The sight reduction calculator page is designed after the United States Power Squadron sight reduction form SR 96a. The goal is that it reduce the sights and some up with the exact same answer as would be achieved if the sight was reduced by hand using the standard rounding rules. Because of the volume of data in the daily pages and the fact that it changes each year, it is not possible to include those tables into this calculator. Thus, the GHA, the increments, declination and the declination correction must be entered from the almanac. The increments may be included at a later time if a simple equation that matches the results from the table is found. All other computations of dates, time conversions, zone descriptor, dip, main correction, Hc, Zn, and the error, a, is computed automatically.
This computation is written in java script and is done in your own browser. This means that you can do a save as on the web page and save a copy to your own machine and do the computations offline. However, you will not get updates and corrections as they are found so if you do this, it would be a good idea to check back for newer copies. This is Beta software so there could be bugs. Some have already been found and corrected including:
The three types of entry boxes fall are as follows:
Tables of the entry boxes, their types, and notes about what should be entered or how the entries have been computed follow.
Form Headings | |
Name | Non-required User Entry |
Squadron | Non-required User Entry |
TIME | ||
Date day | Required User Entry | |
Date month | Required User Entry | |
Date year | Required User Entry | |
Watch Time | ||
WT hours | Required User Entry | |
WT minutes | Required User Entry | |
WT seconds | Required User Entry | |
Watch Error | ||
WE fast/slow | Required User Entry | |
WE minutes | Required User Entry | |
WE seconds | Required User Entry | |
Zone Time | ||
ZT | Computed Entry | Computed from WT and WE |
Zone Descriptor | ||
ZD | Computed Entry | Uses DR Longitude and W/E direction |
Universal Time (GMT) | ||
UT | Computed Entry | Computed from ZT and ZD |
G Date | Computed Entry | Computed from ZT and Date |
The date is used to compute the date and universal time for looking up the data in the almanac and it is used to determine the collect table to use for the Main correction. The calculator attempts to keep track of changes to the date or time and remind the user in a message box at the bottom that the almanac data needs to be updated. Once any field is changed, the message will be cleared whether or not the correct fields are updated.
SIGHT DATA | ||
Sight No. | Non-required User Entry | |
Body | Required User Entry | Current options Sun LL and Sun UL |
Dead Reckoning Position | ||
DR L deg | Required User Entry | Latitude whole number of degrees |
DR L min | Required User Entry | Latitude to the nearest tenth of a minute |
DR L dir | Required User Entry | North or South latitude |
DR Lo deg | Required User Entry | Longitude whole number of degrees |
DR Lo min | Required User Entry | Longitude nearest tenth of a minute |
DR Lo dir | Required User Entry | East or West longitude |
ALTITUDE | ||
Ht of eye | Required User Entry | Height of Eye above the natural horizon in feet |
Hs deg | Required User Entry | Elevation of body in degrees from the sextant |
Hs min | Required User Entry | Elevation of body in minutes to the nearest tenth |
IC + | Required User Entry | Sextant Index correction positive for off limb errors |
IC - | Required User Entry | Sextant Index correction negative for on limb errors |
Dip - | Computed Entry | Computed from Ht of eye |
Total + | Computed Entry | Computed sum of positive corrections |
Total - | Computed Entry | Computed sum of negative corrections |
Corr | Computed Entry | Computed sum of positive and negative corrections |
ha deg | Computed Entry | Apparent height computed from hs and corr |
ha min | Computed Entry | Apparent height computed from hs and corr |
HP min | Computed Entry | Only used for moon sights |
Main + | Computed Entry | Main correction computed from Hs, Date and internal tables |
Main - | Computed Entry | Main correction computed from Hs, Date and internal tables |
Add'l, Pl + | Computed/Manual Entry | Computed for all but Venus and Mars which is manual |
Add'l, Pl - | Computed Entry | Computed except for Venus |
UL'-30.00 | Computed Entry | Computed 0 except UL moon |
Add'l, Ref + | Computed Entry | Not available for T&B yet |
Add'l, Ref - | Computed Entry | Not available for T&B |
Total + | Computed Entry | Computed sum of positive corrections |
Total - | Computed Entry | Computed sum of negative corrections |
Corr | Computed Entry | Computed sum of positive and negative corrections |
Ho deg | Computed Entry | Computed height observed from ha and corr |
Ho deg | Computed Entry | Computed height observed from ha corr |
ALMANAC --- LHA | ||
SHA | Non-required User Entry | Entry ignored for sun sights |
hr | Computed Entry | UT hours from the TIME section |
deg | Required User Entry | The GHA from the almanac daily pages for give date and hours |
min | Required User Entry | The GHA from the almanac daily pages for give date and hours |
min | Computed Entry | UT minutes from the TIME section |
sec | Computed Entry | UT seconds from the TIME section |
deg | Required User Entry | The GHA correction from the almanac increments pages for given minutes and seconds |
min | Required User Entry | The GHA correction from the almanac increments pages for given minutes and seconds entered to nearest tenth |
v(+-) | Non-required User Entry | Ignored for sun sights |
v | Computed Entry | Forced to 0 for sun sights |
v corr(+-) | Computed Entry | Follows the sign of v |
v corr | Computed Entry | Forced to 0 for sun sights |
Tot GHA deg | Computed Entry | Computed from the almanac entries |
Tot GHA min | Computed Entry | Computed from the almanac entries |
DR Lo sign | Computed Entry | Computed from the DR longitude direction |
DR Lo deg | Computed Entry | Follows the Lo in the SIGHT DATA section |
DR Lo min | Computed Entry | Follows the Lo in the SIGHT DATA section |
DR Lo dir | Computed Entry | Follows the Lo dir in the SIGHT DATA section |
LHA deg | Computed Entry | Computed from the total GHA, Dr Lo and sign |
LHA min | Computed Entry | Computed from the total GHA, Dr Lo and sign |
ALMANAC DEC | ||
Dec hr | Computed Entry | Follows UT hours in TIME section |
Dec deg | Required User Entry | The declination from the daily pages |
Dec min | Required User Entry | The declination from the daily pages to nearest tenth |
Dec dir | Required User Entry | The declination direction from the daily pages |
d +/- | Required User Entry | The declination slope from the daily pages + if increasing - if decreasing |
d | Non-required User Entry | The d from the bottom of the daily pages |
d corr +/- | Computed Entry | Follows d +/- above |
d corr | Required User Entry | The d corr from the increments table indexed by d |
Dec deg | Computed Entry | Computed from Dec and d corr and it's sign |
Dec min | Computed Entry | Computed from Dec and d corr and it's sign |
Dec dir | Computed Entry | Computed from Dec direction above |
INTERCEPT and AZIMUTH | ||
LHA deg | Computed Entry | Follows LHA from the ALMANAC ---- LHA section |
LHA min | Computed Entry | Follows LHA from the ALMANAC ---- LHA section |
LHA decimal deg | Computed Entry | Computed from LHA deg + LHA min / 60 |
Lat deg | Computed Entry | Follows DR Lat from the SIGHT DATA section |
Lat min | Computed Entry | Follows DR Lat from the SIGHT DATA section |
Lat dir | Computed Entry | Follows DR direction from the SIGHT DATA section |
Lat decimal deg | Computed Entry | Computed from Lat deg + Lat min / 60 |
Dec deg | Computed Entry | Follows Dec from the ALMANAC ---- Dec section |
Dec min | Computed Entry | Follows Dec from the ALMANAC ---- Dec section |
Dec dir | Computed Entry | Follows Dec direction from the ALMANAC ---- Dec section |
Dec decimal deg | Computed Entry | Computed from Dec deg + Dec min / 60 and the sign from the direction of Lat and Dec |
Hc | Computed Entry | Height computed from the navigation triangle see text |
Z pole dir | Computed Entry | Follows the DR lat direction |
Z | Computed Entry | Height computed from the navigation triangle see text |
Z body dir | Computed Entry | Follows the E/W body direction computed from LHA 0-180 or 180-360 |
Hc deg | Computed Entry | Follows Hc computed above converted from decimal degrees |
Hc min | Computed Entry | Follows Hc computed above converted from decimal degrees |
(optional msg) | Computed Entry | Message box to help notify user when Almanac data needs updating |
Ho deg | Computed Entry | Follows Ho computed in ALTITUDE section |
Ho min | Computed Entry | Follows Ho computed in ALTITUDE section |
a min | Computed Entry | Difference between Hc and Ho in nautical miles |
T/A | Computed Entry | Sign of difference between Hc and Ho in Towards/Away notation |
Zn | Computed Entry | The rounded direction of the body from Z adjusted based on the body E/W location |
The form follows SR 96a and its contents closely, however, the navigation triangle is NOT solved using the law of cosines method in the calculator as stated on the form. Instead, the Abhav method based on Haversines is used. The answers should agree with the cosine method but should avoid the limitations of the cosine method for angles near 90 degrees where precision limits from the inaccuracies of the computer may cause the body to appear on the wrong side of the observer.
Go to the Sight Reduction calculator.