Moonrise Moment

Observing glossary

What is horizon?

Horizon is the apparent boundary where sky meets land or sea. Local terrain and buildings can create a practical horizon above the astronomical horizon.

The Moon above an ocean horizon with altitude and azimuth guide arcs.
A visual guide to horizon. Original Moonrise Moment illustration.

What it means

Horizon is part of the practical language of skywatching. It helps describe where the Moon appears, how it moves across a local sky, or what an observer actually sees. Because every horizon and viewing location is different, observing terms are most useful when paired with a specific place and time.

For a deeper look with practical observing advice, read the Moonrise Moment guide to horizon.

Keep exploring

Observing Altitude

The angle of an object above the observer's horizon, measured from 0 degrees at the horizon to 90 degrees at the zenith.

 Observing Azimuth

A compass direction measured clockwise around the horizon, usually from 0 degrees at north through 90 degrees at east.

 Observing Culmination

The moment a celestial object crosses the observer's meridian and usually reaches its highest altitude for that passage.

 Observing Earthshine

The faint glow on the Moon's unlit portion caused by sunlight reflecting from Earth to the Moon and back toward the observer.

Sources and further reading

This definition was written in plain language and checked against these science references.

Moonrise Moment showing the live direction and countdown screen.

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