an instrument formerly used for measuring the angular distance between two stars
Webster’s New World College Dictionary, Fourth Edition, compiled by the
editors of Webster’s New World Dictionaries. Copyright © 2010 by Houghton
Mifflin Harcourt. Published by Houghton Mifflin Harcourt. All rights reserved.
Word origin of 'heliometer'
: see helio-
: so called because orig. used in measuring the sun's diameter