Definition

1. Noun:
   • An astronomical instrument for measuring small angular distances: A heliometer is a specialized telescope designed to measure the angular separation between two celestial objects, typically stars, that are too widely spaced to be observed simultaneously in the standard field of view of a regular telescope. Its name originates from its initial use in measuring the diameter of the Sun.

Usage and Examples

• Noun:
  • The 19th-century astronomer used a heliometer to precisely measure the parallax of nearby stars.
  • Accurate measurements from the heliometer were crucial for determining stellar distances.

Advanced Usage

• Historical Context: The heliometer was a pivotal instrument in the 18th and 19th centuries for astrometry, the branch of astronomy concerned with measuring the positions and motions of celestial bodies. Its design often involved a split objective lens, allowing the observer to superimpose images of two separate stars to measure their angular distance.

Variants and Related Words

• Heliometric (adj): Pertaining to or measured by a heliometer.
  • The heliometric data provided unprecedented accuracy.
• Astrometry (n): The measurement of the positions and motions of stars and other celestial bodies.

Synonyms

• Astrometric instrument: A general term for devices used in positional astronomy.
• Double-image micrometer: A descriptive term for the heliometer's operating principle.

Notes

• While the name contains "helio-" (Sun), the instrument's primary use expanded beyond solar measurements to general stellar angular separations. It has largely been superseded by modern electronic and space-based telescopes.

Noun

1. an instrument used to measure the angular separation of two stars that are too far apart to be included in the field of view of an ordinary telescope