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    Re: Leap second today
    From: Paul Hirose
    Date: 2012 Jul 03, 20:16 -0700

    Old surveying books say quite a bit about celestial. It even gets a full 
    chapter in a 1989 textbook I own. By that time the main purpose of 
    celestial was to find true north.
    The book lists some useful ephemerides, including the Nautical Almanac 
    (but not the Astronomical), and an almanac sold by the Leitz company.
    An interesting accessory is the Roelof solar prism, which fits over the 
    instrument's objective lens. It dims the Sun and splits it into four 
    images in a square pattern. The separation is such that there's a small 
    diamond shaped area in the center where the images don't touch. That's 
    where you put the crosshairs. It's hard to simultaneously manipulate 
    both slow motions and stay precisely on target, so a common trick is to 
    set one crosshair ahead of the body, track with the other, and mark the 
    time when both crosshairs are "on".
    Polaris can produce an azimuth accurate to 1" by averaging several 
    observations from a first order theodolite, says the book. Sun 
    observations are more convenient, but accuracy is about 10".
    In the optical era the U.S. Coast & Geodetic Survey used elaborate 
    celestial instruments and methods for latitude, longitude, and azimuth 
    determinations at selected stations in their trianulation networks. 
    Their old manual on geodetic astronomy (Special Pub. 237) is now online 
    in the NOAA Library. Some photos of geodetic astronomers working in the 
    field are online too:
    Almanacs and Time Scales.
    Modern fundamental ephemerides, such as the JPL DE series, produce 
    positions of solar system bodies as a function of TDB (Barycentric 
    Dynamical Time). Because TDB is always within 2 milliseconds of TT 
    (Terrestrial Time), for most applications these time scales are identical.
    UT1 is unsuitable for a precise ephemeris due to unpredictable 
    variations in rate. However, in addition to Earth's position, we are 
    interested in its orientation. That requires UT1.
    The different roles for these time scales are seen in The Astronomical 
    Almanac tables, where RA and dec. are a function of TT, and GHA Aries a 
    function of UT1. It's the user's responsibility to know the time in both 
    scales, and compute hour angle from RA and GHA Aries.
    For a navigator that would be too complicated, so the Nautial Almanac 
    uses only UT1 and directly shows the GHA of solar system bodies. A ∆T 
    estimate is necessary to create tables in this format. However, it need 
    not be very accurate. The GHA of the Moon is 30 times less sensitive to 
    TT than UT1. Snce UT1 is the almanac time scale, its exact value is 
    known when the tables are computed. The only time error is in TT, via 
    the ∆T estimate. But a 1 second error would change the Moon GHA only 
    half an arcsecond.
    George Kaplan has speculated that the NA time scale could change to UTC 
    if leap seconds are abolished. In that case the 30:1 ratio works against 
    you. I.e., TT (via the fixed offset from UTC) is known exactly, with UT1 
    computed from a ∆T estimate.
    "In the case of Earth-rotation-dependent data, however, the convenience 
    [of an almanac in UTC] would come at the price of some degradation in 
    accuracy. It seems likely that the increased error (for tabulations 
    computed about two years in advance) would not be worse than that from 
    assuming UT1=UTC now, which is probably a common assumption. 
    Furthermore, if necessary, the error could be removed by application of 
    corrections based on the measured value of UT1–UTC for the date on which 
    the data are needed."
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