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    Re: Interactive Computer Ephemeris (ICE)
    From: Paul Hirose
    Date: 2013 Jul 07, 15:17 -0700

    It is true the program's delta T is inaccurate, but in 2010 I explained
    how that may be fully overcome by adjusting UT1 and the observer's
    longitude. Well, to the extent allowed by the 1 second resolution of the
    time input.
    "Incorrect delta T is another issue. Say the correct value is 66 seconds
    but the program uses 70. In that case, to get the coordinates of the
    Moon at 12:00 UT1 for instance, input 11:59:56 UT1. The program will add
    an extra 4 seconds, thereby arriving at the correct TT. However, your
    position in space relative to the celestial coordinate reference system
    will be too far west, since you gave the computer a too-early UT1. This
    doesn't matter for right ascension and declination, but affects
    topocentric coordinates such as azimuth and altitude.
    "The solution is to multiply the -4 s UT1 correction by 1.002738 to
    convert to sidereal seconds, multiply by 15 to convert time to arc, and
    move the observer's longitude *west* by the resulting angle, i.e., a
    little over a minute east in this case.
    "If utmost accuracy isn't needed, you can simply enter the correct UT1,
    which puts the observer in the correct place relative to the celestial
    reference frame. The body's place in that frame will be slightly off,
    the error amounting to .5 arc second per second of time for the Moon,
    much less for other bodies."
    The same technique may be used with the USNO MICA program.
    See the thread "Errors in USNO celestial data" at the bottom of the May
    2010 message archive. It begins with an apparent bug in the online
    calculator (which was really a browser issue) but changes to a
    discussion of MICA and its delta T error.
    I filter out messages with attachments or HTML.

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