Astrology glossary — every term, in plain English

Astrology has a vocabulary that is two thousand years old and mostly undefined wherever it is used. Here is every word this site relies on, in plain English, with the astronomy separated from the interpretation.

Angle: One of the four cardinal points of a chart: the ascendant, descendant, midheaven and imum coeli. They depend on the birth time and place, not just the date, which is why a chart without a birth time has none.
Arcminute: A sixtieth of a degree, written with a prime: 17°42′. A chart is read to this resolution; the Moon covers about half a degree an hour, so a birth time wrong by ten minutes moves it by roughly five arcminutes.
Ascendant: The point of the ecliptic rising on the eastern horizon at the moment of birth. Its sign is the “rising sign”. It changes about every two hours, which is why it needs a birth time and a birthplace.
Aspect: The angle between two points on the wheel. Seven are traditionally read. Set out in full here.
Ayanamsa: The offset between the tropical and sidereal zodiacs, currently about 24°. Indian astrology subtracts it; Western astrology does not use it.
Cardinal: One of the three modes. The four signs that begin a season: Aries, Cancer, Libra, Capricorn. More here.
Chart: A map of the sky from one place at one moment. Also called a horoscope, from the Greek for “hour-watcher”, which originally referred to the ascendant alone.
Conjunction: Two bodies at the same longitude. The tightest of the aspects and the one with no fixed character of its own.
Cusp: A boundary — between two signs, or between two houses. “Born on the cusp” usually means within a day or two of a sign change, and whether it is one sign or the other is not a matter of opinion: the Sun's longitude decides it.
Decan: A ten-degree third of a sign, also called a face. Thirty-six in the zodiac. All of them here.
Descendant: The point opposite the ascendant, setting on the western horizon. The cusp of the seventh house, which is partnership.
Ecliptic: The plane of the Earth's orbit, and therefore the line the Sun appears to trace against the stars over a year. The zodiac is the band either side of it. Everything in a chart is measured along it.
Element: Fire, earth, air or water. Three signs each. More here.
Ephemeris: A table of where the bodies were on each day. Historically a printed book; on this site, a series of trigonometric terms evaluated in your browser.
Equinox: The two moments each year when day and night are equal, in March and September. The March equinox is the zero point of the tropical zodiac.
Face: The older English word for a decan. This site prefers it.
Fixed: One of the three modes. The four signs at the settled middle of a season: Taurus, Leo, Scorpio, Aquarius.
House: One of twelve divisions of the chart, tied to the horizon rather than to the zodiac, each covering an area of life. There are several competing systems for drawing the boundaries; this site uses whole sign.
Imum coeli: The bottom of the chart, opposite the midheaven. Abbreviated IC. The cusp of the fourth house, which is home and origin.
Longitude, celestial: Position along the ecliptic, measured 0° to 360° from the March equinox. 0° is the start of Aries, 180° the start of Libra.
Luminaries: The Sun and Moon. Not planets, and given wider orbs than the planets in most schools because they carry more weight.
Midheaven: The point where the meridian crosses the ecliptic — the highest point of the ecliptic at that moment. Abbreviated MC. The cusp of the tenth house, which is public standing.
Mode: Cardinal, fixed or mutable. Four signs each. Also called quality or quadruplicity.
Mutable: One of the three modes. The four signs at the end of a season: Gemini, Virgo, Sagittarius, Pisces.
Nodes, lunar: The two points where the Moon's orbit crosses the ecliptic. The north node is where it crosses going north. They move backwards through the zodiac, completing a circuit in about 18.6 years, and eclipses happen when a new or full moon lands near one.
Nutation: A small, fast nodding of the Earth's axis on top of the slow precessional cone, driven mostly by the Moon. Under twenty arcseconds. Included in this site's calculations because leaving it out is a needless approximation, not because anyone would see it.
Obliquity: The tilt of the Earth's axis relative to its orbit — about 23.44°, and very slowly decreasing. It is what produces the seasons and what makes the ascendant depend on latitude.
Ophiuchus: The serpent-bearer, a constellation the Sun crosses for about eighteen days and which has never been a sign in any system. The thirteenth-sign story is here.
Opposition: Two bodies 180° apart. A see-saw rather than a mismatch.
Orb: How far from exact an aspect may be and still count. A matter of school rather than of fact; this site's values are stated on the aspects page.
Precession: The 25,800-year wobble of the Earth's axis, which slides the equinox backwards along the ecliptic by a degree every 72 years. Explained here.
Retrograde: A body appearing to move backwards through the zodiac, which happens because the Earth is overtaking it or being overtaken. Mercury does it about three times a year, for roughly three weeks at a time. Nothing actually reverses.
Rising sign: The sign of the ascendant. Along with the Sun and Moon it is one of the three placements most charts are read from.
Ruler: The planet traditionally assigned to a sign. Before the telescope, seven bodies covered twelve signs, so five ruled two each; Uranus, Neptune and Pluto were added after their discoveries and both readings remain in use.
Sidereal zodiac: A zodiac anchored to the stars rather than to the equinox, used by most Indian astrology. Drifts against the seasons; currently about 24° behind the tropical.
Square: Two bodies 90° apart. The hard aspect that actually builds things.
Stellium: Three or more bodies clustered in one sign or house. Concentrates a great deal of a chart in one register.
Synastry: Comparing two full charts against each other. The reading on this site is not synastry — it works from Sun signs and faces alone, and says so.
Trine: Two bodies 120° apart, necessarily in the same element. The easy angle, and the one most likely to go unexamined.
Triplicity: The three signs of one element, taken as a group. Also the name of the decan scheme this site uses.
Tropical zodiac: A zodiac anchored to the March equinox, used by Western astrology since antiquity. Twelve exactly equal thirty-degree signs, unaffected by precession by construction.
Whole sign houses: The oldest house system: the first house is the entire sign the ascendant falls in, the second the sign after, and so on. Exact everywhere on Earth, including inside the polar circles where other systems break down.

Every termon this site.

Every term
on this site.