An astronomical instrument used to measure the diameter of the sun or the angular distance between celestial objects, using a precise optical system.
From Greek 'helios' (sun) + 'metron' (measure). Invented in the 18th century, heliometers were crucial tools for precise astronomical measurements.
Before GPS and computers, heliometers let astronomers measure distances to distant stars by watching how the sun appeared to move—it's like using the sun as a cosmic ruler!
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