What is the difference between azimuth and heading?

What is the difference between azimuth and heading?

I understand heading, track, radial, course and bearing but I don't know what azimuth is. I rarely hear it used. The Wikipedia definition is a horizontal angle measured clockwise from a north base line or meridian. How does that differ from heading?

Bonus question: when atc tells you there's a plane at your 2 o'clock, is there a word for that?

• duplicate? aviation.stackexchange.com/q/8000/1467 Feb 3, 2016 at 18:25
• @Federico Not sure it's a duplicate, but definitely closely related. Feb 3, 2016 at 18:44
• Not a duplicate because the other question does not mention azimuth, which is what I'm confused about. If somebody wants to edit azimuth into the other answers then close this as a dupe that would work Feb 3, 2016 at 18:58
• The word is relative bearing, a bearing relative to your heading. Azimuth is a bearing relative to North as @voretaq7 mentioned. An azimuth is a type of bearing.
– mins
Feb 3, 2016 at 19:15
• @mins I thought relative bearing was putting up with your in-laws for the weekend :D Feb 3, 2016 at 19:21

A heading (in the general case of moving "forward") is the direction your nose is pointed in. This may not be your course (as discussed here).
Headings are measured from onboard a traveling vehicle or object (e.g. from the cockpit of an aircraft or the bridge of a ship at sea).

An azimuth is a bearing, more precisely a compass bearing from a specific point of observation like a radar station. (The "North" used as a reference may be either magnetic or true depending on the system you're working with, but for purposes of this discussion it doesn't matter.)
Often the point of observation is fixed (ground radar, a control tower, an artillery spotter, etc.), though it need not be as long as the observation can be mapped to a compass bearing somehow.

Bonus Answer: The "O'Clock" positions are an informal type of relative bearing as mins noted: The 360-degree circle is broken up into 12 chunks (30 degrees each) for easier scanning, with "12 O'Clock" being directly in front of you and "6 O'Clock" directly behind.

An important caveat to be aware of is that when ATC provides a "clock bearing" for traffic it's relative to the aircraft's course (12 O'Clock is aligned with the aircraft's track over the ground as shown on the radar). Depending on wind there may be considerable variation between your course (direction of travel) and your heading (direction the nose is pointing), so the called traffic may appear in a different location when you look out your window.

• So azimuth is from the POV of the radar and heading is from the POV of the aircraft? Feb 3, 2016 at 19:04
• @TomMcW Essentially correct: as you can see here the azimuth reported by the two stations is different, but the plane's heading is still roughly west-by-southwest: Heading is always relative to the vehicle you're on. Feb 3, 2016 at 19:24
• Sometimes ATC will use azimuth instead of clock if your airplane is maneuvering.
– rbp
Apr 25, 2016 at 14:52