Epoch - Epoch of observation

Description:

This attribute is used to qualify the co-ordinate system described by a Frame, by giving the moment in time when the co-ordinates are known to be correct. Often, this will be the date of observation.

The Epoch value is important in cases where the co-ordinate system changes with time. For instance, when considering celestial co-ordinate systems, possible reasons for change include: (i) changing aberration of light caused by the observer'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 co-ordinate 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 co-ordinate system as a whole).

The Epoch attribute is stored as a Modified Julian Date, and is not usually changed.

Type:

Floating point.

Notes:

• 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 co-ordinate 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 co-ordinate system.

Input Formats :

The formats accepted when setting an Epoch  value are listed below. They are all case-insensitive and are generally tolerant of extra white space and alternative field delimiters.

• 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).

Output Format :

When enquiring Epoch values, the format used is the ``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.