Celestial navigation

Jump to: navigation, search

See also Air Navigation: State of the Art in 1937.

"Celestial navigation, also known as astronavigation, is a position fixing technique that was devised to help sailors cross the featureless oceans without having to rely on dead reckoning to enable them to strike land. Celestial navigation uses angular measurements (sights) between the horizon and a common celestial object. The Sun is most often measured. Skilled navigators can use the Moon, planets or one of 57 navigational stars whose coordinates are tabulated in nautical almanacs."[1]

Basic celestial theory tells us that we can convert the height of a celestial body above the horizon to a distance from its subpoint. If we know where the subpoint is, we can determine our distance and approximate direction from it. When the body is directly overhead (the sextant reads 90 degrees) we are at the subpoint. When the body is on the horizon (the sextant reads 0) we are 90 x 60 nautical miles or 5400 nautical miles from the subpoint.

This information can tell us that we are somewhere on a circle around the body’s subpoint which may have a radius up to 5400 nautical miles. To make things a bit easier, as far as plotting is concerned, we assume a position on the earth, compute what the height of the body above the horizon should be at that position, and compare it to what we actually observe with the sextant. The difference is then used to adjust the assumed distance of the circle from the subpoint to its actual distance. For practical convenience, only a short segment of this circle is plotted on the chart and it becomes a “line of position”. This circle segment (the LOP) is perpendicular to the direction of the celestial body's subpoint from the observer.

Offset method

When dead reckoning from one line of position (LOP) to an advanced LOP, it is sometimes useful to deliberately aim north or south of the intended destination in order to know for sure which direction to turn when one calculates that the advanced LOP has been reached.

There is good evidence that Noonan did not use the **offset method** on the fatal flight.

Ric Gillespie, Forum 26 June 2010:
Back in AE's days nobody routinely tried to find islands by LOP navigation. Pan Am was the only airline flying the Pacific and they used Radio Direction Finding (RDF) to navigate to their island destinations. For example, when a clipper flight departed California for Hawaii, it tracked outbound on a bearing from the Alameda station until it was out of radio range. The navigator then kept the flight more or less on course until it came within range of the Mokapu station which guided it in. Earhart and Noonan planned to find Howland by the same method.
For Noonan to have used the offset method to find Howland he would have had to know that RDF was not going to work when they were still far enough out to make the offset. Earhart was still trying to use RDF as late as 1930Z (0800 local) after they had already reached the advanced LOP.
At 1912Z (0742 local time) Earhart said, "We must be on you but cannot see you." If you are using the offset method there is no moment when you can say that you "must be" at your destination. Earhart probably meant that, according to the clock, they had now reached the advanced LOP and no island was in sight.

Moreover, it is unlikely that Earhart and Noonan would have reported that they were "running the line north and south" if they had used the offset method. The whole point of the method is to fly so far off course from one's destination that when the advanced LOP is reached, one knows which way to turn on the LOP. The last transmission heard is good evidence that they did not know whether they were north or south of Howland when they reached the LOP.

Related articles