Definitions

1. noun

An optical instrument used to measure the angular diameter of celestial objects, especially the Sun, by comparing the image formed by a telescope with that of a solar disk.

“Astronomers used a heliometer to calculate the size of the Sun during the solar eclipse.”

2. noun

A device for measuring the angular diameter of the Sun or other celestial objects.

“The heliometer was used to observe the Sun’s angular diameter during the total solar eclipse.”