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The true meridian is the chord that goes from one pole to the other, passing through the observer, and is contrasted with the magnetic meridian, which goes through the magnetic poles and the observer. The true meridian can be found by careful astronomical observations, and the magnetic meridian is simply parallel to the compass needle.
The circles parallel to the equator are lines of constant latitude, or parallels. The graticule shows the latitude and longitude of points on the surface. In this example meridians are spaced at 6° intervals and parallels at 4° intervals.
On an ellipsoid of revolution, for short meridian arcs, their length can be approximated using the Earth's meridional radius of curvature and the circular arc formulation. For longer arcs, the length follows from the subtraction of two meridian distances, the distance from the equator to a point at a latitude φ.
In normal aspect, these map the central meridian and parallels as straight lines. Other meridians are curves (or possibly straight from pole to equator), regularly spaced along parallels. Conic In normal aspect, conic (or conical) projections map meridians as straight lines, and parallels as arcs of circles. Pseudoconical
The 0° parallel of latitude is designated the Equator, the fundamental plane of all geographic coordinate systems. The Equator divides the globe into Northern and Southern Hemispheres. The longitude λ of a point on Earth's surface is the angle east or west of a reference meridian to another meridian
The latitude of the circle is approximately the angle between the Equator and the circle, with the angle's vertex at Earth's centre. The Equator is at 0°, and the North Pole and South Pole are at 90° north and 90° south, respectively. The Equator is the longest circle of latitude and is the only circle of latitude which also is a great circle.
In another approach known as the horizontal coordinate system, the meridian is divided into the local meridian, the semicircle that contains the observer's zenith and the north and south points of their horizon, [1] [2] and the opposite semicircle, which contains the nadir and the north and south points of their horizon.
Geodetic latitude measures how close to the poles or equator a point is along a meridian, and is represented as an angle from −90° to +90°, where 0° is the equator. The geodetic latitude is the angle between the equatorial plane and a line that is normal to the reference ellipsoid.