English mathematician who presented a reflecting octant to the Royal Society in 1731
Thomas Godfrey
Philadelphia glazier who independently developed a similar instrument
John Bird
Instrument maker who built the first sextant in 1759
Plate 049
1730s
England and colonial Pennsylvania
No photograph is shown for this exhibition. The museum displays an image only where it can show the object honestly.
About this date
John Hadley in England and Thomas Godfrey in Philadelphia independently developed the octant around 1730–31. The sextant was built from 1757–59.
About credit
Hadley and Godfrey both have a claim. Bird built the first sextant on the suggestion of Captain John Campbell.
Chapter II
The world before
Life before the Octant and Sextant
Mariners measured the sun’s altitude with a backstaff or cross-staff, which was hard to use on a rolling deck.
II
The human achievement
How it came about
A mirror and a half-silvered mirror let the observer see the horizon and the Sun at the same time. The angle between the two mirrors, read from a scale, gives the Sun’s altitude. A sextant spans 60 degrees of arc.
III
Understanding the invention
How it works
The Octant and Sextant, explained in two steps.
01
Step 01 of 02
Two mirrors
The index mirror moves; when its reflected image of the Sun touches the horizon, the angle is read off.
Two mirrors
From angle to position
IV
The transformation
What changed for humanity
Navigation
Greater precision lowered the risk of shipwreck.
Exploration
Charting coasts and oceans became more accurate.
Sailors could find latitude, and with a chronometer longitude, more accurately than before. It stayed in use until satellite navigation arrived.
V
Before and after
Life before and after The Octant and Sextant
Before
A rough estimate with a staff.
After
A precise angle to within a minute of arc.
On larger screens, drag the divider (or use the arrow keys) to give more room to the world before or after.