Lesson 1 · 8 min
What astrology is, and what it isn't
Astrology is a symbolic tradition that reads meaning into the positions of the Sun, Moon and planets. This lesson traces its roots and explains the zodiac it uses.
A language of symbols
Astrology is the practice of reading meaning into where the Sun, Moon and planets sit in the sky at a given moment. A birth chart is a map of that sky at the moment and place a person was born. Astrologers treat it like a text to interpret, full of symbols that describe character, timing and themes.
It helps to be clear about what astrology is not. It is not astronomy, the science that measures and explains the physical universe. It is not a proven method of prediction, and controlled studies have not found that it forecasts events or personality better than chance. What it is, and has been for more than two thousand years, is a rich cultural tradition with its own vocabulary, logic and literature.
In this course we study that tradition on its own terms, the way you might study mythology, poetry or the history of medicine. You will learn what astrologers mean by each symbol, and you will also learn where the evidence stands.
From Babylon to Alexandria
The roots of Western astrology lie in ancient Mesopotamia. Babylonian scholars kept careful records of the sky for centuries and read celestial events as omens, mostly about kings and the state. By around the fifth century BCE they had divided the Sun's yearly path into twelve equal parts of thirty degrees each. This was the first zodiac. The oldest known birth horoscope, written on a clay tablet in Babylon, dates to 410 BCE.
After Alexander the Great's conquests, Greek, Babylonian and Egyptian ideas mixed, especially in the city of Alexandria. Between roughly the second century BCE and the first century CE, this blend produced Hellenistic astrology, the direct ancestor of the system taught here. It added the Ascendant, the twelve houses and much of the interpretive method still used today.
In the second century CE, the Alexandrian scholar Claudius Ptolemy wrote the Tetrabiblos, meaning four books. It tried to give astrology a natural philosophy and became one of the most influential astrological texts in Europe and the Islamic world for well over a thousand years.
The ecliptic and the tropical zodiac
Over a year, the Sun appears to travel once around the sky against the background stars. That path is called the ecliptic. The Moon and planets always stay close to it, because the solar system is fairly flat. The zodiac is simply the ecliptic divided into twelve signs, each spanning thirty degrees, for a full circle of 360.
Western astrology mostly uses the tropical zodiac. It starts the circle at the point where the Sun sits at the March equinox, and calls that point zero degrees Aries. From there the signs follow in a fixed order: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius and Pisces. Because it is tied to the equinoxes and solstices, the tropical zodiac is really a calendar of the seasons, measured in degrees.
Sidereal, tropical and precession
Earth wobbles slowly on its axis, like a spinning top. Because of this wobble, called precession, the equinox point drifts backwards against the stars by about one degree every 72 years, completing a full circle in roughly 26,000 years. The Greek astronomer Hipparchus is credited with discovering it in the second century BCE.
When the zodiac was first defined, the signs and the constellations with the same names roughly lined up. Today they are about 24 degrees apart. The sidereal zodiac, used in Indian astrology (Jyotish) and by some Western astrologers, keeps the signs anchored to the stars. The tropical zodiac keeps them anchored to the seasons. Neither is a mistake. They are two different choices about what the zodiac should measure.
Key terms
- Birth chart
- A map of the sky, as seen from a specific place, at the moment of a person's birth.
- Ecliptic
- The Sun's apparent yearly path around the sky. The zodiac is laid out along it.
- Tropical zodiac
- A zodiac that begins at the March equinox point, so its signs stay tied to the seasons.
- Sidereal zodiac
- A zodiac anchored to the background stars. It now sits about 24 degrees from the tropical zodiac.
- Precession
- The slow wobble of Earth's axis that makes the equinox point drift against the stars, about one degree every 72 years.
Worked example
Finding a sign from a position
Our first teaching sample, Jun, was born on 2 November 1986 at 08:10 in Seoul. Zodiaq's engine places his Sun at 219.2 degrees along the ecliptic, counting from zero degrees Aries. To find the sign, count in steps of thirty: Aries covers 0 to 30, Taurus 30 to 60, and so on. Scorpio, the eighth sign, covers 210 to 240. So Jun's Sun is 9 degrees and 13 minutes into Scorpio.
Our second sample, Maya, was born on 15 March 1992 at 14:30 in Lisbon. Her Sun sits at 355.3 degrees, which is 25 degrees and 15 minutes into Pisces, the twelfth and last sign. In under five more degrees the Sun would have crossed zero Aries, the March equinox point, and the cycle would have started again.
Now try precession. If you subtract the roughly 24 degree gap to move Jun's Sun into the sidereal zodiac, it lands in the middle of Libra instead of Scorpio. Maya's Sun moves back too, but stays in early Pisces. The sky did not change, only the starting point of the measurement. This is why the order of the signs matters so much: every position is counted around the same fixed sequence.
History and traditionSame sky, many traditions
Several of the sign names reach back to Babylonian star figures. The goat-fish that became Capricorn is Babylonian, and the figure the Babylonians called the Hired Man later became the Greek Ram, Aries. The Greeks for a time called the stars of Libra the Claws of the Scorpion before the Scales became standard.
For most of history, astrology and astronomy were practised by the same people. Johannes Kepler, who worked out the laws of planetary motion in the early seventeenth century, also cast horoscopes. The two fields separated gradually during the scientific revolution, as astronomy turned to physical explanation and astrology remained a symbolic art.
Try it: Put the signs in order
Drag the twelve signs into zodiac order, starting with Aries at zero degrees.
Tap the items in order. Tap a placed item to remove it.
Your order
Still to place
Check your understanding
Question 1 of 5
Think about it
If astrology does not predict, what do you think has kept people reading the sky for meaning for more than two thousand years?