A message or image transmitted by heliograph, an early method of sending signals using reflected sunlight.
From Greek 'helios' (sun) + '-gram' (something written or recorded). The term emerged in the 19th century when heliography became a practical military communication method.
Before radio existed, soldiers could send secret messages across miles using just mirrors and sunlight—heliograms were used during wars in India and Africa, making the sun itself a telegraph operator!
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