Celestial Navigation Basics – Hs Sextant Altitude

In this post we will look at how to take a sextant altitude Hs. In the following post, we will look at how to reduce Hs to Ha and then to Ho. Ho will then be compared with Hc to determine an intercept. The complete details are contained in Ref.1. Complete Sight Reduction procedures are contained in the Nautical Almanac Ref.2 starting page 277, and the NAO form on page 319.

Hs = Sextant Altitude

Ha = Apparent Altitude

Ho = Observed Altitude

Before taking a sextant sight, several things have to be done. We need accurate time. The Earth revolves at 360deg/24hours = 15deg/1hour = 15min_arc/min_time = 1min_arc = 4secs_time. A shortwave receiver can be used to get this time using local time stations.

In Toronto, CHU Ottawa or WWV Boulder Colorado can be received. CHU transmits on 3.33MHz/7.85MHz/14.67MHz & WWV on 2.5MHz/5.0MHz/10MHz/15MHz/20MHz. Fig.1 shows the web interface of NIST the US Government agency that runs WWV.

Fig.1 NIST Time

Once our watch is calibrated to within one second, we need to find a location where the Sun or Celestial Body will be visible over a water horizon. If an artificial horizon is available for the sextant, then the sight can be taken anywhere the body is visible.

I have found an excellent location in Trillium Park with a good view of Lake Ontario where Sun measurements can be taken over several hours. I have checked this location using a Garmin GPS38/iPhone10/Google Earth.

Lat = 43deg 37′ 46.2″ North
Long = 79deg 24′ 34.2″ West
Height of Eye = 3.0m
Index Error = 59min = 1min on_limb

Once on location, the sextant Index Error is determined by levelling the water level across the split mirror. Several altitudes are taken every 2minutes and the average taken. The height of eye above the water level is estimated. Date = July 6th_2020.

Time DSTTime UTCHs deg min
12:0316:0363deg 10min
12:0516:0563deg 25min
12:0716:0763deg 38min
12:0916:0963deg 56min
Fig.2 Sextant Altitudes Average = 12:06/16:06 63deg 32.25min

Fig.3 is a short mp4 that covers the measurement of time, determination of Sextant Error, Index Error and measurement of sextant altitude. Through the telescope shots are taken with the camera.

Fig.3 YouTube Video “Celestial Navigation Basics – Hs Sextant Altitude”

Download “Celestial Navigation Basics & Equipment”


#1. “Celestial Navigation Basics & Equipment”, Clark Telecommunications, 2019, ISBN 9780988049086

#2. – “Nautical Almanac 2020 Commercial Edition”, Paradise Cay Publications, ISBN 9781951116033

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By Jeremy Clark

Jeremy Clark is a Senior Telecommunications Engineer and Advanced Amateur Radio Operator VE3PKC. He is the author of E-Books on Telecommunications, Navigation & Electronics.