Method Deck / astronomy

Initial bearing between two points

Computes the initial compass bearing (forward azimuth) from one latitude/longitude point toward another along the great circle.

Published for review. The Founder is checking every card himself; anything he strikes comes down the same hour.

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lat1 (number, degrees, required) min -90 max 90 : Latitude of the starting point.
lon1 (number, degrees, required) min -180 max 180 : Longitude of the starting point.
lat2 (number, degrees, required) min -90 max 90 : Latitude of the destination point.
lon2 (number, degrees, required) min -180 max 180 : Longitude of the destination point.

bearing_degrees (degrees from true north, 0-360): Initial great-circle bearing from point 1 to point 2.

Convert both points' latitudes and longitudes to radians.
theta = atan2( sin(dLon)*cos(lat2), cos(lat1)*sin(lat2) - sin(lat1)*cos(lat2)*cos(dLon) ), where dLon = lon2 - lon1.
Convert theta to degrees and normalize into the range 0 to 360 by adding 360 and taking the result modulo 360.
Round to 2 decimals.
This is the initial bearing only; the bearing changes continuously along a great-circle route except along the equator or a meridian.

None.

Calculate distance, bearing and more between latitude/longitude points (Movable Type Scripts (Chris Veness), 2026-09-30T19:45:00Z) - Standard spherical-trigonometry formula for initial bearing, consistent with Ed Williams' Aviation Formulary.

Name	Inputs	Answer
worked example	`{"lat1":0,"lon1":0,"lat2":0,"lon2":90}`	90.00 degrees
test vector: London to Paris	`{"lat1":51.5074,"lon1":-0.1278,"lat2":48.8566,"lon2":2.3522}`	148.12 degrees

Address: 007fa1b8a433d439095d9ab9e351bea2
Made at: 2026-09-30T22:39:30.722Z
Method sha256: e3b64c85d971f395275b0a0fe8b8a7376170a6e4c9c87a05184c6a6ca420694a