# Celestia/Glossary
## Introduction

This is a glossary of Celestia terminology. Many of the terms used with Celestia are very similar to the corresponding astronomical terminology. For a glossary of astronomical terms, see the Astronomical Terms: Appendix to the Wiktionary

## Glossary
### Barycenter

    A (possibly moving) invisible position in space defined in an STC catalog file, around which other STC and SSC objects can orbit.

### catalog

    A text file used to define objects and their positions. Different catalogs are used to define a Star (STC), a planetary system which orbits around a Star (SSC) and objects not associated with a Star (DSC).

### CMOD

    Celestia MODel file: a proprietary format used to define a 3D object.

### DSC

    Deep Space Catalog; used to define glowing Nebulae or Galaxies or invisible OpenCluster objects around which nothing can orbit. Celestia v1.6 adds Globular Clusters.

### EllipticalOrbit

    One of several different types of periodic trajectories, defined using traditional Keplerian parameters. Used in STC and SSC catalog files.

### Galaxy

    A glowing object defined in a DSC catalog file using a PNG template and astronomical coordinates.

### GlobularCluster

    A spherical group of dots defined in a DSC catalog file using parameters which determine how tightly grouped the dots are.

### Hipparcos

    An astrometric satellite and the database created from its observations; used to define the positions of more than 100,000 stars in Celestia.

### Horizons

    JPL's on-line solar system data and ephemeris computation service. Often used to generate orbit and trajectory data files.

### Julian Date

    Julian dates are a continuous count of days and fractions since noon Universal Time on January 1, 4713 BCE (on the Julian calendar). Almost 2.5 million days have transpired since this date. Julian dates are used in Celestia's SSC and STC catalogs for the fields Epoch, Beginning and Ending. These fields use 64-bit floating point (double precision) variables, and can represent a Julian date to about 1 millisecond of precision. The time scale that is the basis for Julian dates is Universal Time. Starting with v1.5, Celestia uses TDB internally and UT for its on-screen display. 0h UT corresponds to a Julian date fraction of 0.5: Julian days go from Noon to Noon instead of from Midnight to Midnight. This is so that the date doesn't change in the middle of nocturnal observations.

### Location

    A Celestia object: defines a label for a position on a body.

### map

* An image file providing information about the visual characteristics of an object.
* A mathematical transform between the geometry of a spheroidal object and a flat surface. The mapping known as "simple cylindrical projection" or "Plate Carré" is used by Celestia to map between flat images (surface texture files) and spherical planets and moons.

### Mesh

* A 3D model used to display an object with an arbitrary shape.
* A list of connected vertices specified in a CMOD 3D model file.

### NAIF

    NASA's Navigation and Ancillary Information Facility. A source for SPICE trajectory files.

### Nebula

    A glowing or invisible object defined in a DSC catalog file by specifying a Mesh and astronomical coordinates.

### normal

* A vector perpendicular to the surface at a position. I.e. a surface normal vector. Used to indicate irregularities and provide dynamic shading on surfaces.
* conventional or standard surface texture; not an AltSurface.

### object

    A body which has its appearance and position defined in a catalog.

### OpenCluster

    A labeled, fixed position defined in a DSC catalog file. It is not necessarily associated with any particular glowing object.

### path

* A line drawn on the screen to indicate the orbit or trajectory of an object.
* The hierarchical list of objects around which the object currently being defined is orbiting.

### position

    A place in a coordinate system. Not to be confused with a Location.

### ReferencePoint

    A (usually moving) invisible position in space defined in an SSC catalog file, around which other SSC objects can orbit. A ReferencePoint has only positional characteristics. It cannot be used to provide any orientation information.

### SPICE

    Spacecraft, Planet, Instrument, C-matrix and Events kernels; not to be confused with Simulation Program with Integrated Circuit Emphasis. A type of NASA trajectory information file. Use of SPICE kernels was introduced in Celestia v1.5.0.

### SpiceOrbit

    A trajectory defined using NAIF SPICE kernels.

### SSC

    Solar System Catalog; used to define non-glowing objects which orbit around Stars, Barycenters and each other.

### Star

    A glowing object defined in an STC catalog file, around which STC and SSC objects can orbit.

### STC

    STar Catalog; used to define glowing Stars or invisible Barycenter objects around which other objects can orbit.

### trajectory

    The positions traversed by a moving object.

### Universal Coordinate System

    The default coordinate system used by Celestia is Ecliptic J2000:
    The fundamental plane is the J2000 Earth ecliptic, and the preferred direction (x-axis) is the J2000 equinox.