A heading was a common way to describe the direction of travel of a starship. Headings were programmed in at the ship's helm control using a combination of two 360-degree directions on planes that are at a right angles to one another, separated by the word "mark". (TNG: "Datalore")
The terms heading and course were often used as synonyms. (TNG: "Brothers"; TOS: "The Deadly Years", etc.) Starships traveling at warp speeds usually traveled in a straight line to the destination coordinates. (VOY: "Drive") While sometimes, a course could also be a more elaborate flight path, that included changes in heading and parabolic trajectories. (VOY: "Alice"; Star Trek II: The Wrath of Khan)
Examples of headings
- Heading from the Bajoran system to the Klingon Empire was 327-mark-215. (DS9: "Sons of Mogh")
- Heading from Deep Space 9 to the Cardassian Empire was 269-mark-032. (DS9: "The Way of the Warrior")
- Heading from the Federation-Tzenkethi border to the Tzenkethi planet M'kemas III was 015-mark-47 (DS9: "The Adversary")
- Heading from the Romulan Neutral Zone to Vulcan was 143-mark-012. (TNG: "Unification II")
- Heading from Starbase 234 to the Klingon-Romulan border was 118-mark-357. (TNG: "Redemption II")
- Heading from Starbase 375 to the Cardassian border was 054-mark-093. (DS9: "A Time to Stand")
- Heading from Tarsas III to Bynaus was 233-mark-45. (TNG: "11001001")
- Heading from the Vulcan system to the Laurentian system was 151-mark-3. (Star Trek)
- In 2371, USS Voyager was holding a course 351-mark-10 on it's voyage back to Earth from the Delta Quadrant. (VOY: "Time and Again")
The first use of headings in degrees occurred in Star Trek: The Original Series episode "Balance of Terror". The meaning of headings as two 360-degree angles was established in "Datalore". The system of degrees was further explained in reference books, such as Star Trek: The Next Generation Technical Manual (pgs. 36, 37).
The first 360-degree figure is a direction in an imaginary plane similar to the galactic plane, but between the ship and the center of the galaxy. What ever the location of the ship in the galaxy is, heading 000-mark-0 is always a direct course to the center of the galaxy. Heading 180-mark-0 is a direct heading away from it. A heading from Earth directly to the Beta Quadrant would be 090-mark-0 and further into the Alpha Quadrant 270-mark-0.
As explained in "Datalore", the second 360-degree figure is the elevation angle at a right angle to the previous plane. A heading of 000-mark-90 would be a heading directly upwards away from the plane, where as 000-mark-270 a heading directly downwards.