# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**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;