Knowledge centre

Astrology terms explained

Three traditions, one vocabulary list. What each word means, why it changes a reading, and the mistake most people arrive holding.

Western

The tropical zodiac, twelve houses and the aspects between planets — the system most English-speaking readers have met.

19 terms

Vedic

Jyotish — the sidereal zodiac, the Moon as the centre of the chart, and dashas as a timing system with no Western equivalent.

12 terms

Chinese · Ba Zi

Four Pillars of Destiny — a calendar system rather than a positional one, read from eight characters and the balance between them.

10 terms

Terms every tradition uses

Vocabulary that belongs to no single system — how houses get divided, where the positions come from, and why the time on a birth certificate is not quite the time.

Ephemeris

The table of where every body was, at every moment. Yours is computed from NASA JPL data.

What it is: A table of where every celestial body was at every moment. Astrologers worked from printed volumes of them for centuries; this site computes yours from NASA JPL data, the same source used for spacecraft navigation.
Why it matters: It is the boundary between the measurable part of astrology and the interpretive part. Where the planets were is straightforward astronomy anyone can check; what it means is the part under discussion. Getting the first part exactly right is not optional.
Common mistake: Assuming astrological positions are approximate or invented. They are precise to a fraction of a degree and are the same numbers an astronomer would give — any disagreement between astrologers is about interpretation, never about where Saturn was.

Whole-sign houses

The oldest house system: each house is exactly one sign. Simple, and the default here.

What it is: The house system in which each house is exactly one whole sign. If Leo is rising, the whole of Leo is the first house, the whole of Virgo the second, and so on around the chart. No cusps have to be calculated, because the sign boundaries are the house boundaries.
Why it matters: It is the oldest system, used by Hellenistic astrologers from around the first century BCE, and it is standard in Vedic astrology — which together make it the most-used house system in the world. It is also what lets this site set a Western and a Vedic chart side by side, since both then count houses the same way.
Common mistake: Assuming it is a simplified version of something more accurate. It is not a shortcut, it is the original, and it was in continuous use for well over a thousand years before quadrant systems like Placidus existed. It went out of Western fashion, and came back.

House system

The rule deciding where the twelve houses begin. Different rules can put the same planet in different houses.

What it is: Everyone agrees the chart has twelve houses; the disagreement is about where each one starts. Whole Sign uses the sign boundaries. Equal House measures thirty-degree steps from the Ascendant’s exact degree. Placidus and its relatives divide time rather than space, which is why their houses come out unequal.
Why it matters: The house system is the one choice that can move a planet from one department of life to another without anything in the sky changing. Two astrologers can look at the same birth and place your Sun in the ninth house or the tenth, purely because they chose differently.
Common mistake: Expecting a settled right answer. There are dozens of systems, they have been argued over for two thousand years, and competent practitioners use different ones. What matters is knowing which you are reading — which is why this chart tells you.

Placidus

The most common house system in modern Western astrology. It divides time rather than space, so its houses are unequal.

What it is: A quadrant system that divides the time a degree takes to travel from the horizon to the meridian, rather than dividing the circle geometrically. Named after Placidus de Titis, a seventeenth-century monk, and spread largely because it was the system the widely printed house tables happened to use.
Why it matters: It is the default in nearly every Western astrology program and app, so it is what most people have seen if they have had a chart done before. Its houses are unequal, sometimes dramatically, and a planet can land in a different house here than it does under Whole Sign.
Common mistake: Believing it became standard because it was judged the most accurate. It became dominant largely through printing and habit, and its real weakness is high latitude — the houses stretch grotesquely, and above the polar circles it fails altogether.

Equal House

Houses of exactly thirty degrees each, measured from the Ascendant. Simple, and it works at any latitude.

What it is: Take the exact degree of the Ascendant and cut twelve thirty-degree segments from it around the chart. The Ascendant then sits precisely on the first house cusp rather than somewhere inside the first house, and every house is the same size.
Why it matters: It keeps the Ascendant’s exact degree as the starting point, which Whole Sign does not, while avoiding the distortion quadrant systems suffer at high latitude. It is the usual third choice after Placidus and Whole Sign.
Common mistake: Confusing it with Whole Sign because both produce equal houses. They are different: Whole Sign starts each house at zero degrees of a sign, Equal House starts them at the Ascendant’s own degree. With an Ascendant at 22° Leo the two disagree about almost every boundary.

True solar time

Clock time corrected so that noon is when the Sun was actually overhead where you were born.

What it is: Clock time is a civil convention: time zones are wide, their borders are political, and daylight saving shifts the whole thing by an hour. True solar time corrects for the longitude you were really born at, and for the small annual wobble in the Sun’s apparent motion, so that noon means the Sun is genuinely at its highest.
Why it matters: The correction can exceed an hour near the edge of a wide time zone, which is enough to change the hour pillar in Ba Zi and to move the Ascendant by more than half a sign in a Western chart. China is the extreme case, running a single time zone across a span that would naturally take five.
Common mistake: Assuming the time on a birth certificate is astronomically meaningful as written. It records a clock reading, and the clock was set by legislation — turning it into a chart requires knowing the longitude, the time zone in force that day, and whether daylight saving applied.