.. doctest-skip-all

.. _precision:

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Why is my target above/below the horizon at the rise/set time?
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Rise/set/meridian transit calculations in `astroplan` are designed to be fast
while achieving a precision comparable to what can be predicted given the
affects of the changing atmosphere. As a result, there may be some
counter-intuitive behavior in `astroplan` methods like
`astroplan.Observer.target_rise_time`, `astroplan.Observer.target_set_time` and
`astroplan.Observer.target_meridian_transit_time`, that can lead to small
changes in the numerical values of these computed timed (of order seconds).

For example, to calculate the rise time of Sirius, you might write::

    from astroplan import Observer, FixedTarget
    from astropy.time import Time

    # Set up observer, target, and time
    keck = Observer.at_site("Keck")
    sirius = FixedTarget.from_name("Sirius")
    time = Time('2010-05-11 06:00:00')

    # Find rise time of Sirius at Keck nearest to `time`
    rise_time = keck.target_rise_time(time, sirius)

You might expect the altitude of Sirius to be zero degrees at ``rise_time``,
i.e. Sirius will be on the horizon, but this is not the case::

    >>> altitude_at_rise = keck.altaz(rise_time, sirius).alt
    >>> print(altitude_at_rise.to('arcsec'))
    2.70185arcsec

The altitude that you compute on your machine may be different from the number
above by a small amount – for a detailed explanation on where the difference
arises from, see :doc:`iers`. The rise and set time methods use the following
approximation:

* A time series of altitudes for the target is computed at times near ``time``

* The two times when the target is nearest to the horizon are identified, and a
  linear interpolation is done between those times to find the horizon-crossing

This method has a precision of a few arcseconds, so your targets may be slightly
above or below the horizon at their rise or set times.