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    Re: equinox
    From: Paul Hirose
    Date: 2004 Mar 21, 18:55 -0800

    Here's what I got from the JPL Horizons program. Note that 06:50:04 TT is about
    06:49:00 UTC. The USNO MICA program was pretty close to Horizons, about 2
    seconds of TT different.
    
    
    Ephemeris / MAIL_REQUEST Sun Mar 21 16:47:51 2004  Pasadena, USA / Horizons
    *******************************************************************************
    Target body name: Sun (10)                        {source: DE-0406LE-0406}
    Center body name: Earth (399)                     {source: DE-0406LE-0406}
    Center-site name: GEOCENTRIC
    *******************************************************************************
    Start time      : A.D. 2004-Mar-20 06:50:00.0000 TT
    Stop  time      : A.D. 2004-Mar-20 06:51:00.0000 TT
    Step-size       : 60 steps
    *******************************************************************************
    Center geodetic : 0.000000,     0.0000,     0.0000{E-lon(deg),Lat(deg),Alt(km)}
    Center cylindric: 0.000000,     0.0000,     0.0000{E-lon(deg),Dxy(km),Dz(km)}
    Center pole/equ : High-precision EOP model        {East-longitude +}
    Center radii    : 6378.1 x 6378.1 x 6356.8 km     {Equator, meridian, pole}
    Target pole/equ : IAU_SUN                         {East-longitude +}
    Target radii    : 696000.0 x 696000.0 x 696000.0 k{Equator, meridian, pole}
    Target primary  : Sun                             {source: DE-0406LE-0406}
    Interfering body: MOON (Req= 1737.400) km         {source: DE-0406LE-0406}
    Deflecting body : Sun, EARTH                      {source: DE-0406LE-0406}
    Deflecting GMs  : 1.3271E+11, 3.9860E+05 km^3/s^2
    Atmos refraction: NO (AIRLESS)
    RA format       : HMS
    Time format     : CAL
    Units conversion: 1 AU= 149597870.691 km, c= 299792.458 km/s, 1 day= 86400.0 s
    Table cut-offs 1: Elevation (-90.0deg=NO ),Airmass (>38.000=NO), Daylight (NO )
    Table cut-offs 2: Solar Elongation (  0.0,180.0=NO )
    
     Date__(TT)__HR:MN:SC.fff     R.A.__(airls-apparent)__DEC        CT-UT
    
     2004-Mar-20 06:50:00.000     00 00 00.0518 -00 00 00.065    64.185608
     2004-Mar-20 06:50:01.000     00 00 00.0544 -00 00 00.049    64.185608
     2004-Mar-20 06:50:02.000     00 00 00.0569 -00 00 00.032    64.185608
     2004-Mar-20 06:50:03.000     00 00 00.0594 -00 00 00.016    64.185608
     2004-Mar-20 06:50:04.000     00 00 00.0620 +00 00 00.001    64.185608
     2004-Mar-20 06:50:05.000     00 00 00.0645 +00 00 00.017    64.185608
     2004-Mar-20 06:50:06.000     00 00 00.0670 +00 00 00.034    64.185608
     2004-Mar-20 06:50:07.000     00 00 00.0696 +00 00 00.050    64.185608
    
    Column meaning:
    
    TIME
    
      Terrestrial Dynamical Time, "TDT" or "TT", is used here. This uniform time
    scale is based on Atomic Time (TAI) and is extrapolated to pre-1955 dates.
    Any 'b' symbol in the 1st-column denotes a B.C. date. First-column blank (" ")
    denotes an A.D. date.  Calendar dates prior to 1582-Oct-15 are in the Julian
    calendar system.  Later calendar dates are in the Gregorian system.
    
    
      NOTE: "n.a." in output means quantity "not available" at the print-time.
    
     R.A._(airls-apparent)__DEC. =
       Airless apparent right ascension and declination of the target with respect
    to the Earth true-equator and meridian containing the Earth true equinox of
    date. Corrected for light-time, the gravitational deflection of light, stellar
    aberration, precession and nutation. Units: HMS (HH MM SS.ffff) and
    DMS (DD MM SS.fff)
    
     CT-UT =
       Difference between uniform Coordinate Time scale and Earth-rotation
    dependent Universal Time. Prior to 1962, the difference is with respect
    to UT1 (CT-UT1).  For 1962 and later, the delta is with respect to UTC
    (CT-UTC).  Values beyond the next July or January 1st may change if a
    leap-second is introduced. Units: SECONDS
    
    
    The Horizons home page is here:
    
    http://ssd.jpl.nasa.gov/horizons.html
    
    Horizons isn't friendly, at least not the email interface. I've learned to keep
    examples of successful command emails on file, so when I need to do something I
    have a starting point known to work. Today it took me multiple tries to figure
    out Horizons doesn't accept time step values in seconds. "Unknown units
    specification -- re-enter" isn't very helpful if the parameter with illegal
    units isn't stated.
    
    The system is fast, though. The table above is an excerpt from a 60-line table I
    requested from Horizons. Right after dispatching the command email I checked for
    a reply, and it was already waiting.
    
    On the other hand, it took more than a day to receive the "detailed example"
    mentioned at the bottom of the home page.
    
    By the way, I checked to see if lunarians could get precise separation angles
    between the Moon and other bodies via Horizons. No such luck. In fact, only
    solar system objects are available.
    
    
    

       
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