Observing basics
波特尔等级
Bortle scale
A 1–9 scale describing how dark a night sky is.
Class 1 shows the zodiacal light; class 9 leaves a handful of stars.
Related terms
- Lunar eclipse — The Moon passing into Earth's shadow.
- Zodiacal light — Sunlight scattered by interplanetary dust, a faint cone.
- Air mass — Relative air path; 1 at zenith.
- Extinction — Atmosphere dimming starlight.