USPS Sight Reduction Form SR 96a Web Page Calculator Help Page

• This Sight Reduction calculator (Original Sun Only) is an older copy.
• This Sight Reduction calculator (Latest Sun Only) is the latest Sun only version. This one has bold and gray to help identify which entry boxes require inputs to get an answer out.
• This Sight Reduction calculator (Sun, Moon, Planets, Stars) updated11/5/2008 is the latest version updated to support reduction of all navigation bodies.
• NEW This Sight Reduction calculator (Sun, Moon, Planets, Stars) updated 1/26/2009 with CSS is the latest version updated to support reduction of all navigation bodies. It adds cascading style sheets to control the format so that it can be printed in portrait mode on a single page. It does not yet support T & B corrections for tempurature and pressure, but does include planet, star, and moon main corrections and the additional corrections for moon but requires entry of HP from the almaniac. The HP lookups do not interpolate entries from the additional corrections tables. Mars and Venus additional corrections change from year to year and must be entered from the Almaniac. The help page below can be used as a guide, but additional fields, lisk SHA are now active. Latest version fixed SHA not being added into LHA. This is alpha quality. Please forward bugs to webmister at snark dot dhs dot org.

  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:

  • Leading 0s on numbers caused apparent errors because the numbers were interpreted to be octal numbers. Changed parseInt() to parseFloat()
  • The fix above allowed fractional numbers to be entered where they shouldn't be. Added floor() as needed to prevent decimals in these fields. The decimal may show in the field it is entered in, but the calculation will be correct as if the decimal were not there. You cannot enter decimal degrees and leave the minute box as 0, for example.
  • Certain optional fields like watch error now have default values and don't have to have 0s entered into them.
  • Fixed the body field to display the sun symbol when sun is selected (currently the only option).
  • Added bold around fields where entries should be made in to help distinguish them from calculated fields.

  The three types of entry boxes fall are as follows:

  • Computed entry - This box is computed from other entries and cannot be changed by the user. All results and intermediate results appear in these boxes. Computed height, Hc and zone descriptor, ZD are examples.
  • Required User entry - An entry is required for a correct result to be obtained. An example would be height of sight, Hs, and the local date and watch time. If only an intermediate result is needed then only the data needed for that computation needs to be entered.
  • Non-required User entry - These entries are for information purposes and do not change the result. The name, sight number, and squadron are all examples.

  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 (New).