Latin word “sideris” meaning constellation, the sidereal zodiac takes into consideration the position of the stars in the celestial sphere, as well as certain fixed stars. The north and south nodes of the moon, where the moon passes the ecliptic, are also important. While the constellation rising in the East on your birthday determines your Ascendant or Lagna sign, the moon sign is treated as highly significant, more so than in the tropical zodiac.

Because of the precession of the equinoxes, the sidereal and tropical zodiac slowly move away from each other, about one degree every 72 years. The exact discrepancy between the tropical and sidereal zodiac, however, is disputed.

Brent BecVar, who wrote an article on Jyotish astrology for the Chopra Center, estimated that the current difference is about 24 degrees. That’s nearly a whole zodiac sign. If you know the degrees of your signs in the tropical zodiac, you can subtract 24 degrees to find the positions on the sidereal zodiac.

If you don’t know the degrees of your signs, you can visit alabe.com/freechart and enter the moment and place of your birth to generate a birth chart. Otherwise, flip to the horoscopes on page 7. This month, the dates are based on the sidereal zodiac.

The term Jyotish originates from the Sanskrit words “jyoti” meaning light and “ishta” meaning deity. This emphasizes the spiritual significance of this astrological system. Similarly to how Western astrology divides signs into four elements (fire, earth, air and water) and three qualities (cardinal, fixed and mutable), Vedic astrology separates signs into four human motivations and three attributes in nature.

According to Vedic tradition, the three primary attributes in nature are rajas (passion and activity), tamas, (inertia and immobility) and sattva (harmony and goodness). These qualities apply to planetary bodies, which in turn govern the signs. The rajasic signs are Taurus, Cancer, Leo and Libra. The tamasic signs are Aries, Scorpio, Capricorn and Aquarius. Gemini, Virgo, Sagittarius and Pisces are sattvic signs.

The four motivations in human nature are dharma, artha, kama and moksha. Dharma refers to the execution of duty or law through meaningful work, while artha focuses on acquiring wealth and material security. Kama is the desire for sensual enjoyment, and moksha is transcendence of the cycle of karma. Aries, Leo and Sagittarius follow the path of dharma. Taurus, Virgo and Capricorn are ruled by artha. Kama prevails in Gemini, Libra and Aquarius, leaving Cancer, Scorpio and Pisces to follow the path of moksha.

Some modern forms of astrology also recognize a thirteenth sign, Ophiuchus, also known as Serpentarius. Sandwiched between Scorpius and Sagittarius, Ophiuchus can be seen in the northern hemisphere in the summer, across from Orion. One star in the constellation, the red dwarf Barnard’s Star, is one of the closest from Earth, a mere six light years away.

Ophiuchus is Latin for “serpent bearer,” and the constellation depicts a person wrestling a snake. The Babylonians associated the constellation with the god Nirah, who had serpent limbs. Greeks saw Asclepius, a man who learned the secret of immortality from watching serpents treat each other with herbs. Ophiuchus is also associated with multiple other snake-related heroes, healers and deities.

Pronounced “Ah-fee-YOU-kuss” or “Oh-fee-YOU-kuss,” Ophiuchus boomeranged into consciousness in 2016 when NASA released an article on NASA Space Place, a website that in its own words “demonstrates and explains scientific concepts related to astronomy in a fun, hands-on manner.” The article casually mentioned that Babylonians recognized thirteen constellations in the zodiac but threw out Ophiuchus.

Tonya Riley from the 2015-founded technological news source Inverse.com posited that one possible reason for the original exclusion of Ophiuchus might have been that the constellation is invisible during winter in most northern hemisphere latitudes. Another reason–the sun aligns with the constellation for only about 18 days. According to the tropical zodiac, those days are Nov. 29 to Dec. 17.

While some astrologists call for an updated zodiac based on the current position of constellations, the proposal seems based on an inaccurate view of the origins of astrology. The tropical zodiac didn’t take into account the varied length of constellations but rather mathematically divided sections of sky to fit a numerical scheme that correlated with the seasons.

Although the constellations have moved, the astrological schema was internally consistent. Made up of physically unrelated stars anyway, the constellations themselves weren’t important. Rather, the symbolic meaning behind them was what powered, and still speaks to, their influence on human life.