Editing Geodesy (section)

== Observational concepts ==
[[File:AxialTiltObliquity.png|285px|thumb|Axial tilt (or [[Obliquity]]), rotation axis, plane of [[orbit]], [[celestial equator]] and [[ecliptic]]. [[Earth]] is shown as viewed from the [[Sun]]; the orbit direction is counter-clockwise (to the left).]]
[[File:Global Gravity Anomaly Animation over OCEANS.gif|285px|thumb|right|Global [[gravity anomaly]] animation over oceans from the NASA's GRACE (Gravity Recovery and Climate Experiment)]]

As defined in geodesy (and also [[astronomy]]), some basic observational concepts like angles and coordinates include (most commonly from the viewpoint of a local observer):

* '''[[Plumbline]]''' or '''vertical''': (the line along) the direction of local gravity.
* '''[[Zenith]]''': the (direction to the) intersection of the upwards-extending gravity vector at a point and the [[celestial sphere]].
* '''[[Nadir]]''': the (direction to the) antipodal point where the downward-extending gravity vector intersects the (obscured) celestial sphere.
* '''Celestial horizon''': a plane perpendicular to the gravity vector at a point.
* '''[[Azimuth]]''': the direction angle within the plane of the horizon, typically counted clockwise from the north (in geodesy and astronomy) or the south (in France).
* '''[[Elevation]]''': the angular height of an object above the horizon; alternatively: [[zenith distance]] equal to 90 degrees minus elevation.
* '''Local topocentric coordinates''': azimuth (direction angle within the plane of the horizon), elevation angle (or zenith angle), distance.
* '''North [[celestial pole]]''': the extension of Earth's ([[precession|precessing]] and [[nutation|nutating]]) instantaneous spin axis extended northward to intersect the celestial sphere. (Similarly for the south celestial pole.)
* '''Celestial equator''': the (instantaneous) intersection of Earth's equatorial plane with the celestial sphere.
* '''[[meridian (geography)|Meridian]] plane''': any plane perpendicular to the celestial equator and containing the celestial poles.
* '''Local meridian''': the plane which contains the direction to the zenith and the celestial pole.