Epoch

class sofia_redux.scan.coordinate_systems.epoch.epoch.Epoch(equinox=None, immutable=False)

Bases: ABC

Initialize an astronomical epoch.

An epoch defines a time or set of times when an observation occurred. This is important when converting between an apparent equinox (time at when a celestial object was observed) and a standard equinox such as J2000.0 (January 1, 2000 at 12:00 TT). The equinox if defined as the two places on the celestial sphere at which the ecliptic intersects with the celestial equator. The Sun’s ascending node is used as this origin for celestial coordinate systems, but moves westward wrt the celestial sphere due to perturbing forces. An epoch defines the date for which the position of a celestial object applies. Therefore, astronomical coordinates require both the date of the equinox and the epoch.

The current standard for the equinox and epoch is J2000.0 with “J” signifying the Julian epoch. Before 1984, the standard was B1950.0 with “B” signifying the Besselian epoch.

Parameters:

equinoxparameter, optional

The given equinox can be a string, int, float, Time, Header, FK4, FK5, or Epoch, or numpy.ndarray or list

immutablebool, optional

If True, the equinox will be fixed and cannot be altered by standard methods.

Attributes Summary

besselian_year	Return the Julian year.
default_epoch
default_format
is_julian	Return whether the epoch if Julian (True) or Besselian (False).
julian_year	Return the Julian year.
mjd	Return the equinox MJD.
ndim	Return the number of dimensions of the time.
shape	Return the shape of the time array.
singular	Return True if the epoch represents a single time.
size	Return the number of time measurements.
year	Return the equinox year.

Methods Summary

copy()	Return a copy of the Epoch.
edit_header(header[, alt])	Edit a FITS header with the epoch data.
empty_copy()	Return a copy of the epoch without times.
get_besselian_epoch()	Get a Besselian representation of the epoch.
get_epoch(epoch)	Return an epoch for a given input.
get_equinox([equinox])	Return an astropy equinox frame from the given input.
get_equinox_from_header(header[, alt])	Return an equinox from a FITS header.
get_indices(indices)	Return selected times for given indices.
get_julian_epoch()	Get a Julian representation of the epoch.
set_mjd(mjd)	Set the MJD for the epoch.
set_year(year)	Set the year values.

Attributes Documentation

besselian_year

Return the Julian year.

Returns:

float or numpy.ndarray

default_epoch = 'J2000'

default_format = 'jyear'

is_julian

Return whether the epoch if Julian (True) or Besselian (False).

Returns:

bool

julian_year

Return the Julian year.

Returns:

float or numpy.ndarray

mjd

Return the equinox MJD.

Returns:

float or numpy.ndarray (float)

ndim

Return the number of dimensions of the time.

Returns:

int

shape

Return the shape of the time array.

Returns:

tuple (int)

singular

Return True if the epoch represents a single time.

Returns:

bool

size

Return the number of time measurements.

Returns:

int

year

Return the equinox year.

Returns:

float or numpy.ndarray (float)

Methods Documentation

copy()

Return a copy of the Epoch.

Returns:

Epoch

edit_header(header, alt='')

Edit a FITS header with the epoch data.

Parameters:

headerfits.header.Header

altstr, optional

The optional alternate system.

Returns:

None

empty_copy()

Return a copy of the epoch without times.

Returns:

Epoch

get_besselian_epoch()

Get a Besselian representation of the epoch.

Returns:

BesselianEpoch

classmethod get_epoch(epoch)

Return an epoch for a given input.

Parameters:

epochthing

The str, int, float, Time, Epoch for which to get the epoch.

Returns:

JulianEpoch or BesselianEpoch

classmethod get_equinox(equinox=None)

Return an astropy equinox frame from the given input.

Parameters:

equinoxparameter

The given equinox can be a string, int, float, Time, Header, FK4, FK5, or Epoch, or numpy.ndarray or list

Returns:

equinoxastropy.units.Time

classmethod get_equinox_from_header(header, alt='')

Return an equinox from a FITS header.

Parameters:

headerastropy.io.fits.header.Header

The header to read.

altstr, optional

The optional alternate system.

Returns:

equinoxastropy.units.Time

get_indices(indices)

Return selected times for given indices.

Parameters:

indicesslice or list or int or numpy.ndarray (int) or None

The indices to extract. If None, an empty copy of the epoch will be returned

Returns:

Epoch

get_julian_epoch()

Get a Julian representation of the epoch.

Returns:

JulianEpoch

set_mjd(mjd)

Set the MJD for the epoch.

Parameters:

mjdint or float or Time or numpy.ndarray

Note that if a int or float value is provided, those MJD values will be assumed to be in UTC scale. If necessary, conversion to TT (Terrestrial Time) will occur if the current equinox is also in TT (default).

Returns:

None

set_year(year)

Set the year values.

Parameters:

yearint or float or numpy.ndarray or Time

Returns:

None