Definition
A navigation instrument that determines true direction (true heading) by sighting on a celestial body — typically the sun, moon, or a known star — and using the body's known position in the sky relative to the observer's latitude, longitude, and the exact time. Unlike a magnetic compass, it is unaffected by magnetic variation, magnetic deviation, or proximity to the magnetic poles.
Plain English
A direction-finding tool that uses the sun or stars instead of magnetism to tell you which way is true north. You point it at a celestial object, and the instrument works out your true heading from where that object is in the sky at that moment.
Context Anchor
Seen in long-range, remote-area, and polar navigation discussions, where normal magnetic compass indications may be weak or misleading.
Derivation
From 'astro-' (Greek 'astron', meaning star) and 'compass' (a direction-finding instrument). The name signals the key idea: a compass that takes its reference from the stars rather than from the Earth's magnetic field.
Why Pilots Care
Gives accurate true heading independent of magnetic variation or polar anomalies, allowing reliable navigation when other references fail.
Intuition Check
An astrocompass is not a magnetic compass with a special display. It finds direction from the known position of a sky object, not from a magnetic needle.
Example Sentence 1
On flights crossing the Arctic, the crew used an astrocompass to confirm their true heading when the magnetic compass became unreliable.
Example Sentence 2
On the long overwater leg the navigator cross-checked the astrocompass against the directional gyro every thirty minutes.