The 4 Best Modern Sextants – Reviews & Buying Guide

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A marine sextant is used for measuring the angular distance between the horizon and an astronomical body such as the Moon, the Sun, or any other celestial body.

Its name is derived from its shape – which looks like an arc which is one-sixth of a circle, 60 degrees.

Below are four of the best modern sextants that are worth every penny you spend.

 

 

 

1. Davis Instruments 26 Mark 15 Sextant – Best Overall

 

The first and the best on the list is the Davis Instruments 26 Mark 15 Sextant, made by Davis Instruments. This instrument weighs just about 3.4 pounds and has a dimension of 13 x 11 x 3.45 inches.

 

 

 

Features of the instrument include:

• A star telescope that has a dimension of 3 x 27mm.
• The instrument also includes 7 sun shades to aid your measurement during the daytime.
• It also includes a traditional half-silver split image mirror which is especially useful for a form of coastal piloting called sextant surveying.
• The instrument is also made of stabilized high-impact weather-resistant plastic which will help you to endure different weather times.

 

2. Davis Instruments Mark 25 Deluxe Sextant

 

The second on the list is the Davis Instruments Mark 25 Deluxe Sextant, also made by Davis Instruments. This modern sextant weighs just 3.5 pounds and has a dimension of 13 x 11 x 5 inches. Its increase in dimension explains the little addition in weight, as compared to the first one.

 

 

 

Feature of this instrument includes:

• Full-field Beam Converger mirror. This mirror replaces the conventional half-silvered mirror and gives the sailor more reliable sightings even under the most unpleasant situations.
• LED lights to illuminate the arc and drum. This gives better lighting when used in low-light conditions.
• The instrument also includes 7 sun shades to aid your measurement during the daytime.
• A star telescope that has a dimension of 3 x 27mm.

 

3. Marine Sextant, Davis

 

The third modern sextant is the Marine Sextant, Davis. This modern sextant weighs about 0.2kilogram (200 grams) with a dimension of 10 x 9 x 3 inches.

 

The modern sextant, often referred to as the “lifeboat” sextant is inexpensive and has been used by many sailors of the world.

It does not have any optical magnification but is full-sized and like the previous ones, includes sunshades. Alongside its professional purposes, it can also be used for training and which is why it includes an instruction booklet.

 

4. Celestaire Astra III Professional Sextant

 

The last on our list is the Celestaire Astra III Professional Sextant. Produced by Celestaire Inc., this high-quality modern sextant weighs 3.76 Kilograms and has a dimension of 13 x 11 x 6.5 inches.

To meet its high-quality standard, the sextant has a metal build, and guess what; it is also affordable. In the US alone, more than 25,000 sextants of this model have been sold in the past 31 years, which is more than the average sale of any other sextant.

 

Due to its popularity worldwide, the Celestaire Astra III Professional Sextant is regarded as the most supported and recognized sextant.

 

 

How To Choose Modern Sextant?

 

Now, do you know what to look out for when buying your next sextant? Choosing a modern sextant requires considering some factors. These factors include:

 

New or Used?

A new sextant is the best option you can consider as this saves you worries like a bent arc, small or dirty scopes, action of wear and tear, rust, and so on.

 

Mirror size

Good quality modern sextant often comes with large mirror sizes as this allows for more movement during use, and lessens the possibility of losing an image during angle measurement.

 

Scopes

Depending on your type of use, scopes are also a good thing to consider. For example, a scope of 3.5 x 40 is good for stars, while that of 6 x 30 0r 7 x 35 is good for sun sights.

 

Accuracy

Metal sextants, oftentimes, do not include errors like refraction, data tabulation, or the oblateness of the earth. Plastic sextants, however, are prone to error even when used carefully. Starters, therefore, are advised to get sextants of the metal build.

 

Horizon mirror

There are two types of mirrors, whole horizon mirror and split horizon (traditional) mirror.

The split horizon is divided vertically into two halves, and then has one of the half-silvered while the other is clear glass. Typically used by people on stable platforms like large ships.

The whole horizon mirror has no split image and has a full view. Used by people on vessels like yachts.

 

FAQs

1. What is a marine sextant?

 

A marine sextant is an instrument used for determining the angular distance between two objects.

Particularly, the marine sextant is used for measuring the angular distance between the horizon and an astronomical body such as the Moon, the Sun, or any other celestial body.

 

Its name is derived from its shape – which looks like an arc which is one-sixth of a circle, 60 degrees. The marine sextant is composed of a set of complex mirrors which enhances the navigator’s ability to measure distances.

 

2. How does a marine sextant work?

 

The sextant works by using two mirrors. The first of these mirrors is half-silvered which the sailor uses to see the horizon. The second mirror is attached to an arm that is moveable around the arc of the instrument.

This arm, which the second mirror is attached to, allows for the correct positioning.

The marine sextant works on the principle of mirrors which states that “when a ray of light is reflected by a plane mirror, the angle of the incident ray is equal to the angle of the reflected ray, when the incident ray, reflected ray, and the normal lie on the same plane”

 

3. What is the error of sextant?

 

While the sextant is a good tool for measurement, some errors can arise as one uses the instrument. These errors are largely classified into two parts:

 

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  Adjustable errors:

   

These are errors that are bound to occur when using the sextant. The good news, however, is that these errors can be fixed by the navigator of the instrument.

 

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  Side error:

This error occurs when the plane of the instrument and the mirror of the horizon are not perpendicular to each other.

To detect if the error is present in the device, follow these steps.

Reduce the index of the index arm to zero, and then observe any celestial object through the instrument.

Then, you move the tangent pin in a back-and-forth manner to allow the reflected image to pass below and above the direct view alternatively.

If the reflected image passes directly over the direct image while you change position from one side to another, then this side error does not occur.

But, if the direct passing of the reflected over the direct image does not occur, then there is a side error. The navigator can then place the instrument on its side and view the horizon, if two horizons show, then this also indicates that the side error occurs.

To fix this, the navigator should adjust the mirror of the horizon until both direct and reflected images collide into one image directly on top of each other.

The side error is a matter of convenience as it does not necessarily cause alteration in the measurement.

 

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  Index error:

This error occurs when the index arm is set to a zero position and then both index and horizon are not parallel to each other.

To check if an index error occurs, you should set the index arm to zero and then observe the horizon.

If the direct image and the reflected image are on the same line, then the index error isn’t present.

But if both images are not on the same line and one of them is above the other, then the index error is present.

To fix this error, you have to adjust the index mirror until both direct and reflected images of the horizon are on the same line. This is especially possible using the moon at night, or possibly a star.

 

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  Perpendicular error:

This error occurs when the index mirror of the instrument isn’t perpendicular to its plane.

To check if an index error occurs, move the index arm to a position of about 60 degrees on the instrument’s arc, then hold the instrument alongside its arc away from you.

If the arc of the sextant appears as though it were continuous, then there is no perpendicular error.

But if the arc appears broken, there is an error.

To fix this, adjust the mirror until the direct image and the reflection appear to be continuous.

 

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  Collimation error:

This error occurs if the eyepiece used to view the mirror is not parallel to the plane of the sextant.

To check if there an index error occurs, observe two celestial bodies at 90 degrees or more from each other, and then bring these objects to a side of the view. Then move the sextant slightly to move the celestial body to the other side of the view. If these bodies separate at this other side, then the collimation error is present.

 

This type of error however is not usually encountered as modern sextants do not use adjustable telescopes.

 

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  Non-adjustable errors:

 

These are errors that are usually hard to fix by the navigator.

 

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  Centering error

 

This error occurs if the index bar’s pivot is not located at the arc’s center. This is usually caused by careless handling of the instrument or sometimes a factory fault.

 

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  Shade error

 

This error occurs if the shades are mounted in a way that their surfaces aren’t normal to the rays of light passing through them. This error causes distortion and usually increases with the number of shades used.

 

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  Graduation error

 

This error occurs due to inaccurate graduation of the scale on the instrument’s arc or the micrometer.

 

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  Optical error

 

This error is caused by improper cut of the telescope lenses

 

• Wear on the worm

This occurs due to lack of lubrication, careless handling, or the collection of dust particles on the rack.

 

4. Can you use a sextant during the day?

 

Since the purpose of the marine sextant is to measure the angular distance between a celestial object and the horizon, using it during the day might not be convenient.

This is because the only available celestial object during the clear skies of daytime is the Sun, and depending on the time of the day, especially midday, the position of the Sun is directly over us.

One can easily infer that the marine sextant has little to no use as the range of the Sun’s distance from the horizon can span more than the range of the instrument.

 

 

The best time however to use a sextant is about 30 minutes before sunrise or 30 minutes after sunset. This doesn’t only offer additional celestial bodies for measurement but also makes sure that there is sufficient light for observing the horizon and the celestial object.

Perhaps you are on the sea and you have no access to a GPS or you want to know how far away two objects are from each other, or probably you just have to have fun by putting theory into practice, what modern sextants are best in the market.

 

 

5. Why is it important to know the different sextant corrections?

 

Like any other instrument, knowing how to correct them aid the user in the process of reading values and also ensures the precision and accuracy of the values measured.

 

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