How Things Work: Celestial Navigation
Knowing where you are going in space.
- By Joe Henderson
- Air & Space magazine, November 2001
(Page 2 of 2)
You can see from the chart that my calculated position—in the triangle formed by the intersection of three lines of position—is only about five nautical miles away from the actual position, given to me by the aircraft’s navigation system. On our North Atlantic plotting charts, 10 nautical miles is about a sixteenth of an inch. At our ground speeds, that size circle of error puts us within one to one and a half minutes of the exact position.
The process of navigation may be complicated, but its concept is simple. As my mom, now 82, puts it, “It’s when you leave home, you know how to get back.”
The Navigational Triangle
To describe the locations of celestial objects, astronomers imagined a celestial sphere, whose surface is of infinite distance from the Earth. Early navigators used the sphere to plot a navigational triangle, the points of which coincide with the celestial body, the elevated pole, and the zenith, a point directly above the observer. Using spherical trigonometry, the navigator solved the angle at the pole, and from that could calculate his longitude. By referring to charts that gave the star’s angular distance from the celestial equator, or declination, and determining the angle at the star, he could pinpoint his latitude.
Modern navigators have it easier. They find the altitude of a star and know they are somewhere on a circle, where from every point the star is the same angle above the horizontal.