Old instruments
Heliometer
Direct answer
Heliometer is a telescope whose divided objective produces movable duplicate images for precise small-angle measurement.
Heliometer is a split-objective angular micrometer used in historical astronomy, timekeeping, or navigation.
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At a glance
- Found in
- Historical Almanacs & Instruments
- Related
- Zenith Sector, Equatorial Telescope, Micrometer Eyepiece, Filar Micrometer
Heliometer is a split-objective angular micrometer used in historical astronomy, timekeeping, or navigation.
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The story behind Heliometer
Heliometers became important for solar-diameter work, double stars, and the first successful stellar-parallax measurements.
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Why Heliometer matters
Heliometer matters because image displacement lets an observer compare angular separations more finely than a simple scale in the focal plane.
Heliometer in practice: an example
An observer can slide the half-objectives until two duplicated star images meet and read their separation.
Where readings of Heliometer go wrong
Optical distortion, scale calibration, seeing, and observer judgment limit the measurement.
Sources and editorial method
Research references used for this entry include:
- Royal Museums Greenwich: Astronomical and Navigational Instruments - Museum reference collection for historical observing, calculating, timekeeping, and navigation instruments.
- Source
- The Great Almanac Dictionary
- Category
- Historical Almanacs & Instruments
- Last updated
- 2026-09-26
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More in Historical Almanacs & Instruments
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Common questions
What is a heliometer?
Heliometer is a telescope whose divided objective produces movable duplicate images for precise small-angle measurement.
What do people get wrong about a heliometer?
Optical distortion, scale calibration, seeing, and observer judgment limit the measurement.
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