An angle on a named reference circle
Ecliptic longitude is an eastward angular coordinate around the ecliptic. The US Naval Observatory defines it from the dynamical equinox; Skyfield places 0° at the March equinox and reports longitude from 0° to 360°. A longitude is incomplete unless its reference frame and origin are known.
Longitude and ecliptic latitude are separate coordinates. Two objects can have similar longitudes while lying different angular distances north or south of the ecliptic, and neither coordinate alone supplies distance. A chart label is a transformation of a declared coordinate, not a measurement of an object’s physical size, nearness, or influence.
US Naval Observatory · Astronomical Almanac Glossary ↗Skyfield · Coordinates ↗
Turn a longitude into a zodiac label
A twelve-sign zodiac divides its circle into equal 30° sectors after choosing an origin. In an invented equinox-based example, 73°18′ is 13°18′ after the 60° boundary, so it is Gemini 13°18′. The sign boundary occurs at the exact underlying longitude: 89°59′ remains Gemini, while 90°00′ begins Cancer.
A sidereal result requires a separately named sidereal reference and offset convention. The underlying object need not have moved when the displayed sign changes. Keep the raw longitude, zodiac setting and any ayanamsha together when comparing reports.
Record enough to reproduce it
A usable calculation record includes the object, instant and time scale, observer choice where relevant, ephemeris or computation model, coordinate frame or epoch, and zodiac origin. Apparent and geometric positions, geocentric and topocentric positions, and date-specific reference frames can yield different coordinates without any arithmetic mistake.
Use Chart data and conventions to compare the displayed longitude before comparing a sign, house, or aspect. Do not round a value before testing a boundary; carry precision through the calculation and round only for display.
Keep coordinate and reading distinct
Longitude can establish a position under declared conventions. Traditional meanings assigned to signs, aspects, or houses require a source within the selected system; they do not follow from the angle by astronomical necessity. A tropical reading and a sidereal reading also require their own stated interpretive methods.
For a comparison, first preserve the common birth or event data, then list every setting that changes. This makes a changed label inspectable and prevents an interpretation from being carried across a coordinate convention without acknowledgement.
Word origin · etymology
Classical roots in an English compound.
Longitude comes from Latin longitudo, length; ecliptic specifies the reference circle along which this angular coordinate is measured.
How we know · epistemology
Coordinate convention
This angle can be reproduced from a position and a declared zodiac origin. A different origin changes the number without moving the object; longitude alone omits latitude and distance.
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Use the citations beside each section to distinguish a calculation convention, a historical teaching and an editorial example.