Observing basics
天文暗
Astronomical darkness
Sun more than 18° below the horizon, when twilight is gone.
Around the summer solstice Beijing barely reaches true darkness.
Related terms
- Meteor shower — Many meteors when Earth crosses a comet's dust trail.
- ZHR — The theoretical hourly rate of a shower under perfect skies.
- Sidereal day — One rotation relative to the stars, about 4 minutes short of a solar day.
- Solar eclipse — The Moon covering the Sun.