Hemelvlak

Hemelvlak is a term used in Dutch astronomy to describe the plane of the sky—the imaginary two‑dimensional surface onto which directions to celestial objects are projected from a given observer and moment. It is a practical construct for describing the positions and motions of objects as seen from Earth.

In everyday observational astronomy, the hemelvlak is effectively the plane perpendicular to the observer’s line of sight. Coordinates on this plane are usually given as altitude (height above the horizon) and azimuth (direction along the horizon). As the Earth rotates, objects move across the hemelvlak, tracing paths across the sky that depend on the observer’s latitude and the object's celestial coordinates.

The concept is connected to several reference planes that appear on the celestial sphere, such as the celestial equator and the ecliptic. These great circles define directions on the sphere and serve as reference lines when transforming observations between coordinate systems (for example, from horizontal coordinates to equatorial coordinates). While the hemelvlak itself is a projection surface for practical observation, discussions about the celestial equator or the ecliptic are often framed in relation to this plane.

Usage and nuance: in Dutch texts, hemelvlak may refer broadly to the plane of the sky used for visual observation, imaging, and telescope alignment, or more generally to the idea of projecting celestial directions onto a two‑dimensional surface. The exact interpretation can depend on context, but it always centers on describing and mapping how the sky appears from a specific location and time.

See also: celestial sphere, horizon, celestial equator, ecliptic, equatorial coordinate system.