The division goes like this: fire for Aries, Leo, and Sagittarius; earth for Taurus, Virgo, and Capricorn; air for Gemini, Libra, and Aquarius; water for Cancer, Scorpio, and Pisces.
Place the signs on a circle and you see that the three of a single family draw a triangle: from one to the next, there are always 120 degrees. Hence the name trine, which designates both this triangle and the corresponding aspect.
Greek philosophy gives each element two qualities. According to Aristotle, fire is hot and dry, air hot and moist, water cold and moist, earth cold and dry. Two elements sharing a quality echo each other better, hence the old idea that fire and air agree, as do earth and water, while fire and water oppose. It is a way of thinking, not a measurement.
As for reading, tradition links fire to drive, earth to the concrete, air to exchange, and water to sensitivity. These traits are conventions of tradition, and each has its flip side: drive can scatter, the concrete can harden, exchange can stay on the surface, sensitivity can overflow. A chart almost always contains all four elements; you simply look at which dominate, according to the number of planets in them.
Family
The signs
The diagram
The signs by element
Read next
The modalities (cardinal, fixed, mutable)
History, debates, and sources
A little history
Empedocles, in the 5th century BCE, speaks of the four “roots” of all things: fire, air, earth, and water.
Aristotle then associates qualities with them, in his treatise on generation and corruption. Ptolemy, in the Tetrabiblos, sorts the signs into triplicities according to these elements. In the 20th century, authors drew a parallel between the four elements and the four psychological functions described by Jung, which spread the modern use, more psychological than physical.
Sources
3
Oldest
Empedocles, 5th century BCE
Most recent
Claudius Ptolemy, 2nd century
What is debated
For some astrologers, the elements describe a way of acting and perceiving, useful in psychology.
For others, they remain physical qualities inherited from antiquity, to be handled with the dignities. Chinese astrology uses five elements and a cycle of its own, which are best not mixed in. Finally, the methods for finding the dominant element differ: some count the planets, sometimes the rising sign, sometimes the luminaries only.
Schools do not all agree: the site sets out the positions without deciding for you, and states the setting it uses by default.
Where this concept comes from
The authors and texts that establish or discuss this concept, with the passage when it is known.
5th century BCEEmpedoclesOn Nature (fragments), all the fragments
The four “roots”: fire, air, earth, water.
4th century BCEAristotleOn Generation and Corruption, Book II
Pairs each element with two qualities (hot, cold, dry, moist).
2nd centuryClaudius PtolemyTetrabiblos, Book I, 18
Fire: Aries, Leo, Sagittarius. Earth: Taurus, Virgo, Capricorn. Air: Gemini, Libra, Aquarius. Water: Cancer, Scorpio, Pisces. The three signs of an element are 120 degrees apart and are linked by the aspect called the trine.
Where does the theory of the four elements come from?
From Greek philosophy: Empedocles made them the four roots of the world, in the 5th century BCE, and Aristotle gave them qualities. Greek astrologers, including Ptolemy, then sorted the signs into four families according to these elements.
How do I find the dominant element of a chart?
You count the planets, sometimes the rising sign too, in the signs of each element, then see which one dominates. Methods vary, notably in the weight given to the Sun, the Moon, and the rising sign angle: the result therefore remains an indication from tradition, not a measurement.