The TimeFrame class supports the following System values and
associated coordinate systems (the default is "MJD"):
- "MJD": Modified Julian Date (d)
- "JD": Julian Date (d)
- "JEPOCH": Julian epoch (yr)
- "BEPOCH": Besselian (yr)
The default value for the Unit attribute for each system is shown
in parentheses. Strictly, these systems should not allow changes
to be made to the units. For instance, the usual definition of
"MJD" and "JD" include the statement that the values will be in
units of days. However, AST does allow the use of other units
with all the above supported systems (except BEPOCH), on the
understanding that conversion to the "correct" units involves
nothing more than a simple scaling (1 yr = 365.25 d, 1 d = 24 h,
1 h = 60 min, 1 min = 60 s). Besselian epoch values are defined
in terms of tropical years of 365.2422 days, rather than the
usual Julian year of 365.25 days. Therefore, to avoid any
confusion, the Unit attribute is automatically cleared to "yr" when
a System value of BEPOCH System is selected, and an error is
reported if any attempt is subsequently made to change the Unit
attribute.
Note that the default value for the ActiveUnit flag
is .TRUE.
for a TimeFrame, meaning that changes to the Unit attribute for
a TimeFrame will result in the TimeFrame being re-mapped within
its enclosing FrameSet in order to reflect the change in units
(see AST_SETACTIVEUNIT routine for further information).