David Green, Age 11, Of Tucson, Ariz., for his question:
What is a sextant?
Sir Isaac Newton (1643-1727) invented the principle of the doubly reflecting navigation instrument but never published it. Two men independently developed the octant around 1730: John Hadley (1682-1744), an English mathematician, and Thomas Godfrey (1704-1749), a glazier in Philadelphia. The octant and later the sextant, replaced the Davis quadrant as the main instrument for navigation.
The latitudes and longitudes that girdle the globe are taken from circles that span the celestial sphere of the heavens. A pilot can figure his position on the lonely ocean from the position of the heavenly bodies wheeling overhead. Chances are he will use a sextant to do the job.
It is called a sextant because it measures the degrees on a sixth or sextus of a circle. A pilot who can use it can locate his position on land or sea. The sextant measures the angular distance between points on the celestial sphere of the sky and gives the pilot the answer in degrees of latitude.
A sextant looks like a triangle with an extra leg standing upon a curved base. The middle leg is the movable index arm that pivots from the top and moves around the degrees marked on the curved arc. Along one of the fixed legs is an eye piece that points to a small glass set into the opposite leg. This is the horizon glass, half c1ear and half mirrored. A small mirror ca11ed the index glass is set at the top of the movable index arm.
The sextant works by a duplicated law of optics. The reflections of an object in two mirrors can be lined up to the same plane and used to figure the angular distance
Between the object and its image. The gadget can be used to measure the distances altitude between two stars. Pilots as a rule use it to figure the angle of the sun.
The sextant uses the horizon as a perpendicular plane. The index arm is pivoted to tilt the top index mirror until it throws an image of the sun onto the lower horizon mirror. The observer first spies the sun through the horizon glass at which point the base of the index arm points to zero degrees. Then he moves the arm, tilting the index mirror until the sun is reflected into the horizon mirror.
As the arm moves its base swings around the graduated arc. The pilot notes where it stops and uses this degree to compute his degree of latitude. One sixth of a circle, or course, is 60 degrees. But the one sixth arc of a sextant is marked with 120 degrees. This is because it shows just half the angular distance between the two points. Each half degree equals a whole degree of latitude in the sky.
A pilot can stay on course by using a compass and keeping a detailed record of distance traveled and time elapsed. This method of depending upon the earth to chart a course is called dead reckoning. When the pilot uses a sextant, he locates his position from the motions of the sun or stars across the heavenly sphere. This method of charting a course is called celestial navigation.