Month: July 2025

CELESTIAL EQUATOR
The projection of Earth’s equator onto the celestial sphere.

MERIDIAN
An imaginary line passing through the zenith and celestial poles.

NADIR
The point directly below an observer on Earth.

ZENITH
The point directly overhead in the sky.

HORIZON
The boundary where the sky meets the ground.

CELESTIAL SPHERE
An imaginary sphere surrounding Earth onto which celestial objects are projected.

METONIC CYCLE
A 19-year cycle aligning the lunar and solar calendars.

SIDEREAL DAY
The time it takes for Earth to rotate relative to the stars.