What is nautical twilight?
Nautical twilight is when the Sun's centre is between 6° and 12° below the horizon. It is too dark for most outdoor work without light, but at sea the horizon is still visible while the brightest stars have appeared — exactly what a navigator needs to measure star altitudes with a sextant. That's where the name comes from.
| Phase | Sun below horizon | What you see |
|---|---|---|
| Civil twilight | 0° to 6° | Enough light to work outdoors; brightest planets appear |
| Nautical twilight | 6° to 12° | Horizon still visible at sea; bright stars visible |
| Astronomical twilight | 12° to 18° | Sky looks dark to most people; faint glow remains |
| Night | more than 18° | No sunlight scattered in the sky |