Celestial Navigation Basics – Ha Apparent Altitude

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

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 dip angle based on geometry and also using the Nautical Almanac formula on p.280. The dip = 3.0 min for a height of eye = 3.0m. Fig.3 shows a spreadsheet used to calculate Ha from Hs.

Ha = Hs +/-SextantError +/-IndexError – Dip = 63deg 28.25min

Fig.2 Dip Calculation
Fig.3 Ha Calculation

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

Fig.4 YouTube Video “Celestial Navigation Basics – Ha Apparent Altitude”

Celestial Navigation Basics & Equipment Ebk:

Please send your comments, questions and suggestions to:


#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”

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