Astronomy · Institutional reference
The Astronomical Almanac glossary
US Naval Observatory, Astronomical Applications Department
Definitions of astronomical coordinates, reference planes and time. Ecliptic longitude and the equinox provide the coordinate vocabulary for the tropical-sign demonstration.
Open the source text ↗Where to begin
Find “longitude, ecliptic”, then “ecliptic” and “equinox”. Compare the coordinate definition with the twelve equal intervals in the layers explorer.
Keep in mind
An astronomy reference defines measurements; it does not establish astrological interpretations. The historical zodiac and correspondence texts remain separate sources.