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The celestial-sphere equivalent of longitude, measured in hours (0–24h) eastward from the vernal equinox.

Celestial latitude in degrees. +90° is the north celestial pole, −90° the south.

Angle measured along the plane of Earth's orbit — the coordinate we use for planet positions.

A logarithmic brightness scale where each 5 steps = 100× flux. Lower = brighter.

A continuous day count since 4713 BCE. All astronomical time math uses it internally.

A clock based on Earth's rotation relative to the stars, ~4 min shorter than a solar day.

The beam of light leaving an eyepiece. Larger than your dark-adapted pupil = wasted light.

Empirical resolving power of a telescope: 116″ ÷ aperture(mm).

The central bright spot of a diffraction pattern. Smaller with more aperture.

Closest / farthest point of an orbit around Earth (used for the Moon).

When a planet is opposite the Sun in our sky — usually its best viewing window.

When two bodies share the same ecliptic longitude — they appear close together.