heliometer
A specialized astronomical instrument for measuring the sun's diameter or angular distances between celestial bodies. Technical term.
19th-century astronomers used a heliometer to precisely measure the sun's diameter.
Compound of 太陽 (taiyō, 'sun') + 儀 (gi, 'instrument'). The exact historical derivation is uncertain; the spelling is conventionally associated with the meaning 'heliometer'.