Civil twilight and twilight charts
Civil twilight: transition between full daylight and full darkness, timed by sun's disc angle below horizon.
| Twilight | Begins | Ends | Length |
|---|---|---|---|
| Morning | Centre of sun's disc 6° below horizon and ascending | Sunrise | about 25 min |
| Evening | Sunset | Centre of sun's disc 6° below horizon and descending | about 25 min |
Day vs night:
| Term | From | To | Source |
|---|---|---|---|
| Day (daylight) | Beginning of morning civil twilight | End of evening civil twilight | TC AIM |
| Night | End of evening civil twilight | Beginning of morning civil twilight | CARs 101.01(1) |
- Day is not sunrise to sunset; night is its exact reciprocal.
Twilight chart: gives twilight local time for a latitude and date, on the observer's own meridian. Convert to zone (standard) time:
| Observer position | Correction |
|---|---|
| WEST of standard meridian | ADD 4 min per degree |
| EAST of standard meridian | SUBTRACT 4 min per degree |
Zone standard meridians listed in time-zones.
Trap: correction backwards (west subtracts, east adds), or 1 min/degree instead of 4.
References
- TC AIM GEN 1.5.2
- CARs 101.01(1)