Octant (oct)

General & non-SICurrent unit

One eighth of a full turn (45 degrees).

1 octant = 0.125 turns (exactly).

About the octant

Octant is best known as the name of a navigation instrument. Its arc is an eighth of a circle, 45 degrees, but because mirrors double the measured angle it reads up to 90 degrees. Isaac Newton showed a reflecting quadrant to the Royal Society in 1699, though its description was not published until 1742. Around 1730 the English mathematician John Hadley and the Philadelphia glazier Thomas Godfrey developed the instrument independently, and Hadley's version published in 1731 took the familiar octant form. It let a navigator see the horizon and a star together, which made sightings easier.

Lunar distance measurements can exceed 90 degrees, so a larger instrument was needed, and the sextant followed. Octants stayed in use into the 19th century, especially on merchant and fishing vessels. As a plain angle, an octant is 1/8 of a turn: 45 degrees, π/4 radian, or 4 points on a 32-point compass rose, which is the spacing between neighboring principal winds such as north and northeast.

Sources: Wikipedia: Octant (instrument); Wikipedia: Points of the compass

Octant conversion chart

1 octant expressed in every other angle unit:

More General & non-SI units

• degree (angle)
• gradian (angle)
• turn (angle)
• arcminute (angle)
• arcsecond (angle)
• right angle (angle)
• sextant (angle)
• piece (counting & quantity)
• pair (counting & quantity)
• score (counting & quantity)
• hundred (counting & quantity)
• thousand (counting & quantity)
• million (counting & quantity)
• billion (short scale) (counting & quantity)
• trillion (short scale) (counting & quantity)
• billion (long scale) (counting & quantity)

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