Sight Calc Version 2 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.

Version 2 is a complete re-write in SwiftUI with a simplified user interface. Inputs can be entered in any sequence, but the normal order is to start with LHA degrees and advance to the next field by pressing "Enter". You may enter LHA, Latitute, and Declination in Degrees with a decimal fraction (e.g., 44.48833), or as a whole number of degrees with minutes (e.g., 32.5) in the Minutes field. Sight Calc will prompt you to enter six numbers in sequence:

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

but you can skip any field. Results are continuously displayed above the entry fields. Be sure to 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. To start over fresh at any time, press "C" to clear everything. "CE" key clears the Current Entry. Optionally, use the "o" key to convert minutes to decimal degrees, add that to the degrees field, and clear the minute field. This has no effect on the calulation.



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 degrees and minutes.

Enter LHA degrees LHA minutes Press o to convert minutes Enter Latitude+/-
Latitude minutes Press o Enter Declination +/- Dec minutes
Press o  

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)))