Choose the reference before the number
A height needs a surface datum; an angle needs a reference direction. “Drop” without a specified tangent or chord can refer to different distances. Use the definitions below when comparing two calculations.
Earth-shape and measurement glossary
Find the meaning of horizon dip, tangent drop, geoid, refraction, azimuth and other terms used in Earth-shape calculations.
16 terms
- Angular diameter
- The angle between the two apparent edges of an object, measured at the observer.
- Azimuth
- Compass direction measured clockwise from north; east is 90° and south is 180°.
- Celestial pole
- The sky direction of Earth’s rotation axis, about which stars appear to turn.
- Chord
- A straight segment joining two points on a circle or sphere.
- Ellipsoid
- A smooth reference surface with different equatorial and polar radii.
- Equipotential
- A surface on which gravitational potential has the same value.
- Geoid
- A gravity-related equipotential surface approximating undisturbed global mean sea level.
- Great circle
- The intersection of a sphere with a plane through its centre; its shorter arc gives a shortest spherical route.
- Hidden height
- The minimum target height that meets the observer’s limiting sight line at the chosen distance.
- Horizon dip
- The angle below the observer’s local horizontal to the geometric horizon.
- Local horizontal
- A plane perpendicular to local gravity at one location.
- Meridian
- A line of longitude through both poles, used for north–south surface comparisons.
- Refraction
- A change in ray direction as refractive index varies along its path.
- Standard uncertainty
- An uncertainty expressed as an estimated standard deviation, distinct from a stated coverage interval.
- Tangent drop
- The surface’s displacement below a specified local tangent plane.
- Zenith angle
- The angular separation from the local upward vertical; its sign can identify which side of zenith a solar observation lies.