Bearing is regularly defined as the direction (relative to true north) in which your destination lies. If you could go in a straight line to your destination, this would be the most direct way of getting there.
You also have relative bearing, where the front of the vehicle is considered "north"; you'll see this when pilots claim, for example, that something is "at your 3 o'clock"; in naval terminology, port (left) and starboard (right) are always relative to the bow (front of the vessel).
Heading is the direction your face/nose/front/bow is currently pointing at (relative to true north), assuming default movement by your vehicle's impulse mechanism is forward. If there are no obstacles between you and your destination, and nothing aside from your vehicle's own impulse mechanism is moving it, heading should always be equal to bearing.
Course describes the entire planned route to get to your destination. A course correction implies that your planned route encountered an obstacle, and that you have to correct that plan.
As an example, let's assume a car is the vehicle, and it's navigating in a standard grid-like street structure. If you're at point (0, 0) with your car facing north and your destination is at (3, 3), your course would be "Three blocks north, then three blocks east". Your bearing at the start of your journey would be north-east, and your heading would be north, since that's the way your car is facing.
After you travel three blocks north, you're stopped at a red light. You should plan to turn right, to change your heading to east, since your bearing is now east. Your remaining course is "3 blocks east".
In aviation, where winds can and often will push you off-course, heading is rarely equal to bearing when the total travel distance is large. This makes for frequent course corrections to account for wind.
In my opinion, direction is the most basic term of all, and describes a directed vector between two points.
For your example, it seems like the proper phrasing would be "We are heading north, but we're being pushed off-course eastwards."
The word twilight actually means what you're looking for:
twilight, n.: the light from the sky between full night and sunrise or between sunset and full night
Animals that are active during twilight are said to be crepuscular, so if you're looking for an adjective to apply to that period, crepuscular (which is itself derived from the Latin for twilight) should serve.
Edit: Lacking the context in which you wish to use the term you're looking for, I can offer these suggestions:
Sunset/sunrise occur at the instant that the sun's upper limb is tangent to the horizon. You could then, for example, have science officer of some enterprising starship reporting, "Two degrees to limb tangency, Captain" to mark some impending sunrise/sunset/moonrise/moonset/et ceteraset.
Sunset/sunrise occur at a precise zenith distance of 90.8333 degrees. You could coin a term for this value, so that the same science officer could report, "Two arc seconds from zenith distance alpha, Captain."
Best Answer
Occupants
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