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 DST | Time UTC | Hs deg min |
| 12:03 | 16:03 | 63deg 10min |
| 12:05 | 16:05 | 63deg 25min |
| 12:07 | 16:07 | 63deg 38min |
| 12:09 | 16:09 | 63deg 56min |
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.4 is a short mp4 that explains the reduction of Hs to Ha.
Celestial Navigation Basics & Equipment Ebk:
https://www.clarktelecommunications.com/celnav.htm
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References
#1. “Celestial Navigation Basics & Equipment”, Clark Telecommunications, 2019, ISBN 9780988049086
https://www.clarktelecommunications.com/celnav.htm
#2. – “Nautical Almanac 2020 Commercial Edition”, Paradise Cay Publications, ISBN 9781951116033
https://www.paracay.com/nautical-almanac/
#3. – “Celestial Navigation Basics – Hs Sextant Altitude”
