Geocentric coordinates
Encyclopedia
Geocentric coordinates are an Earth
-centered system of locating objects in the solar system
in three-dimensions along the Cartesian
X, Y and Z axes. They are differentiated from topocentric coordinates which use the observer's location as the reference point for bearings in altitude
and azimuth. Both systems, however, share a common difficulty in that the Earth is constantly moving, which requires the addition of a time
component to fix objects.
For nearby stars astronomers use heliocentric coordinates, with the center of the Sun
as the origin. The reference plane can either be aligned with the Earth's equator
, the plane of the ecliptic
or the plane of the Milky Way
galaxy. The distances involved are so great compared to the relative velocities of the stars that for most purposes the time component can be neglected.
, "Earth Centred Earth Fixed" coordinate system.
- Earth Centered Inertial
,or ECI
, "Earth Centred Inertial" coordinate system
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
-centered system of locating objects in the solar system
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
in three-dimensions along the Cartesian
Cartesian coordinate system
A Cartesian coordinate system specifies each point uniquely in a plane by a pair of numerical coordinates, which are the signed distances from the point to two fixed perpendicular directed lines, measured in the same unit of length...
X, Y and Z axes. They are differentiated from topocentric coordinates which use the observer's location as the reference point for bearings in altitude
Altitude
Altitude or height is defined based on the context in which it is used . As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The reference datum also often varies according to the context...
and azimuth. Both systems, however, share a common difficulty in that the Earth is constantly moving, which requires the addition of a time
Time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....
component to fix objects.
For nearby stars astronomers use heliocentric coordinates, with the center of the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
as the origin. The reference plane can either be aligned with the Earth's equator
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....
, the plane of the ecliptic
Plane of the ecliptic
The plane of the ecliptic is the plane of the Earth's orbit around the Sun. It is the primary reference plane when describing the position of bodies in the Solar System, with celestial latitude being measured relative to the ecliptic plane. In the course of a year, the Sun's apparent path through...
or the plane of the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
galaxy. The distances involved are so great compared to the relative velocities of the stars that for most purposes the time component can be neglected.
See also
- ECEFECEF
ECEF stands for Earth-Centered, Earth-Fixed, and is a Cartesian coordinate system, and is sometimes known as a "conventional terrestrial" system. It represents positions as an X, Y, and Z coordinate. The point is defined as the center of mass of the earth, hence the name Earth-Centered...
, "Earth Centred Earth Fixed" coordinate system.
- Earth Centered Inertial
Earth Centered Inertial
Earth-centered inertial coordinate frames have their origins at the center of mass of the Earth. ECI frames are called inertial in contrast to the Earth-centered, Earth-fixed frames which rotate in inertial space in order to remain fixed with respect to the surface of the Earth...
,or ECI
ECI
The three letter acronym ECI can refer to:* East Central Indiana, a region in Indiana east of Indianapolis, Indiana, and borders the Ohio state line.* Etobicoke Collegiate Institute, a public high school in Ontario, Ontario, Canada...
, "Earth Centred Inertial" coordinate system