Observing basics
凌日
Transit
An inner planet crossing the Sun's disc.
Mercury transits come every 7–13 years.
Related terms
- Zodiacal light — Sunlight scattered by interplanetary dust, a faint cone.
- Air mass — Relative air path; 1 at zenith.
- Extinction — Atmosphere dimming starlight.
- Dark adaptation — Eyes adjusting to darkness over 20–40 minutes.