A faint glow of light in the night sky seen exactly opposite the Sun. The gegenschein is sunlight back-scattered off millimeter-sized dust particles along the ecliptic. In comparison with zodiacal lig
ht (forward-scattered sunlight), which is triangular in shape and found near the horizon, the gegenschein is roughly circular.
GEI Geocentric Equatorial Inertial - Acoordinate system where the Z axis is along Earth's spin vector, positivenorthward. X axis points towards the first point of Aries (from the Earthtowards the Sun
at the vernal equinox). See Russell, 1971. When the X axisis the direction of the mean vernal equinox of J2000, the coordinate systemis also called GCI. Then the Z axis is also defined as being normal to themean Earth equator of J2000.
This system has its Z axis parallel to the Earth's rotation axis (positive to the North) and its X axis towards the First Point of Aries (the direction in space defined by the intersection between the
Earth's equatorial plane and the plane of its orbit around the Sun (the plane of the ecliptic). This system is (to first order) fixed with respect to the distant stars. It is convenient for specifying the orbits (and hence location) of Earth-orbiting spacecraft as one can specify a Keplerian orbit in this frame.
However note that the GEI system is subject to second order change with time owing to the various slow motions of the Earth's rotation axis with respect to the fixed stars. Thus for GEI coordinates one must specify the date (normally termed the epoch) to which the coordinate system applies. For space physics work one should use the epoch-of-date GEI system, i.e. the system applying at the same time as the data were taken. (Thus the rotation axis in GEI is identical with the GEO rotation axis.)
Spacecraft orbits and locations are often made available in geocentric equatorial inertial coordinates for a fixed epoch, e.g. the standard astronomical epoch known as J2000.0, which is 12:00 UT on 1
January 2000. We treat this as a separate coordinate system (with the qualified acronym GEI2000) and specify how to transform from this to other systems.