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Glossary

Plain-language definitions of astronomy and astrology terms.

Overview

The glossary defines the terms used across the site, clearly separating scientific astronomy vocabulary from astrological and mythological terms so the two are never confused.

  • Astronomical terms have agreed technical definitions; several are routinely misused in popular writing.
  • Some words — 'sign', 'house', 'aspect' — belong to astrology and have no astronomical meaning.
  • Units in astronomy are chosen for the scale of the problem, not for consistency with everyday measures.

Terms that are commonly confused

  • Magnitude is a logarithmic brightness scale running backwards: lower numbers are brighter, and a difference of 5 magnitudes is a factor of 100 in brightness. Apparent magnitude is how bright something looks from Earth; absolute magnitude is how bright it would look at a standard distance.
  • A light-year is a distance, not a time — the distance light travels in a year, about 9.46 trillion kilometres.
  • A parsec is the distance at which one astronomical unit subtends one arcsecond, about 3.26 light-years. Professional literature uses parsecs; popular writing usually uses light-years.
  • A solar system is a specific system; the Solar System with capitals is ours. A planetary system is the general term.
  • A meteoroid is the object in space, a meteor is the streak of light it makes in the atmosphere, and a meteorite is what survives to the ground.
  • An asteroid and a comet differ by composition and behaviour, not size — comets develop a coma and tail when heated because they contain volatiles.

Units astronomers actually use

  • Astronomical unit (au): the mean Earth–Sun distance, defined exactly as 149,597,870,700 metres. Used for Solar System scales.
  • Parsec, kiloparsec, megaparsec: stellar, galactic and cosmological distance scales respectively.
  • Solar mass, solar radius, solar luminosity: stellar properties are almost always quoted relative to the Sun.
  • Jansky: the flux-density unit of radio astronomy.
  • Arcminute and arcsecond: 1/60 and 1/3600 of a degree, the working units of angular size and position.
  • Redshift z: a dimensionless measure of wavelength shift, used as a distance and time proxy at cosmological scales.

Vocabulary that belongs to astrology, not astronomy

Several words appear in both contexts with entirely different meanings, and conflating them is the most common source of confusion for readers arriving from either direction.

  • Sign: a 30-degree division of the ecliptic used in astrology. It is not a constellation, and in Western tropical astrology it is not tied to one.
  • House: a division of the local sky used in astrological chart construction. It has no astronomical counterpart.
  • Aspect: an angular relationship between two bodies to which astrology assigns meaning. Astronomers measure the same angles but attach no interpretation.
  • Constellation: an astronomical term with a precise modern definition — one of 88 regions of the sky with boundaries fixed by the International Astronomical Union in 1922 and 1930.

How this glossary is organised

Individual terms are published as entries under this category, each with a definition, the context in which it is used, and links to the catalogue pages where the concept appears in real data. Terms drawn from the interpretive tradition are labelled as such on their own pages, so a reader always knows which register a definition belongs to.

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In-depth, individual pages in this category.

Continue in the data

Catalogues, hubs, and reference pages that hold the underlying records for this topic.

Frequently asked

Why are brighter stars given smaller magnitude numbers?
The scale descends from Hipparchus's second-century-BCE classification, which called the brightest stars 'first magnitude' and the faintest visible ones 'sixth'. When the system was made quantitative in the nineteenth century, that ordering was kept and calibrated so that five magnitudes equal exactly a factor of 100 in brightness. The inversion is historical inheritance, not a physical convention.
What is the difference between a light-year and a parsec?
Both are distances. A light-year is how far light travels in a year, about 9.46 trillion kilometres. A parsec is the distance at which one astronomical unit subtends an angle of one arcsecond — about 3.26 light-years — and it comes directly from parallax measurement, which is why professional astronomy prefers it.
Is a zodiac sign the same as a constellation?
No. A constellation is one of 88 sky regions with boundaries defined by the International Astronomical Union, of very unequal size. A zodiac sign is a 30-degree slice of the ecliptic used in astrology, all equal in size, and in the Western tropical system anchored to the equinox rather than to the stars. The two have drifted apart by roughly one sign over the last two thousand years.

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