The Epoch attribute is stored as a Modified Julian Date, but when setting its value it may be given in a
variety of formats. See the " Input Formats" section (below) for details. Strictly, the Epoch
value should be supplied in the TDB timescale, but for some purposes (for instance, for
converting sky positions between different types of equatorial system) the timescale is not
significant, and UTC may be used.
’ s velocity (e.g. due to the Earth’ s motion around the Sun), (ii) changing
gravitational deflection by the Sun due to changes in the observer’ s position with time, (iii)
fictitious motion due to rotation of non-inertial coordinate systems (e.g. the old FK4 system),
and (iv) proper motion of the source itself (although this last effect is not handled by the
SkyFrame class because it affects individual sources rather than the coordinate system as a
whole).
The default Epoch value in a SkyFrame is B1950.0 (Besselian) for the old FK4-based coordinate systems (see the System attribute) and J2000.0 (Julian) for all others.
Care must be taken to distinguish the Epoch value, which relates to motion (or apparent motion) of the source, from the superficially similar Equinox value. The latter is used to qualify a coordinate system which is itself in motion in a (notionally) predictable way as a result of being referred to a slowly moving reference plane (e.g. the equator).
See the description of the System attribute for details of which qualifying attributes apply to each celestial coordinate system.
" typical" time spanned by the TimeFrame. The default Epoch value for a TimeFrame will
be the TDB equivalent of the current value of the TimeFrame’ s TimeOrigin attribute. If
no value has been set for TimeOrigin, then the default Epoch value is J2000.0. Besselian Epoch: Expressed in decimal years, with or without decimal places (" B1950" or " B1976.13"
for example).
Julian Epoch: Expressed in decimal years, with or without decimal places (" J2000" or " J2100.9" for
example).
Year: Decimal years, with or without decimal places (" 1996.8" for example). Such values
are interpreted as a Besselian epoch (see above) if less than 1984.0 and as a Julian epoch
otherwise.
Julian Date: With or without decimal places (" JD 2454321.9" for example).
Modified Julian Date: With or without decimal places (" MJD 54321.4" for example).
Gregorian Calendar Date: With the month expressed either as an integer or a 3-character abbreviation,
and with optional decimal places to represent a fraction of a day (" 1996-10-2" or " 1996-Oct-2.6" for
example). If no fractional part of a day is given, the time refers to the start of the day (zero
hours).
Gregorian Date and Time: Any calendar date (as above) but with a fraction of a day expressed as
hours, minutes and seconds (" 1996-Oct-2 12:13:56.985" for example). The date and time can
be separated by a space or by a " T" (as used by ISO8601 format).
" Year" format described under "
Input Formats" . This is a value in decimal years which will be a Besselian epoch if less
than 1984.0 and a Julian epoch otherwise. By omitting any character prefix, this format
allows the Epoch value to be obtained as either a character string or a floating point value.