In ancient times, navigators knew how to determine their latitude, or position north or south of the equator, by the position of Polaris (the North Star). When setting sail, the navigator would simply measure the altitude of Polaris as he left port, and when returning after a long voyage, needed only to sail north or south to bring Polaris to the altitude of home port, then turn and “sail down the latitude,” keeping Polaris at a constant angle.
The early Arabs used a finger’s width held at arms length to sight the horizon and Polaris. Later, they used a kamal, a knotted cord attached to a wooden transom, to make the observation. The navigator would hold the cord in his teeth and sight Polaris along the top of the transom and the horizon along the bottom. As the years passed, advancements in astronomy and astrology led to the development of two important astronomical instruments in the 10th century; the quadrant and the astrolabe.
The mariner’s quadrant – a quarter of a circle made of wood or brass – came into widespread use for navigation around 1450. Like the Arab kamal, the quadrant measured, in degrees, the altitude of Polaris or the sun to determine the geographic position. The motion of the ship made it difficult to keep the quadrant aligned with the stars, and it was impossible to keep the wind from blowing the plumb bob off-line. The astrolabe was simply a circular scale marked in degrees, and a rotating alidade with sighting pinnules used to sight the star and read its altitude. The next step in the evolution of celestial navigation instruments was the cross-staff. This was replaced in 1711 by the Davis back-staff which used the shadow of the sun and a sight of the horizon to determine altitude.
In the late 1600s and early 1700s, instrument makers started shifting their focus to optical systems based on mirrors and prisms. This led to the idea to use two mirrors to make a doubly reflecting instrument. The sextant was invented independently, and almost simultaneously, by John Hadley of England and Thomas Godfrey of Philadelphia around 1730.
The sextant (pictured) is an optical system consisting of a telescope and two mirrors, one fixed and one moveable. A mounted telescope points toward a fixed mirror, half transparent and half silvered. An observer looking through the telescope sees the horizon through the clear part of the mirror while seeing the reflection of the star or sun on the mirror’s silvered portion, as reflected from a movable mirror on the level. By moving the level, the image on the mirror is brought into coincidence with the image of the horizon. The angular distance between the two can then be read on a scale engraved on the sextant. This scale is an arc of one sixth of a circle (60°); hence the sextant’s name from the Latin word “sextans” for one sixth.
Today, sextants are used only for back-up navigation. They make ideal nautical gifts for those who love the sea, or are collectors of historic navigational instruments.
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