Twilight is the period between dawn and sunrise, or between sunset and dusk, when the Sun is below the horizon but its rays still illuminate the upper atmosphere.
Astronomers and meteorologists divide twilight into three distinct legal and scientific phases based on the Sun's angle below the horizon.
The 3 Stages of Twilight Defined
1. Civil Twilight (Sun 0° to 6° Below Horizon)
- Morning Civil Dawn: Begins when the Sun reaches 6° below the horizon until sunrise.
- Evening Civil Dusk: Begins at sunset until the Sun sinks 6° below the horizon.
- Visibility: Artificial street lighting is generally not required for outdoor activities. The brightest stars (like Venus and Jupiter) become visible in the sky.
2. Nautical Twilight (Sun 6° to 12° Below Horizon)
- Origin: Historically crucial for sea navigators measuring star positions against a visible sea horizon line.
- Visibility: The horizon line remains distinguishable at sea, while major navigation stars shine brightly. General ground activities require artificial illumination.
3. Astronomical Twilight (Sun 12° to 18° Below Horizon)
- Stargazing Boundary: Marks the transition into true astronomical night.
- Visibility: Sky brightness is nearly indistinguishable from complete dark night. Deep-sky objects like nebulae, distant galaxies, and faint Milky Way stars become visible to astronomers once Astronomical Twilight ends.
Summary Comparison Table
| Phase | Solar Angle | Natural Light Level | Key Use Case |
|---|---|---|---|
| Civil Twilight | 0° to -6° | Bright enough for outdoor reading | Legal outdoor lighting standards |
| Nautical Twilight | -6° to -12° | Sea horizon line still visible | Marine navigation & sailing |
| Astronomical Twilight | -12° to -18° | Faint atmospheric glow | Deep-sky astronomy & stargazing |