Sight Calc Support Page

Sight Calc is a specialized calculator that eases the task of calculating sight reductions via the "Law of Cosines" method. It does not use Nautical Almanac data, so you must first compute LHA (Local Hour Angle) and declination, and determine your approximate latitude. It is designed particularly for students learning celestial navigation, such as the Celestial Navigation courses taught by the US Power Squadrons®. If you are unfamiliar with how to do sight reductions, some good resources are available from the U.S. Power Squadrons® at In particular, this calculator is useful in conjunction with Form SR96a.

Inputs are always entered as two numbers: Degrees - left display, Minutes - right display. If you already know the value in decimal degrees (e.g., 44.48833), key it in followed by ENTER, then press ENTER again to indicate zero minutes. Sight Calc will prompt you to enter six number in sequence:

LHA (degrees) - Local Hour Angle
LHA (minutes)
Lat (degrees) - Latitude
Lat (minutes)
Dec (degrees) - Declination
Dec (minutes)

After you enter each value, it will be displayed in decimal degrees in the corresponding register. After you enter the last value (Dec minutes), Hc and Z will be computed and displayed. ** In version 1.3 and later, enter North Latitude as a positive number and South Latitude as a NEGATIVE number. Sight Calc will take the absolute value of Latitude in the law of cosines forumula, but needs the hemisphere to calculate azimuth (Zn). The sign of Declination is positive if Latitude and Declination are the same sense (both North or both South). If they are different, key the minus sign before the first declination digit. If you want to do repeated calculations with some of the same numbers, just enter the changed numbers and press ENTER to accept the remembered values from the previous sight reduction. To start over fresh at any time, press "C" to clear everything. "CE" key clears the Current Entry.



Sight Data


Date: 1 Dec 2011
Watch Time: 10:50:05
Watch Error: 0:00
Zone Time: 10:50:05
Zone Description: +5W
UT: 15:50:05
G Day/Mo: 1 Dec
Almanac --- LHA
15hr 47°45.6'
50m 05s 12°31.3'
Tot GHA: 60°16.9'
DR Lo: - 82°01.3W
LHA: 338°15.6'
Body: Sun UL
Lat: 26°26.9' N
DR Long: 82°01.3' W
Almanac --- Dec
Dec: 15 hr 21°48.4S
d: +0.4'
d corr: +0.3'
Dec: 21°48.7'S
Height of eye: 14.0'
hs: 37°43,7'
IC: 0.0'
Dip: 3.6'
Correction: -3.6'
ha: 37°40.1
Main: -17.3'
Ho: 37°22.8'

The three numbers we need are LHA, DR Lat, and Dec. Starting with LHA degrees, enter each number in degree and minutes. Notice that after you type "338" and press enter, the prompt above the input registers changes to "Enter LHA (minutes)":


Please report any bugs or suggestions to:

Law of Cosines

Hc = arcsin((cos(LHA) * cos(Lat) * cos(Dec)) + (sin(Lat) * sin(Dec)))
Z = arccos((sin(Dec) - (sin(Lat) * sin(Hc)) / (cos(Lat) * cos(Hc)))