Observing basics
子午线
Meridian
The great circle from north through zenith to south.
An object peaks and steadies as it crosses the meridian.
Related terms
- Twilight — The interval after sunset when the sky still glows.
- Astronomical darkness — Sun more than 18° below the horizon, when twilight is gone.
- Seeing — How much the atmosphere smears a star's image.
- Transparency — How well the sky passes starlight, set by water vapour and dust.