meridian plane

The axis lies within the Greenwich observatory's meridian plane.

The intersection of the meridian plane with the horizontal contains the true north-south geographic reference bearing required.

A meridian plane is any plane perpendicular to the celestial equator and containing the celestial poles.

The spacecraft was to be spin-stabilized at 1.8 revolutions per second, the spin direction approximately perpendicular to the geomagnetic meridian planes of the trajectory.

(The prime vertical is the vertical plane orthogonal to the meridian plane at a point on the ellipsoid).

The angle may be expressed as negative east of the meridian plane and positive west of the meridian plane, or as positive westward from 0 to 360 .

Planes which contain the rotation axis intersect the surface in the meridians and the angle between any one meridian plane and that through Greenwich (the Prime Meridian) defines the longitude: meridians are lines of constant longitude.

The hour angle of a point is the angle between two planes: one containing the Earth's axis and the zenith (the meridian plane), and the other containing the Earth's axis and the given point (the hour circle passing through the point).