Sextant
 © Museum of the History of Science, Oxford
A sextant is a very important mathematical instrument for navigating at sea. It can also be used in astronomy. It is used to measure the angle between two far away objects. Most often these objects are the sun and the horizon, or the pole star and the horizon. Sometimes the angle between the horizon and stars or the moon can also be measured with a sextant.

This sextant was made in England at the end of the 18th century. The biggest problem for sailors in the 18th century was finding their position in the middle of the oceans during long journeys. Sailors needed to be able to find both their latitude (which was their position north or south on the earth) and their longitude (which was their position east or west on the earth). Sextants helped them to find out both of these things. Before the invention of satellites and electronic satellite navigation systems it was very hard for sailors to find their position at sea. Many shipwrecks occurred because captains got the positions of their ships wrong. This not only caused the deaths of many seamen but also had big political and economic implications for the Government because lots of battles happened at sea or valuable cargo could be lost in shipwrecks.

Sextants are made with a circular curve that is one sixth of a circle. The curve is divided up like a protractor. It is used for measuring angles so it is labelled with degrees. The other important parts of a sextant are a telescope, a piece of glass which is half see-through and half mirror (called the horizon glass), and a moving arm which has another mirror fixed to it.

To use a sextant to measure the angle between two far away objects you use the telescope to look at one of the objects through the piece of glass which is half see-though and half mirror. While you are looking through the telescope you then adjust the moving arm until a reflection of the other object you want to measure becomes visible next to the first object. The second object becomes visible because it is reflected first off the mirror at the end of the moving arm and then off the mirrored part of the horizon glass. You have to line up the reflection of the second object exactly next to the image of the first object. Then you can read off the angle between the two objects on the degree scale.

Although this sextant was made in the 18th century other sextants like this are still being made today so that sailors can use them to find their position if they don't have satellite navigation or in an emergency when the satellite breaks down. In the 20th century, before satellite navigation was invented, more complicated electronic versions of sextants were used in aeroplanes. Electronic sextants are also put into missiles because they allow the missiles to know their position without needing to rely on external radio signals from satellites, which might be jammed by the enemy.

In all these different types of sextant, no matter how complicated they are, they are all doing one simple mathematical operation: measuring the angle between far away objects.