API Reference / progressedLongitude · function

Function: progressedLongitude()

function progressedLongitude(
   engine, 
   body, 
   natalJd, 
   targetJd, 
   yearLength?, 
   zodiac?): number;

Secondary-progressed longitude of a body for a given target date: the "day-for-a-year" method, where one day of real motion after birth maps to one year of life. Equivalent to taking the body's longitude at progressedJd.

Parameters

Parameter	Type	Default value	Description
`engine`	`Engine`	`undefined`	The engine used to evaluate positions.
`body`	`BodyId`	`undefined`	A body id from Engine.bodies.
`natalJd`	`number`	`undefined`	Natal Julian Day (UT).
`targetJd`	`number`	`undefined`	The date to progress to, Julian Day (UT).
`yearLength`	`number`	`TROPICAL_YEAR`	Days per year of life. Defaults to the tropical year.
`zodiac`	`Zodiac`	`"tropical"`	Zodiac for the result. Defaults to tropical.

Returns

number

The progressed ecliptic longitude in degrees, [0, 360).

Example

const natal = julianDay(1990, 6, 10, 14, 30);
progressedLongitude(engine, "moon", natal, julianDay(2025, 6, 10));

See

solarArc and directedLongitude for solar-arc directions.

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