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Specifying a Particular Time System

By setting the System attribute appropriately, the TimeFrame can represent Julian Date, Modified Julian Date, Julian Epoch or Besselian Epoch (the time scale is specified by a separate attribute called TimeScale).

Selection of a particular coordinate system is performed simply by setting a value for the TimeFrame's (character string) System attribute. This setting is most conveniently done when the TimeFrame is created. For example, a TimeFrame representing Julian Epoch would be created by:

timeframe = astTimeFrame( "System=JEPOCH" );

Note that specifying ``System$=$JEPOCH'' also changes the associated default Unit (from days to years). This is because the default value of the TimeFrame's Unit attribute depends on the System attribute setting.

You may change the System value at any time, although this is not usually needed. The values supported are set out in the attribute's description in [*].


next up previous 264
Next: Attributes which Qualify Time Coordinate Systems
Up: Time Systems (TimeFrames)
Previous: Creating a TimeFrame

AST A Library for Handling World Coordinate Systems in Astronomy
Starlink User Note 211
R.F. Warren-Smith & D.S. Berry
24th May 2011
E-mail:ussc@star.rl.ac.uk

Copyright (C) 2009 Science \& Technology Facilities Council