Celestial Navigation Basics – Aim Off

The sun is shining, it’s early morning and I’m on my deck, Colombian coffee in hand, reading a fantastic book “China Clipper: The Age of the Great Flying Boats” by Robert Gandt (Ref.1). It is the amazing story of the American airline Pan Am crossing the Pacific in 1935. In order to fly safely with the Martin M-130, Pan Am had to build intermediate bases starting in Alameda California, Honolulu Hawaii, Midway Island, Wake Island, Guam and finally Manila (Ref.2). One paragraph got my attention. It was the description of how the famous navigator Fred Noonan found the tiny Wake Island. He did it by “Aiming Off”. Fred Noonan later was Amelia Earhart’s navigator. I immediately remembered the story of Sir Francis Chichester and how he crossed the Tasman Sea in 1931 using exactly the same technique (Ref.3).

Fig.1 Pan Am China Clipper M-130 Flying Boat (Ref.4)
Fig.2 Sir Francis Chichester Flight Paragareng Harbour to Norfolk Island (Ref.3)

Let’s examine what Sir Francis did, and this will give us an idea of what Fred Noonan may have done. The following data is taken from Ian Strathcarron’s book (Ref.3):

Sir Francis Leaves Parengarenga = 11:50 March 28th_1931
Turned towards Norfolk Island = 16:30 (04:30GMT)
Sights Norfolk Island = 17:14 (05:14GMT)
Lands Norfolk Island = 17:40 (05:40GMT)
Approx. Time to Way Point = 4hrs 40min
Distance Norfolk/Parengarenga = 417Nmi
Avg Spd = 89Knots = 164Km/hr
Way Point Offset = Approx. 1hr = 89Nmi = 164Km
Direct Course to Parengarenga = 321.4deg
Course Offset to Western Way Point = (89/417)rad = 0.213rad = 12.2deg
Course to Western Way Point = 309.2deg
Norfolk Island: Lat = 29deg 2′ 14″S Long = 167deg 57′ 18″ E
Local NZ time = GMT + 12hrs

We can use the Nautical Almanac for 2020 (Ref.5) & NavPac as a check to calculate the SunGP, Hc, Azimuth Zn and Sumner Line/LOP Course for a grid of times and the location of Norfolk Island. Sir Francis would have flown approx. NW for four hours all the while using his bubble sextant and measuring Ho and comparing this to his grid of values. At 16:30 he measured 28d 56.7min and he knew he was located on the 0430 Sumner/LOP line from Norfolk Island. He turned off his NW course to the grid course of 22.0d. After 45 minutes he sighted the island.

0400hrs1600hrs238d 44.5′
3d 9.0′
34d 53.8′297.6d27.6d
0430hrs1630hrs246d 14.5′
3d 9.5′
28d 56.7′292.3d22.3d
0500hrs1700hrs253d 44.7′
3d 10.0′
22d 46.4′287.6d17.6d
Fig.3 Sun Hc & Zn to Norfolk Island for 1600/1630/1700 NZ Times
Fig.4 Planning Grid Hc_Time_Azimuth
Fig.5 NavAlgos & NavPac Grid Solutions Hc & Zn & Sumner LOP

Please send your comments, questions and suggestions to:


#1. – “China Clipper: The Age of the Great Flying Boats”, Robert Gandt, ISBN 1591143039

#2. – “Across The Pacific”, PBS Documentary Series about Pan Am

#3. – “Never Fear: Reliving the Life of Sir Francis Chichester”, Ian Strathcarron, ASIN B07289LJSW

#4. – “Martin M-130 China Clipper”

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

#6. – “Norfolk to Lord Howe Islands Great Circle Distance”, https://jeremyclark.ca/wp/nav/norfolk-to-lord-howe-islands-great-circle-distance/

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

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