Time¶

class astropy.time.Time(val, val2=None, format=None, scale=None, precision=None, in_subfmt=None, out_subfmt=None, location=None, copy=False)[source]¶

Bases: TimeBase

Represent and manipulate times and dates for astronomy.

A Time object is initialized with one or more times in the val argument. The input times in val must conform to the specified format and must correspond to the specified time scale. The optional val2 time input should be supplied only for numeric input formats (e.g. JD) where very high precision (better than 64-bit precision) is required.

The allowed values for format can be listed with:

>>> list(Time.FORMATS)
['jd', 'mjd', 'decimalyear', 'unix', 'unix_tai', 'cxcsec', 'gps', 'plot_date',
 'stardate', 'datetime', 'ymdhms', 'iso', 'isot', 'yday', 'datetime64',
 'fits', 'byear', 'jyear', 'byear_str', 'jyear_str']

Parameters:

valpython:sequence, ndarray, number, python:str, bytes, or Time object: Value(s) to initialize the time or times. Bytes are decoded as ascii.
val2python:sequence, ndarray, or number; optional: Value(s) to initialize the time or times. Only used for numerical input, to help preserve precision.
formatpython:str, optional: Format of input value(s)
scalepython:str, optional: Time scale of input value(s), must be one of the following: (‘tai’, ‘tcb’, ‘tcg’, ‘tdb’, ‘tt’, ‘ut1’, ‘utc’)
precisionpython:int, optional: Digits of precision in string representation of time
in_subfmtpython:str, optional: Unix glob to select subformats for parsing input times
out_subfmtpython:str, optional: Unix glob to select subformat for outputting times
locationEarthLocation or python:tuple, optional: If given as an tuple, it should be able to initialize an an EarthLocation instance, i.e., either contain 3 items with units of length for geocentric coordinates, or contain a longitude, latitude, and an optional height for geodetic coordinates. Can be a single location, or one for each input time. If not given, assumed to be the center of the Earth for time scale transformations to and from the solar-system barycenter.
copybool, optional: Make a copy of the input values

Attributes Summary

`FORMATS`	Dict of time formats
`SCALES`	List of time scales
`T`	Return an instance with the data transposed.
`cache`	Return the cache associated with this instance.
`delta_tdb_tt`	TDB - TT time scale offset
`delta_ut1_utc`	UT1 - UTC time scale offset
`format`	Get or set time format.
`in_subfmt`	Unix wildcard pattern to select subformats for parsing string input times.
`info`	Container for meta information like name, description, format.
`isscalar`
`jd1`	First of the two doubles that internally store time value(s) in JD.
`jd2`	Second of the two doubles that internally store time value(s) in JD.
`mask`
`masked`
`ndim`	The number of dimensions of the instance and underlying arrays.
`out_subfmt`	Unix wildcard pattern to select subformats for outputting times.
`precision`	Decimal precision when outputting seconds as floating point (int value between 0 and 9 inclusive).
`scale`	Time scale
`shape`	The shape of the time instances.
`size`	The size of the object, as calculated from its shape.
`value`	Time value(s) in current format
`writeable`

Methods Summary

`argmax`([axis, out])	Return indices of the maximum values along the given axis.
`argmin`([axis, out])	Return indices of the minimum values along the given axis.
`argsort`([axis])	Returns the indices that would sort the time array.
`copy`([format])	Return a fully independent copy the Time object, optionally changing the format.
`diagonal`(args, *kwargs)	Return an instance with the specified diagonals.
`earth_rotation_angle`([longitude])	Calculate local Earth rotation angle.
`flatten`(args, *kwargs)	Return a copy with the array collapsed into one dimension.
`get_delta_ut1_utc`([iers_table, return_status])	Find UT1 - UTC differences by interpolating in IERS Table.
`insert`(obj, values[, axis])	Insert values before the given indices in the column and return a new `Time` or `TimeDelta` object.
`isclose`(other[, atol])	Returns a boolean or boolean array where two Time objects are element-wise equal within a time tolerance.
`light_travel_time`(skycoord[, kind, ...])	Light travel time correction to the barycentre or heliocentre.
`max`([axis, out, keepdims])	Maximum along a given axis.
`mean`([axis, dtype, out, keepdims, where])	Mean along a given axis.
`min`([axis, out, keepdims])	Minimum along a given axis.
`now`()	Creates a new object corresponding to the instant in time this method is called.
`ptp`([axis, out, keepdims])	Peak to peak (maximum - minimum) along a given axis.
`ravel`(args, *kwargs)	Return an instance with the array collapsed into one dimension.
`replicate`([format, copy, cls])	Return a replica of the Time object, optionally changing the format.
`reshape`(args, *kwargs)	Returns an instance containing the same data with a new shape.
`sidereal_time`(kind[, longitude, model])	Calculate sidereal time.
`sort`([axis])	Return a copy sorted along the specified axis.
`squeeze`(args, *kwargs)	Return an instance with single-dimensional shape entries removed
`strftime`(format_spec)	Convert Time to a string or a numpy.array of strings according to a format specification.
`strptime`(time_string, format_string, **kwargs)	Parse a string to a Time according to a format specification.
`swapaxes`(args, *kwargs)	Return an instance with the given axes interchanged.
`take`(indices[, axis, out, mode])	Return a new instance formed from the elements at the given indices.
`to_datetime`([timezone])	Convert to (potentially timezone-aware) `datetime` object.
`to_string`()	Output a string representation of the Time or TimeDelta object.
`to_value`(format[, subfmt])	Get time values expressed in specified output format.
`transpose`(args, *kwargs)	Return an instance with the data transposed.

Attributes Documentation

FORMATS = {'byear': <class 'astropy.time.formats.TimeBesselianEpoch'>, 'byear_str': <class 'astropy.time.formats.TimeBesselianEpochString'>, 'cxcsec': <class 'astropy.time.formats.TimeCxcSec'>, 'datetime': <class 'astropy.time.formats.TimeDatetime'>, 'datetime64': <class 'astropy.time.formats.TimeDatetime64'>, 'decimalyear': <class 'astropy.time.formats.TimeDecimalYear'>, 'fits': <class 'astropy.time.formats.TimeFITS'>, 'gps': <class 'astropy.time.formats.TimeGPS'>, 'iso': <class 'astropy.time.formats.TimeISO'>, 'isot': <class 'astropy.time.formats.TimeISOT'>, 'jd': <class 'astropy.time.formats.TimeJD'>, 'jyear': <class 'astropy.time.formats.TimeJulianEpoch'>, 'jyear_str': <class 'astropy.time.formats.TimeJulianEpochString'>, 'mjd': <class 'astropy.time.formats.TimeMJD'>, 'plot_date': <class 'astropy.time.formats.TimePlotDate'>, 'stardate': <class 'astropy.time.formats.TimeStardate'>, 'unix': <class 'astropy.time.formats.TimeUnix'>, 'unix_tai': <class 'astropy.time.formats.TimeUnixTai'>, 'yday': <class 'astropy.time.formats.TimeYearDayTime'>, 'ymdhms': <class 'astropy.time.formats.TimeYMDHMS'>}¶: Dict of time formats

SCALES = ('tai', 'tcb', 'tcg', 'tdb', 'tt', 'ut1', 'utc', 'local')¶: List of time scales

T¶

Return an instance with the data transposed.

Parameters are as for T. All internal data are views of the data of the original.

cache¶: Return the cache associated with this instance.

delta_tdb_tt¶: TDB - TT time scale offset

delta_ut1_utc¶: UT1 - UTC time scale offset

format¶

Get or set time format.

The format defines the way times are represented when accessed via the .value attribute. By default it is the same as the format used for initializing the Time instance, but it can be set to any other value that could be used for initialization. These can be listed with:

>>> list(Time.FORMATS)
['jd', 'mjd', 'decimalyear', 'unix', 'unix_tai', 'cxcsec', 'gps', 'plot_date',
 'stardate', 'datetime', 'ymdhms', 'iso', 'isot', 'yday', 'datetime64',
 'fits', 'byear', 'jyear', 'byear_str', 'jyear_str']

in_subfmt¶: Unix wildcard pattern to select subformats for parsing string input times.

info¶: Container for meta information like name, description, format. This is required when the object is used as a mixin column within a table, but can be used as a general way to store meta information.

isscalar¶

jd1¶: First of the two doubles that internally store time value(s) in JD.

jd2¶: Second of the two doubles that internally store time value(s) in JD.

mask¶

masked¶

ndim¶: The number of dimensions of the instance and underlying arrays.

out_subfmt¶: Unix wildcard pattern to select subformats for outputting times.

precision¶: Decimal precision when outputting seconds as floating point (int value between 0 and 9 inclusive).

scale¶: Time scale

shape¶

The shape of the time instances.

Like shape, can be set to a new shape by assigning a tuple. Note that if different instances share some but not all underlying data, setting the shape of one instance can make the other instance unusable. Hence, it is strongly recommended to get new, reshaped instances with the reshape method.

Raises:

ValueError: If the new shape has the wrong total number of elements.
AttributeError: If the shape of the jd1, jd2, location, delta_ut1_utc, or delta_tdb_tt attributes cannot be changed without the arrays being copied. For these cases, use the Time.reshape method (which copies any arrays that cannot be reshaped in-place).

size¶: The size of the object, as calculated from its shape.

value¶: Time value(s) in current format

writeable¶

Methods Documentation

argmax(axis=None, out=None)¶

Return indices of the maximum values along the given axis.

This is similar to argmax(), but adapted to ensure that the full precision given by the two doubles jd1 and jd2 is used. See argmax() for detailed documentation.

argmin(axis=None, out=None)¶

Return indices of the minimum values along the given axis.

This is similar to argmin(), but adapted to ensure that the full precision given by the two doubles jd1 and jd2 is used. See argmin() for detailed documentation.

argsort(axis=-1)¶

Returns the indices that would sort the time array.

This is similar to argsort(), but adapted to ensure that the full precision given by the two doubles jd1 and jd2 is used, and that corresponding attributes are copied. Internally, it uses lexsort(), and hence no sort method can be chosen.

copy(format=None)¶

Return a fully independent copy the Time object, optionally changing the format.

If format is supplied then the time format of the returned Time object will be set accordingly, otherwise it will be unchanged from the original.

In this method a full copy of the internal time arrays will be made. The internal time arrays are normally not changeable by the user so in most cases the replicate() method should be used.

Parameters:

formatpython:str, optional: Time format of the copy.

Returns:

tmTime object: Copy of this object

diagonal(*args, **kwargs)¶

Return an instance with the specified diagonals.

Parameters are as for diagonal(). All internal data are views of the data of the original.

earth_rotation_angle(longitude=None)[source]¶

Calculate local Earth rotation angle.

Parameters:

longitudeQuantity, EarthLocation, python:str, or None; optional: The longitude on the Earth at which to compute the Earth rotation angle (taken from a location as needed). If None (default), taken from the location attribute of the Time instance. If the special string ‘tio’, the result will be relative to the Terrestrial Intermediate Origin (TIO) (i.e., the output of era00).

Returns:

Longitude: Local Earth rotation angle with units of hourangle.

See also

astropy.time.Time.sidereal_time

Notes

The difference between apparent sidereal time and Earth rotation angle is the equation of the origins, which is the angle between the Celestial Intermediate Origin (CIO) and the equinox. Applying apparent sidereal time to the hour angle yields the true apparent Right Ascension with respect to the equinox, while applying the Earth rotation angle yields the intermediate (CIRS) Right Ascension with respect to the CIO.

The result includes the TIO locator (s’), which positions the Terrestrial Intermediate Origin on the equator of the Celestial Intermediate Pole (CIP) and is rigorously corrected for polar motion. (except when longitude='tio').

References

IAU 2006 NFA Glossary (currently located at: https://syrte.obspm.fr/iauWGnfa/NFA_Glossary.html)

flatten(*args, **kwargs)¶

Return a copy with the array collapsed into one dimension.

Parameters are as for flatten().

get_delta_ut1_utc(iers_table=None, return_status=False)[source]¶

Find UT1 - UTC differences by interpolating in IERS Table.

Parameters:

iers_tableIERS, optional: Table containing UT1-UTC differences from IERS Bulletins A and/or B. Default: earth_orientation_table (which in turn defaults to the combined version provided by IERS_Auto).
return_statusbool: Whether to return status values. If False (default), iers raises IndexError if any time is out of the range covered by the IERS table.

Returns:

ut1_utcpython:float or python:float array: UT1-UTC, interpolated in IERS Table
statuspython:int or python:int array: Status values (if return_status=`True`):: astropy.utils.iers.FROM_IERS_B astropy.utils.iers.FROM_IERS_A astropy.utils.iers.FROM_IERS_A_PREDICTION astropy.utils.iers.TIME_BEFORE_IERS_RANGE astropy.utils.iers.TIME_BEYOND_IERS_RANGE

Notes

In normal usage, UT1-UTC differences are calculated automatically on the first instance ut1 is needed.

Examples

To check in code whether any times are before the IERS table range:

>>> from astropy.utils.iers import TIME_BEFORE_IERS_RANGE
>>> t = Time(['1961-01-01', '2000-01-01'], scale='utc')
>>> delta, status = t.get_delta_ut1_utc(return_status=True)  
>>> status == TIME_BEFORE_IERS_RANGE  
array([ True, False]...)

insert(obj, values, axis=0)¶

Insert values before the given indices in the column and return a new Time or TimeDelta object.

The values to be inserted must conform to the rules for in-place setting of Time objects (see Get and set values in the Time documentation).

The API signature matches the np.insert API, but is more limited. The specification of insert index obj must be a single integer, and the axis must be 0 for simple row insertion before the index.

Parameters:

objpython:int: Integer index before which values is inserted.
valuesnumpy:array_like: Value(s) to insert. If the type of values is different from that of quantity, values is converted to the matching type.
axispython:int, optional: Axis along which to insert values. Default is 0, which is the only allowed value and will insert a row.

Returns:

outTime subclass: New time object with inserted value(s)

isclose(other, atol=None)¶

Returns a boolean or boolean array where two Time objects are element-wise equal within a time tolerance.

This evaluates the expression below:

abs(self - other) <= atol

Parameters:

otherTime: Time object for comparison.
atolQuantity or TimeDelta: Absolute tolerance for equality with units of time (e.g. u.s or u.day). Default is two bits in the 128-bit JD time representation, equivalent to about 40 picosecs.

light_travel_time(skycoord, kind='barycentric', location=None, ephemeris=None)[source]¶

Light travel time correction to the barycentre or heliocentre.

The frame transformations used to calculate the location of the solar system barycentre and the heliocentre rely on the erfa routine epv00, which is consistent with the JPL DE405 ephemeris to an accuracy of 11.2 km, corresponding to a light travel time of 4 microseconds.

The routine assumes the source(s) are at large distance, i.e., neglects finite-distance effects.

Parameters:

skycoordSkyCoord: The sky location to calculate the correction for.
kindpython:str, optional: 'barycentric' (default) or 'heliocentric'
locationEarthLocation, optional: The location of the observatory to calculate the correction for. If no location is given, the location attribute of the Time object is used
ephemerispython:str, optional: Solar system ephemeris to use (e.g., ‘builtin’, ‘jpl’). By default, use the one set with astropy.coordinates.solar_system_ephemeris.set. For more information, see solar_system_ephemeris.

Returns:

time_offsetTimeDelta: The time offset between the barycentre or Heliocentre and Earth, in TDB seconds. Should be added to the original time to get the time in the Solar system barycentre or the Heliocentre. Also, the time conversion to BJD will then include the relativistic correction as well.

max(axis=None, out=None, keepdims=False)¶

Maximum along a given axis.

This is similar to max(), but adapted to ensure that the full precision given by the two doubles jd1 and jd2 is used, and that corresponding attributes are copied.

Note that the out argument is present only for compatibility with np.max; since Time instances are immutable, it is not possible to have an actual out to store the result in.

mean(axis=None, dtype=None, out=None, keepdims=False, *, where=True)[source]¶

Mean along a given axis.

This is similar to mean(), but adapted to ensure that the full precision given by the two doubles jd1 and jd2 is used, and that corresponding attributes are copied.

Note that the out argument is present only for compatibility with np.mean; since Time instances are immutable, it is not possible to have an actual out to store the result in.

Similarly, the dtype argument is also present for compatibility only; it has no meaning for Time.

Parameters:

axispython:None or python:int or python:tuple of python:int, optional: Axis or axes along which the means are computed. The default is to compute the mean of the flattened array.
dtypepython:None: Only present for compatibility with mean(), must be None.
outpython:None: Only present for compatibility with mean(), must be None.
keepdimsbool, optional: If this is set to True, the axes which are reduced are left in the result as dimensions with size one. With this option, the result will broadcast correctly against the input array.
wherenumpy:array_like of bool, optional: Elements to include in the mean. See reduce for details.

Returns:

mTime: A new Time instance containing the mean values

min(axis=None, out=None, keepdims=False)¶

Minimum along a given axis.

This is similar to min(), but adapted to ensure that the full precision given by the two doubles jd1 and jd2 is used, and that corresponding attributes are copied.

Note that the out argument is present only for compatibility with np.min; since Time instances are immutable, it is not possible to have an actual out to store the result in.

classmethod now()[source]¶

Creates a new object corresponding to the instant in time this method is called.

Note

“Now” is determined using the utcnow function, so its accuracy and precision is determined by that function. Generally that means it is set by the accuracy of your system clock.

Returns:

nowtimeTime: A new Time object (or a subclass of Time if this is called from such a subclass) at the current time.

ptp(axis=None, out=None, keepdims=False)¶

Peak to peak (maximum - minimum) along a given axis.

This is similar to ptp(), but adapted to ensure that the full precision given by the two doubles jd1 and jd2 is used.

Note that the out argument is present only for compatibility with ptp; since Time instances are immutable, it is not possible to have an actual out to store the result in.

ravel(*args, **kwargs)¶

Return an instance with the array collapsed into one dimension.

Parameters are as for ravel(). Note that it is not always possible to unravel an array without copying the data. If you want an error to be raise if the data is copied, you should should assign shape (-1,) to the shape attribute.

replicate(format=None, copy=False, cls=None)¶

Return a replica of the Time object, optionally changing the format.

If format is supplied then the time format of the returned Time object will be set accordingly, otherwise it will be unchanged from the original.

If copy is set to True then a full copy of the internal time arrays will be made. By default the replica will use a reference to the original arrays when possible to save memory. The internal time arrays are normally not changeable by the user so in most cases it should not be necessary to set copy to True.

The convenience method copy() is available in which copy is True by default.

Parameters:

formatpython:str, optional: Time format of the replica.
copybool, optional: Return a true copy instead of using references where possible.

Returns:

tmTime object: Replica of this object

reshape(*args, **kwargs)¶

Returns an instance containing the same data with a new shape.

Parameters are as for reshape(). Note that it is not always possible to change the shape of an array without copying the data (see reshape() documentation). If you want an error to be raise if the data is copied, you should assign the new shape to the shape attribute (note: this may not be implemented for all classes using NDArrayShapeMethods).

sidereal_time(kind, longitude=None, model=None)[source]¶

Calculate sidereal time.

Parameters:

kindpython:str: 'mean' or 'apparent', i.e., accounting for precession only, or also for nutation.
longitudeQuantity, EarthLocation, python:str, or None; optional: The longitude on the Earth at which to compute the Earth rotation angle (taken from a location as needed). If None (default), taken from the location attribute of the Time instance. If the special string ‘greenwich’ or ‘tio’, the result will be relative to longitude 0 for models before 2000, and relative to the Terrestrial Intermediate Origin (TIO) for later ones (i.e., the output of the relevant ERFA function that calculates greenwich sidereal time).
modelpython:str or None; optional: Precession (and nutation) model to use. The available ones are: - apparent: [‘IAU1994’, ‘IAU2000A’, ‘IAU2000B’, ‘IAU2006A’] - mean: [‘IAU1982’, ‘IAU2000’, ‘IAU2006’] If None (default), the last (most recent) one from the appropriate list above is used.

Returns:

Longitude: Local sidereal time, with units of hourangle.

Navigation

Time¶