In astronomy and celestial navigation, an ephemeris (plural: ephemerides; from the Greek word ἐφημερίς ephēmeris "diary", "journal") gives the positions of astronomical objects in the sky at a given time or times. The class Ephemeris can be used to retrieve the positions of the sun and the moon. It also computes transformations which can be used to convert between different astronomical coordinate system. Further, the light contributions of the sun and the moon are estimated.

The input for all computations are the position (specified using Latitude, Longitude and Altitude) and the current Time. All derived values are computed when Update is called. That means, Update must be called every time the input properties are changed. It is not called automatically, so Update must be called at least once.

Following coordinate systems are used. All coordinate system are right-handed and can be used with cartesian coordinates (X, Y, Z) or polar coordinates (latitude, longitude).

Ecliptic Coordinate System:
This coordinate system is relative to the plane defined by the path of the sun or (which is the same) the plane in which the earth moves around the sun. That means, in the ecliptic system the latitude of the sun or the earth is always 0.
Latitude and longitude are 0 at the vernal equinox. Latitude in this space is also called declination. Longitude is also called right ascension.
Regarding Cartesian coordinates, the x and y axes are in the plane of the earth orbit. x is the axis where latitude and longitude are 0, which is equal to the vernal equinox. +z points north. The origin of the coordinate system can be the sun (heliocentric) or the earth (geocentric).

Equatorial Coordinate System:
This coordinate system is relative to the plane defined by the earth's equator. Latitude and longitude are 0 at the vernal equinox.
Regarding Cartesian coordinates, the x and y axes are in the plane of the equator. x is the axis where latitude and longitude are 0, which is equal to the vernal equinox. +y points east. +z points north. The origin of the coordinate system can be the sun (heliocentric) or the earth (geocentric).

Geographic Coordinate System:
This coordinate system is relative to the plane defined by the earth's equator. This system is like the Equatorial system but the longitude is 0 at Greenwich. This means, the difference to the Equatorial system is a constant longitude offset. This coordinate system is well known from school and globes. The properties Latitude, Longitude are relative to the Geographic Coordinate System.
Regarding Cartesian coordinates, the x and y axes are in the plane of the equator. x is the axis where latitude and longitude are 0, which is in the line of Greenwich. +y points east. +z points north.

World Space:
This coordinate system is relative to a place on the earth. Computer game levels use this coordinate system. It is also known as "Horizontal Coordinate System" or "Horizon Coordinates". The origin of this space is defined by Latitude, Longitude (in the Geographic coordinate system) and Altitude.
Regarding Cartesian coordinates, +x points east, +y points up, -z points north.

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