8-inch Double Scoped & Verniered "Round" Semi-Professional Sextant

This is an antique reproduction of a round sextant. It has a slow motion control on both sides to allow for fine adjustment of the elevation angle. There are four tinted swing-arm glass filters, and a hard wood handle on the back of the sextant. As far as recallibrations of sextants go, this one works double time -  it can be used from both sides because it has two vernier scales and two telescopes for measurements. This is a fully functional high quality model. however this is not designed for professional-grade applications.

Some Sextant Basics

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.

The original Ross London sextants (on which this replica unit is authentically based) were 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.

 In earlier times, marine sextants had a fixed telescope leveled on the horizon and a radial arm is moved against an arc scaled in degrees. The radial arm is adjusted to get the reflection, of a known star, from index mirror and then off the horizon mirror down the telescope until it lines up with the horizon. The position of the radial arm on the scale gives the stars elevation. The nautical sextant range includes: slow motion nautical sextant, octants, slow motion nautical sextants and round dubble telescope nautical.

In modern navigation sextants, the light ray from the celestial body is reflected in two mirrors (in series) one of which is adjustable and the other is half silvered. By rotating one mirror and its attached index bar, the image of the body is brought down to the horizon. The rotation measures the altitude on the limb.

Using a Sextant

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-through 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.

When the sextant is set to zero degrees, the image through both optical paths as viewed through the telescope align to produce a single image. Moving the index arm causes the image to split in half.  When viewing two objects, such as the horizon and a celestial body, the index arm is adjusted until the two are superimposed. The micrometer drum reads out the angle between the two objects.

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.

Why Bigger is Better

The size of the mirrors on a sextant generally vary directly with the quality of the instrument. Large index and horizon mirrors are desirable because larger mirrors admit more light, making it easier to obtain sights in marginal conditions. Larger mirrors also lessen the possibility of losing the image as the body is brought down to the horizon.

The heavier weight of a brass sextant provides greater steadiness and hence more accurate readings, especially if it is windy. As the observer develops proficiency and speed in sight taking, fatigue becomes less of a factor.

The 3.5 to 4 power magnification helps you find and maintain stars in view in both calm or pitching seaways. A 6x30 or 7x35 monocular of greater magnification is well suited for sun sights, or the greater heights of eye associated with large ships.The increased magnification allows the sun's diameter to appear larger, and better defines a more distant horizon. This helps the navigator determine the point of tangency of the sun's limb and the horizon. The increased magnification however makes finding and holding sights more difficult on a moving deck. A Sight Tube of zero magnification affords a wider field of view for rough weather, horizontal angles, and finding stars. If your sextant is to have only one scope, a 3.5x or 4x would be the logical choice for yacht sized vessels.

Many sextants have an option of either the traditional (half-silvered) horizon mirror or what is called a "whole horizon mirror". With the traditional mirror, the horizon glass is divided vertically into two halves producing a "split image." The half nearest the frame is a silvered mirror and the other half is clear glass. In some cases this clear glass is eliminated. A later development in sextant technology is the whole horizon mirror. Using specially coated optics, the whole horizon mirror superimposes both the horizon and the celestial body on the entire mirror with no split image. This greatly simplifies "bringing down" the celestial body and makes it easier to hold the body in view. A draw back to this system is a very slight reduction in light transmission and reflection which may affect marginally lighted observations. Some feel these two aspects are a trade off; that is, one can more quickly take observations with the whole horizon mirror, and be finished before marginal conditions occur. In general, professional navigators tend to favor the traditional horizon mirror while beginners tend to favor the whole horizon mirror.