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    Re: Celestial Navigation Data from USNO--alternative
    From: Frank Reed
    Date: 2019 Dec 1, 10:51 -0800

    Jim Rives, you wrote:
    " I was directed to http://www.usno.navy.mil/USNO/astronomical-applications/publications/circ-181 for updates to delta-T, which no longer works.  When did USNO shut down the almanac service?  Is there an alternative?"

    All web services and sites under "usno.navy.mil" were pulled about a month ago without explanation. These sites had suffered frequent, lengthy outages during the past couple of years and in recent months, especially, and I think it's likely that they had problematic security (which conceivably could not be repaired and is being re-built from the bottom up). This might make sense considering the disparity in security requirements for a low-security government agency (USNO, the US Naval Observatory) stuck under the umbrella of the US Navy's ".mil" Internet structure, which requires the highest security.

    When they shut down, a notice stated that the sites would be unavailable for six months. I would not be surprised if they're back more quickly. I also would not be surprised if they never come back at the original addresses and are instead offloaded to another low-security US government agency like NOAA or NASA. The whole idea of the "US Naval Observatory" is anachronistic, a quaint legacy of earlier centuries. Their astronomical operations could have been transferred to NASA decades ago.

    Assuming that MICA allows for manual entry of Delta-T, then you can get the values you need from prior entries in this thread. Quoting my own message from yesterday:

    Here's a list of Delta-T values (these are the ones used in my code):
    DeltaT(2010) = 66.1 
    DeltaT(2020) = 69.5 
    DeltaT(2030) = 73
    ...

    There is a subjective issue with Delta-T in celestial navigation and astronomical computing more generally. Many computation enthusiasts become obsessed with acquiring "accurate" Delta-T values. Values to the nearest second are plenty good enough for all purposes in celestial navigation, and even my values of 66.1 and 69.5 above are over-kill. High accuracy in Delta-T is un-necessary. Some also become obsessed with the "prediction" (or historical computation) of Delta-T by "polynomials" which are often said to be "from NASA". This is magical thinking. Historical datapoints are just approximate values; running a polynomial through them serves no purpose. Except for very general behavior (Delta-T is currently climbing, with long-term quadratic behavior), Delta-T has to be observed, and it happily defies prediction. Nothing in normal, manual celestial navigation, except highest-accuracy sights involving the Moon, requires a Delta-T value more accurate than the nearest ten seconds. So if MICA's current value for Delta-T is 78 seconds insteaad of 68 seconds, there's no harm except for the Moon (and even for the Moon, only for sights demanding the highest accuracy, like lunars!).

    Frank Reed


    PS:
    Some tasty morsels for the computationally-obsessed. Why re-invent the wheel? Grab the open-source code for Stellarium. Open StelCore.hpp and dig around... 

    enum DeltaTAlgorithm
    {
    WithoutCorrection,                  //!< Without correction, DeltaT is Zero. Like Stellarium versions before 0.12.
    Schoch,                             //!< Schoch (1931) algorithm for DeltaT
    Clemence,                           //!< Clemence (1948) algorithm for DeltaT
    IAU,                                //!< IAU (1952) algorithm for DeltaT (based on observations by Spencer Jones (1939))
    AstronomicalEphemeris,              //!< Astronomical Ephemeris (1960) algorithm for DeltaT
    TuckermanGoldstine,                 //!< Tuckerman (1962, 1964) & Goldstine (1973) algorithm for DeltaT
    MullerStephenson,                   //!< Muller & Stephenson (1975) algorithm for DeltaT
    Stephenson1978,                     //!< Stephenson (1978) algorithm for DeltaT
    SchmadelZech1979,                   //!< Schmadel & Zech (1979) algorithm for DeltaT
    MorrisonStephenson1982,             //!< Morrison & Stephenson (1982) algorithm for DeltaT (used by RedShift)
    StephensonMorrison1984,             //!< Stephenson & Morrison (1984) algorithm for DeltaT
    StephensonHoulden,                  //!< Stephenson & Houlden (1986) algorithm for DeltaT
    Espenak,                            //!< Espenak (1987, 1989) algorithm for DeltaT
    Borkowski,                          //!< Borkowski (1988) algorithm for DeltaT
    SchmadelZech1988,                   //!< Schmadel & Zech (1988) algorithm for DeltaT
    ChaprontTouze,                      //!< Chapront-Touzé & Chapront (1991) algorithm for DeltaT
    StephensonMorrison1995,             //!< Stephenson & Morrison (1995) algorithm for DeltaT
    Stephenson1997,                     //!< Stephenson (1997) algorithm for DeltaT
    ChaprontMeeus,                      //!< Chapront, Chapront-Touze & Francou (1997) & Meeus (1998) algorithm for DeltaT
    JPLHorizons,                        //!< JPL Horizons algorithm for DeltaT
    MeeusSimons,                        //!< Meeus & Simons (2000) algorithm for DeltaT
    MontenbruckPfleger,                 //!< Montenbruck & Pfleger (2000) algorithm for DeltaT
    ReingoldDershowitz,                 //!< Reingold & Dershowitz (2002, 2007) algorithm for DeltaT
    MorrisonStephenson2004,             //!< Morrison & Stephenson (2004, 2005) algorithm for DeltaT
    Reijs,                              //!< Reijs (2006) algorithm for DeltaT
    EspenakMeeus,                       //!< Espenak & Meeus (2006) algorithm for DeltaT (Recommended, default)
    EspenakMeeusZeroMoonAccel,          //!< Espenak & Meeus (2006) algorithm for DeltaT (but without additional Lunar acceleration. FOR TESTING ONLY, NONPUBLIC)
    Banjevic,                           //!< Banjevic (2006) algorithm for DeltaT
    IslamSadiqQureshi,                  //!< Islam, Sadiq & Qureshi (2008 + revisited 2013) algorithm for DeltaT (6 polynomials)
    KhalidSultanaZaidi,                 //!< M. Khalid, Mariam Sultana and Faheem Zaidi polynomial approximation of time period 1620-2013 (2014)
    StephensonMorrisonHohenkerk2016,    //!< Stephenson, Morrison, Hohenkerk (2016) RSPA paper provides spline fit to observations for -720..2016 and else parabolic fit.
    Custom                              //!< User defined coefficients for quadratic equation for DeltaT
    };

    And that's some wacko stamp-collecting!

    //! Compute DeltaT estimation for a given date.
    //! DeltaT is the accumulated effect of earth's rotation slowly getting slower, mostly caused by tidal braking by the Moon.
    //! For accurate positioning of objects in the sky, we must compute earth-based clock-dependent things like earth rotation, hour angles etc.
    //! using plain UT, but all orbital motions or rotation of the other planets must be computed in TT, which is a regular time frame.
    //! Also satellites are computed in the UT frame because (1) they are short-lived and (2) must follow paths over earth ground.
    //! (Note that we make no further difference between TT and DT, those might differ by milliseconds at best but are regarded equivalent for our purpose.)
    //! @param JD the date and time expressed as a Julian Day
    //! @return DeltaT in seconds
    //! @note Thanks to Rob van Gent who created a collection from many formulas for calculation of DeltaT: http://www.staff.science.uu.nl/~gent0113/deltat/deltat.htm
    //! @note Use this only if needed, prefer calling getDeltaT() for access to the current value.
    //! @note Up to V0.15.1, if the requested year was outside validity range, we returned zero or some useless value.
    //!       Starting with V0.15.2 the value from the edge of the defined range is returned instead if not explicitly zero is given in the source.
    //!       Limits can be queried with getCurrentDeltaTAlgorithmValidRangeDescription()

    double computeDeltaT(const double JD);
    //! Get current DeltaT.
    double getDeltaT() const;

       
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