Ecliptic longitude
The angle along the ecliptic measured from the vernal point, 0 to 360° — the coordinate behind every position in a chart.
The ecliptic is the Sun's apparent path. Measured from the vernal point (0° Aries) around the full circle, that angle is ecliptic longitude. Saying "Venus at 12° Taurus" is saying "Venus at longitude 42°".
Signs are just twelve equal cuts of it: 0–30° Aries, 30–60° Taurus, and so on. Aspects are differences between longitudes. Houses are longitudes sliced by twelve cusps. The whole apparatus rests on this one number.
The classic programming trap is wraparound: 359° and 1° are 2° apart, but subtracting gives 358. Every angular comparison has to normalise the difference back into 0–360 or −180–180 first.
Related
- AspectThe angular distance between two points on the ecliptic, counted only when it lands on one of a few specific values.
- HouseOne of twelve divisions of the sky, cut from your birth time and place, each covering an area of life.
- IngressA body crossing one of the 30° boundaries on the ecliptic, moving from one sign into the next.
- EphemerisA table of where celestial bodies are at each moment — or the code that computes one.