LONG-TERM ALMANAC FOR MOON, STARS, AND LUNAR DISTANCES V1.11
Copyright © 2003–2004 Henning Umland
Almanac data:
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Description:
This computer almanac is based upon formulas published in Astronomical
Algorithms by Jean Meeus and Textbook on Spherical Astronomy by
W. M. Smart. Primarily designed for navigational applications, the program
calculates Greenwich hour angle (GHA), siderial hour angle (SHA), and
declination (Dec) for the moon and 58 bright stars including Polaris.
Further, the geocentric semidiameter (SD) and equatorial horizontal
parallax (HP) of the moon, the phase of the moon (illuminated fraction
of the moon's disk), and the Greenwich hour angle of the vernal point
(Aries) are calculated.
In addition, the program calculates the geocentric angular distance between
the chosen star and the center of the moon (lunar distance). Lunar distances
have been used in the past to determine the approximate time. It should be
observed, however, that not every star is suitable for this method. The rate of
change of the lunar distance should be as high as possible. This can be checked
by calculating the lunar distance for a time x and a time x + 1h. Usually, stars
being near the path of the moon, i. e., in the vicinity of the ecliptic, yield
the best results.
The almanac can be used for many decades, provided the DT
value (= TT–UT1)
for the given date is known. An accuracy of approx. ±1s is sufficient for most
applications. Errors in DT have a significant influence on the coordinates
of the moon. The effect on the coordinates of stars, however, is negligible.
DT is obtained through the following formula:
DT = 32.184s + (TAI – UTC) – DUT1
Current values for TAI–UTC and DUT1 (= UT1–UTC) are published on the web site
of the IERS Rapid Service / Prediction
Center (IERS Bulletin A).
Reliable long-term predictions for DT are not possible. Here are some
DT values of the past:
1970.0: +40.2s
1975.0: +45.5s
1980.0: +50.5s
1985.0: +54.3s
1990.0: +56.9s
1995.0: +60.8s
2000.0: +63.8s
The program regards any blank dialog box in the time input field as zero. A
missing year, month, or day will result in an error message (program must be
restarted). The number of the year must be entered in 4-digit format.
A "(+)" behind the value for the illuminated fraction of the moon's disk
indicates a waxing moon, a "(–)" a waning moon.
SHA and GHAAries refer to the true equinox of date.
Accuracy (approximate values):
| GHA and SHA of the moon: |
±10'' |
| Dec of the moon: |
±4'' |
| GHA and SHA of stars: |
±0.1'' (Polaris: ±1.5'') |
| Dec of all stars: |
±0.1'' |
| GHA Aries: |
±0.1'' |
| HP and SD: |
±0.1'' |
| Lunar distances: |
±10'' |
With a small program like this, it is not possible to
obtain more precise data for the moon. One should remember, however, that
the coordinates of the moon extracted from the Nautical Almanac have about
the same precision (±0.3') when using the interpolation tables and are
considered accurate enough for celestial navigation at sea.
This program is freeware and may be copied and distributed in unmodified form
and on a non-profit basis. For your private use, you may modifiy the source code
according to your requirements. Any commercial distribution requires the author's
permission.
Henning Umland N 53° 20' 34'' E 9° 52' 00''
Check this web site for updated versions:
http://www.celnav.de/index.htm
Warranty disclaimer:
Results obtained
with this program have been cross-checked with Interactive Computer Ephemeris
0.51 by USNO. However, the program is still experimental and may
contain errors leading to unexpected results in rare cases. Use at your own
risk.
Warning:
Editing and modifying the JavaScript code
may lead to unpredictable results. Users who are not familiar with JavaScript
and calculations of ephemerides are strongly advised to leave the code unchanged.