Our information on this is based upon the actual charts Noonan and Harry
Manning used while going from Oakland to Honolulu in March. Typically,
two stars were used to determine position, with each star approximately
90 degrees in azimuth from one another. Given only one side window in the
fuselage of the plane, this may explain some anecdotal stories that Harry
Manning was a real pain, because he was in and out of the cockpit quite
a bit, probably getting star sights directly ahead of the plane, whereas
the sights to the side of the plane were made in the back.
Why not more stars? The idea of navigation is to monitor where you are,
and absolute precision is not necessary, only good to 10 miles or so. Two
quality star sights can give you that precision. Occasionally, FN and HM
would make a couple of sightings within a half hour or so, to double-check
their readings. All together on that flight, there were less than a dozen
actual fixes. There is no evidence on any charts of any wind vector determinations.
By using the stars and (during the day) the sun, Noonan could calculate,
by means of pre-figured angles and a printed table, a position on a map.
During the day, multiple sun sightings were necessary; with only one sun
shot, only a Line of Position could be calculated. They would know they
were somewhere on that line, but not exactly where in terms of north/south.
The sun shot Noonan took at dawn on the way to Howland was such a LOP.
That LOP was then advanced, on paper, until it fell through the destination,
Howland Island. A calculation was made to estimate how long it would take
to get there, and the flight continued until then, at which time....
“WE MUST BE ON YOU BUT CANNOT SEE YOU....”