readme.aa

来自「c源码」· AA 代码 · 共 497 行 · 第 1/2 页

                    AA.ARC v5.5

   This program computes the orbital positions of planetary
bodies and performs rigorous coordinate reductions to apparent
geocentric and topocentric place (local altitude and azimuth).
It also reduces star catalogue positions given in either the FK4
or FK5 system.  Most of the algorithms employed are from The
Astronomical Almanac (AA) published by the U.S. Government
Printing Office.
   Source code listings in C language are supplied in the file
aa.arc.  The file aaexe.arc contains an IBM PC executable
version.

              Reduction of Celestial Coordinates

   aa.exe follows the rigorous algorithms for reduction of
celestial coordinates exactly as laid out in current editions of
the Astronomical Almanac.  The reduction to apparent geocentric
place has been checked by a special version of the program (aa200)
that takes planetary positions directly from the Jet Propulsion
Laboratory DE200 numerical integration of the solar system. The
results agree exactly with the Astronomical Almanac tables from
1987 onward (earlier Almanacs used slightly different reduction
methods).
   Certain computations, such as the correction for nutation,
are not given explicitly in the AA but are referenced there. In
these cases the program performs the full computations that are
used to construct the Almanac tables (see the references at the
end of this document).


              Running the Program

   Command input to aa.exe is by single line responses to
programmed prompts. The program requests date, time, and which
of a menu of things to do.  Menu item 0 is the Sun, 3 is the
Moon.  The other values 1-9 are planets; 99 opens an orbit
catalogue file; 88 opens a star catalogue. Each prompt indicates
the last response you entered; this will be kept if you enter
just a carriage return.
    Input can also be redirected to come from an ASCII file.  For example,
invoking the program by "aa <command.txt >answer.txt" reads commands
from the file command.txt and writes answers to answer.txt.  Menu
item -1 causes the program to exit gracefully, closing the output
file.
   Entering line 0 for a star catalogue causes a jump back to the
top of the program.

 
             Initialization
       
  The following items will be read in automatically from a disc file
named aa.ini, if one is provided.  The file contains one ASCII
string number per line so is easily edited.  A sample initialization
file is supplied.

Terrestrial longitude of observer, degrees East of Greenwich
Geodetic latitude of observer (program calculates astronomical latitude)
Height above sea level, meters
Atmospheric temperature, degrees Centigrade
Atmospheric pressure, millibars
Input time type: 1 = TDT, 2 = UT, 0 = TDT set equal to UT
Value to use for deltaT, seconds; if 0 then the program will compute it.


              Orbit Computations

   Several methods of calculating the positions of the planets
have been provided for in the program source code.  These range
in accuracy from a built-in computation using perturbation formulae
to a solution from precise orbital elements that you supply from
an almanac.
   The program uses as a default a set of trigonometric
expansions for the position of the Earth and planets.  These have
been adjusted to match the Jet Propulsion Laboratory's DE404 Long
Ephemeris (1995) with a precision ranging from about 0.1" for the
Earth to 1" for Pluto. The adjustment was carried out on the
interval from 3000 B.C. to 3000 A.D. for the outer planets.  The
adjustment for the inner planets is strictly valid only from 1350
B.C. to 3000 A.D., but may be used to 3000 B.C. with some loss of
precision.  See "readme.404" for additional information.  The true
accuracy of positions calculated for prehistoric or future dates
is of course unknown.
   The Moon's position is calculated by a modified version of the
lunar theory of Chapront-Touze' and Chapront.  This has a precision
of 0.5 arc second relative to DE404 for all dates between
1369 B.C. and 3000 A.D.  The real position of the Moon in ancient
times is not actually known this accurately, due to uncertainty
in the tidal acceleration of the Moon's orbit.

   In the absence of an interpolated polynomial ephemeris such
as the DE200, the highest accuracy for current planetary
positions is achieved by using the heliocentric orbital elements
that are published in the Astronomical Almanac. If precise
orbital elements are provided for the desired epoch then the
apparent place should be found to agree very closely with
Almanac tabulations.
   Entering 99 for the planet number generates a prompt for the
name of a file containing human-readable ASCII strings specifying
the elements of orbits. The items in the specification are
(see also the example file orbit.cat):

   First line of entry:
epoch of orbital elements (Julian date)
inclination
longitude of the ascending node
argument of the perihelion
mean distance (semimajor axis) in au
daily motion

   Second line of entry:
eccentricity
mean anomaly
epoch of equinox and ecliptic, Julian date
visual magnitude B(1,0) at 1AU from earth and sun
equatorial semidiameter at 1au, arc seconds
name of the object, up to 15 characters


Angles in the above are in degrees except as noted.  Several
sample orbits are supplied in the file orbit.cat.  If you read
in an orbit named "Earth" the program will install the Earth
orbit, then loop back and ask for an orbit number again.
  The entry for daily motion is optional.  It will be calculated
by the program if it is set equal to 0.0 in your catalogue.
Almanac values of daily motion recognize the nonzero mass of the
orbiting planet; the program's calculation will assume the mass
is zero.
  Mean distance, for an elliptical orbit, is the length of the
semi-major axis of the ellipse. If the eccentricity is given to
be 1.0, the orbit is parabolic and the "mean distance" item is
taken to be the perihelion distance.  Similarly a hyperbolic
orbit has eccentricity > 1.0 and "mean distance" is again
interpreted to mean perihelion distance.  In both these cases,
the "epoch" is the perihelion date, and the mean anomaly is
set to 0.0 in your catalogue.
  Elliptical cometary orbits are usually catalogued in terms of
perihelion distance also, but you must convert this to mean
distance to be understood by the program. Use the formula

  mean distance = perihelion distance / (1 - eccentricity)

to calculate the value to be entered in your catalogue for an
elliptical orbit.
  The epoch of the orbital elements refers particularly to the
date to which the given mean anomaly applies.  Published data
for comets often give the time of perihelion passage as a
calendar date and fraction of a day in Ephemeris Time.  To
translate this into a Julian date for your catalogue entry, run
aa.exe, type in the published date and decimal fraction of a
day, and note the displayed Julian date. This is the correct
Julian Ephemeris Date of the epoch for your catalogue entry.
Example (Sky & Telescope, March 1991, page 297): Comet Levy
1990c had a perihelion date given as 1990 Oct 24.68664 ET.  As
you are prompted separately for the year, month, and day, enter
1990, 10, 24.68664 into the program. This date and fraction
translates to JED 2448189.18664.  For comparison purposes, note
that published ephemerides for comets usually give astrometric
positions, not apparent positions.


              Ephemeris Time and Other Time Scales

   Exercise care about time scales when comparing results against
an almanac.  The orbit program assumes input date is Ephemeris
Time (ET or TDT).  Topocentric altitude and azimuth are
calculated from Universal Time (UT).  The program converts
between the two as required, but you must indicate whether your
input entry is TDT or UT.  This is done by the entry for input
time type in aa.ini.  If you are comparing positions against
almanac values, you probably want TDT.  If you are looking up at
the sky, you probably want UT.  Ephemeris transit times can be
obtained by declaring TDT = UT.  The adjustment for deltaT = ET
minus UT is accurate for the years 1620 through 2011, as the
complete tabulation from the Astronomical Almanac is included in
the program. Outside this range of years, approximate formulas
are used to estimate deltaT.  These formulas are based on
analyses of eclipse records going back to ancient times
(Stephenson and Houlden, 1986; Borkowski, 1988) but they do not
predict future values very accurately.  For precise calculations,
you should update the table in deltat.c from the current year's
Almanac. Note the civil time of day is UTC, which is adjusted by
integral leap seconds to be within 0.9 second of UT.

   Updated deltaT predictions can be obtained from this network archive:
       ftp://maia.usno.navy.mil/ser7/deltat.preds

   In addition, the IAU has adopted several other definitions of
time, but this program does not distinguish among them.  The
International Earth Rotation Service is in charge of UT. Precise
data on Earth orientation are available at the IERS anonymous ftp
site mesiom.obspm.fr.



              Rise and Set Times

   Each calculation of the time of local rising, meridian transit, and
setting includes a first order correction for the motion in right
ascension and declination of the object between the entered input time
and the time of the event.  Even so, the calculation has to be
iterated, or repeated with successively closer estimates of the event
time.  In view of the first order correction the iteration has a
second-order convergence characteristic and arrives at a precise
result in just two or three steps.

   The program reports the transit that is nearest to the input time.
Rise and set times ordinarily precede and follow the transit.  Check
the date displayed next to the rise, set, or transit time to be sure
the results are for the desired date and not for the previous or next
calendar day.  For the Sun and Moon, rise and set times are for the
upper limb of the disc; but the indicated topocentric altitude always
refers to the center of the disc.  The computed event times include
the effects of diurnal aberration and parallax.

   Age of the Moon, in days from the nearest Quarter, also has a
correction for orbital motion, but does not get the benefit of
iterative improvement and may be off by 0.1 day (the stated Quarter is
always correct, however). The estimated time can be made much more
precise by entering the input date and time of day to be near the time
of the event.  In other words, the rigorous calculation requires
iterating on the time; in this case the program does not do so
automatically, hence if you want maximum accuracy you must do the
iteration by hand.



              Ocean Tides

   The time of high tide is easy to estimate with an accuracy of a few
minutes.  The tide occurs slightly ahead of the local meridian transit
of the Moon.  The time difference between tide and transit does not
vary much at a fixed location.  You can calibrate this difference for
your location by checking the local newspaper for tide times and
subtracting off the time of the meridian transit.  Then the tides on
any other date bear the same time offsets to the transit on that date.

    Predicting the height of the tide is much more difficult.
The calculations are about as complicated as planetary theory.



              Stars

   Positions and proper motions of the 57 navigational stars
were taken from the Fifth Fundamental Catalogue (FK5). They are
in the file star.cat.  For all of these, the program's output of
astrometric position agreed with the 1986 AA to the precision of