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Correct UT in calculation of TDB; closes #1938 #1939

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Merged
taldcroft merged 2 commits into from
Jan 3, 2014
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Correct UT in calculation of TDB; closes #1938 #1939

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5c5d52e
Correct UT in calculation of TDB; closes #1938
mhvk Dec 31, 2013
af6bdf6
Also update TDB, TCB in test done in documentation
mhvk Dec 31, 2013
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2 changes: 2 additions & 0 deletions CHANGES.rst
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Expand Up @@ -96,6 +96,8 @@ API Changes

- ``astropy.time``

- Correct use of UT in TDB calculation [#1938, #1939].

- ``astropy.units``

- ``Quantity`` now converts input to float by default, as this is
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9 changes: 5 additions & 4 deletions astropy/time/core.py
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Expand Up @@ -810,12 +810,13 @@ def _get_delta_tdb_tt(self, jd1=None, jd2=None):
jd2 = self._time.jd2

# First go from the current input time (which is either
# TDB or TT) to an approximate UTC. Since TT and TDB are
# pretty close (few msec?), assume TT.
# TDB or TT) to an approximate UT1. Since TT and TDB are
# pretty close (few msec?), assume TT. Similarly, since the
# UT1 terms are very small, use UTC instead of UT1.
njd1, njd2 = erfa_time.tt_tai(jd1, jd2)
njd1, njd2 = erfa_time.tai_utc(njd1, njd2)
# TODO: actually need to go to UT1 which needs DUT.
ut = njd1 + njd2
# subtract 0.5, so UT is fraction of the day from midnight
ut = day_frac(njd1 - 0.5, njd2)[1]

# Compute geodetic params needed for d_tdb_tt()
elon = 0. if self.lon is None else self.lon.to(u.radian).value
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4 changes: 2 additions & 2 deletions astropy/time/tests/test_basic.py
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Expand Up @@ -168,8 +168,8 @@ def test_transforms(self):
assert t.tai.iso == '2006-01-15 21:25:10.500000'
assert t.tt.iso == '2006-01-15 21:25:42.684000'
assert t.tcg.iso == '2006-01-15 21:25:43.322690'
assert t.tdb.iso == '2006-01-15 21:25:42.684372'
assert t.tcb.iso == '2006-01-15 21:25:56.893951'
assert t.tdb.iso == '2006-01-15 21:25:42.684373'
assert t.tcb.iso == '2006-01-15 21:25:56.893952'

def test_creating_all_formats(self):
"""Create a time object using each defined format"""
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4 changes: 2 additions & 2 deletions docs/time/index.rst
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Expand Up @@ -572,9 +572,9 @@ TT, UT1, UTC). This requires auxilliary information (latitude and longitude).
>>> t.tcg.iso
'2006-01-15 21:25:43.322690'
>>> t.tdb.iso
'2006-01-15 21:25:42.684372'
'2006-01-15 21:25:42.684373'
>>> t.tcb.iso
'2006-01-15 21:25:56.893951'
'2006-01-15 21:25:56.893952'

Sidereal Time
-------------
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