heliometer
noun
he·li·om·e·ter ˌhē-lē-ˈä-mə-tər 
: a visual telescope that has a divided objective designed for measuring the apparent diameter of the sun but also used for measuring angles between celestial bodies or between points on the moon
heliometric
ˌhē-lē-ō-ˈme-trik
adjective heliometrically
ˌhē-lē-ō-ˈme-tri-k(ə-)lē
adverb 
Word History
Etymology
French héliomètre, from hélio- heli- entry 1 + -mètre -meter
First Known Use
1753, in the meaning defined above