Sir Isaac Newton and the Sextant

Up until the voyages of discovery began in the late 15th Century, ship captains rarely sailed out of sight of land. Why? Even though they knew what direction they were heading, mariners were not able to accurately “fix” their position.

Shipboard inertial navigation systems (INS) weren’t invented until the 1950s and GPS came much later. Even with the early INS on board, ship captains and navigators combined dead reckoning which calculates ones position based on course, speed and time with sightings through a sextant to accurately fix the ship’s position.

Columbus, for example, used a magnetic company and headed west. He proved that the earth was not flat. Columbus used a device called a quadrant to measure the angle between the horizon and the North Star. It gave him a rough estimation of his latitude, but not his longitude.

In 1594, John Davis, the English explorer who discovered the Falkland Islands, refined the quadrant to improve its accuracy, particularly when using the sun. Around 1731, the British mathematician Thomas Godfrey and the Pennsylvanian John Davis independently came up with what is known as the reflecting quadrant or octant around 1731. The two men added a small, low power telescope, filters to reduce the brightness of the sun and mirrors or prisms arranged so the user can see two images – point on the horizon and the celestial object.

The octant’s index arm calibrated in degrees is moved to align the two images. The Davis/Godfrey octant had two advantages over the Davis quadrant. One – accuracy and two, its “lightweight” design made it easy to align the two images on the pitching and rolling deck of a sailing ship.

It turns out that Davis and Godfrey weren’t the first to come up with the concept of the octant. While studying optics, Isaac Newton studied the Davis quadrant and in 1599 – 50 years before Davis and Godfrey – theorized how the light could be reflected more accurately through what we know as the “reflecting quadrant.” His ideas and drawings were given to the astronomer Edmund Halley (as in Halley’s Comet) and not published until well after Halley’s death in 1742. By then, the Godfrey/ Davis’s octant was already being used to measure latitude.

The octant is limited to measuring angles of up to 90 degrees and is best used when sighting objects well above the horizon. In 1759, John Bird modified the octant so it could measure angles of up to 120 degrees that enables sailors to sight the moon, sun or stars at any time during the day.

Now navigators could measure the moon in all its phases. By the end of the 18th Century, the relationship of the sun to the moon was relatively well documented and at sea, a way to tell the time of day. Bird’s sextant gave mariners a rough, not precise way to estimate their longitude. Now, sailors could, with reasonable certainty, plot their position on a Mercator chart. Accurate estimation of longitude would come later when inventors created the first accurate chronographs.

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