Sky science
Prime Vertical
Direct answer
Prime Vertical: The Prime Vertical is the great circle through zenith, nadir, and the east and west horizon points.
It is the east-west vertical circle of the local sky.
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At a glance
- Found in
- Astronomy & Skywatching
- Related
- Zenith, Astronomical Nadir, Great Circle, Small Circle
It is the east-west vertical circle of the local sky.
A scientific orientation to scale, light, or motion; the article supplies the precise definition.
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The story behind Prime Vertical
Prime-vertical instruments measured stellar positions and latitude from east-west crossings.
A one-minute story
The Wandering Stars
How the moving planets were watched, named and found, and what a transit is.
Why Prime Vertical matters
Prime Vertical supplies a geometric counterpart to the north-south meridian.
Prime Vertical in practice: an example
A star on the prime vertical lies due east or west in azimuth.
Where readings of Prime Vertical go wrong
The usual misreading is to confuse it with the celestial equator or ecliptic.
Sources and editorial method
Research references used for this entry include:
- U.S. Naval Observatory: Astronomical Almanac Glossary - Definitions for astronomical coordinates, time, reference systems, and observable phenomena.
- International Astronomical Union: Standards of Fundamental Astronomy - IAU-maintained standards and algorithms for astronomical reference systems and coordinate transformations.
- Source
- The Great Almanac Dictionary
- Category
- Astronomy & Skywatching
- Last updated
- 2026-09-26
Explore the east-west sky circle
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More in Astronomy & Skywatching
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Common questions
What is the Prime Vertical?
The Prime Vertical is the great circle through zenith, nadir, and the east and west horizon points.
What do people get wrong about the Prime Vertical?
The usual misreading is to confuse it with the celestial equator or ecliptic.
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