Celestia/Glossary

From Wikibooks, open books for an open world
Jump to navigation Jump to search

Introduction[edit | edit source]

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[edit | edit source]

Barycenter[edit | edit source]

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

catalog[edit | edit source]

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[edit | edit source]

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

DSC[edit | edit source]

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[edit | edit source]

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

Galaxy[edit | edit source]

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

GlobularCluster[edit | edit source]

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

Hipparcos[edit | edit source]

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

Horizons[edit | edit source]

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

Julian Date[edit | edit source]

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[edit | edit source]

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

map[edit | edit source]

  1. An image file providing information about the visual characteristics of an object.
  2. 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[edit | edit source]

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

NAIF[edit | edit source]

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

Nebula[edit | edit source]

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

normal[edit | edit source]

  1. 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.
  2. conventional or standard surface texture; not an AltSurface.

object[edit | edit source]

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

OpenCluster[edit | edit source]

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

path[edit | edit source]

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

position[edit | edit source]

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

ReferencePoint[edit | edit source]

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[edit | edit source]

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[edit | edit source]

A trajectory defined using NAIF SPICE kernels.

SSC[edit | edit source]

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

Star[edit | edit source]

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

STC[edit | edit source]

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

trajectory[edit | edit source]

The positions traversed by a moving object.

Universal Coordinate System[edit | edit source]

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.