Celestial Navigation Basics – Ho Observed Altitude

In this post we will look at how to reduce a sextant apparent altitude Ha to an observed altitude Ho. In the following post, we will look at how to calculate Hc. 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

Fig.1 shows the result of our sextant altitudes taken on Monday July 6th_2020 at Trillium Park (Ref.3):

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

Hs average = 63deg 32.25min at 12:06DST/16:06UTC

Time DSTTime UTCHs deg min
12:0316:0363deg 10min
12:0516:0563deg 25min
12:0716:0763deg 38min
12:0916:0963deg 56min
Fig.1 Sextant Altitudes Average = 12:06/16:06 63deg 32.25min

Fig.2 shows a ScicosLab routine used to calculate the refraction based on the Nautical Almanac formula on p.280. The formula compensates for temperature. SD is taken from the Nautical Almanac for Monday July 6th_2020. Fig.3 shows a spreadsheet used to calculate Ho from Ha (Ref.4).

Ho = Ha – Refraction +PA +/- SD = 63deg 28.25min – 0.5min + 0min + 15.7min = 63deg 43.45min = 63.724deg

Fig.2 Refraction Calculation R = 0.5min
Fig.3 Ho Calculation Ho = 63.724deg

Fig.4 is a short mp4 that explains the reduction of Ha to Ho.

Fig.4 YouTube Video “Celestial Navigation Basics – Ho Observed 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

#3. – “Celestial Navigation Basics – Hs Sextant Altitude”

#4. – “Celestial Navigation Basics – Ha Apparent Altitude”

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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.