(28S) Ephemeredes of the Sun and Determination of Position
01-31-2019, 10:55 PM
Post: #1
 SlideRule Senior Member Posts: 1,251 Joined: Dec 2013
(28S) Ephemeredes of the Sun and Determination of Position
An extract from Ephemeredes of the Sun and Determination of Position, A Program for the HP-28S Scientific Calculator.

The program outlined below will run on an HP-28S calculator and will provide quick and accurate ephemeris of the sun without the bulk or cost of the yearly almanac. The program can be used to calculate the declination (Dec) and Greenwhich hour angle (GHA) of the sun for any time between March 1, 1900 and February 28, 2100. If the sunís observed altitude and your assumed position are also entered the program will provide the calculated altitude (Hc), azimuth angle (Azn) and intercept (Ic) thus allowing you to plot a line of position (St Hilaireís method). Although it has not been tested exhaustively, several test cases have been checked with good results against the U.S. Naval Observatory ís Interactive Computer Ephemeris program (ICE Version 0.51).
The formulas used for this program where sourced from Henning Umlandís website at ìhome.tonline.de/home/h.umland/î and, as I have not explained them here in any detail you may want to peruse this site for a better understanding of what is happening. This is an excellent site and is great for those with a bit of mathematics behind them who want to understand the why of everything (rather than the how covered by most books on the subject). The Interactive Computer Ephemeris program mentioned above is also available from his website.

MAIN ROUTINE: SUN

SUBROUTINES:
RANG SUBROUTINE: ANGLE CONVERT
RLLD SUBROUTINE: LATITUDE/LONGITUDE CONVERSION
AZn ROUTINE: CALCULATES THE AZIMUTH ANGLE RELATIVE TO TRUE NORTH

CALCULATIONS:
The following are the calculations contained in the main routine SUN. Some notes are included.
T: TIME VARIABLE CALCULATION
GMT: GREENWHICH MEAN TIME CALCULATION
GHAA: GREENWHICH HOUR ANGLE (ARIES) CALCULATION
G: MEAN ANOMALY OF THE SUN CALCULATION
LM: MEAN LONGITUDE OF THE SUN CALCULATION
LT: TRUE LONGITUDE OF THE SUN CALCULATION
GE: OBLIQUITY OF THE ECLIPTIC CALCULATIONDECL: DECLINATION OF THE SUN CALCULATION
RA: RIGHT ASCENSION OF THE SUN CALCULATION
GHAS: GREENWHICH HOUR ANGLE OF THE SUN CALCULATION
LHA: LOCAL HOUR ANGLE CALCULATION
HC: CALCULATED ALTITUDE OF THE SUN FROM APPARENT LAT/LONG
AZ: AZIMUTH ANGLE CALCULATION
IC: CALCULATES THE INTERCEPT

BEST!
SlideRule
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