Worlds of David Darling
Encyclopedia of Science
Home > Encyclopedia of Science


The term "heliograph" may apply to either of two devices:
  1. A 19th-century instrument used for mainly military signaling, comprising essentially a mirror and a shutter to cut off the sunlight reflected from it. A further mirror permitted messages to be sent even when the Sun was behind the sender. The signals could be interpreted up to 50 km away.

  2. A device for recording the positions of sunspots.

Related category