How to Use a Sextant
The sextant is a navigational aid that consists of a small sighting scope, mirrors, a movable arm, and a mirror 60-degree wooden or metal arc (a sixth of a circle, from which the instrument’s name is derived).You can use a sextant to determine the position (called the “shoot”) in the sky of the sun, moon, or other celestial body, as well as your longitude. Although its design looks complicated, with an understanding of how it works and practice, you can reliably use it to find your position.
Finding Angle of Elevation for an Observed Object
Know, if possible, your position above sea level.If you’re not using the sextant from aboard a ship at sea, you’ll have to correct your sighting for your height above sea level.We’ll discuss the correction later; for now, you just need to know your elevation above sea level.
Sight the horizon by looking through the horizon mirror.The horizon mirror is only partially silvered, allowing you to look through it and through the sighting scope beyond it.
- The horizon line forms the baseline for the angle of elevation for the object you’re determining the position of.
- Your sextant may not regard the horizon line as being 0 degrees. If it doesn’t, you’ll have to correct the angle measure of the object you’re trying to determine by the same amount as the horizon line error. This error is called index error.
Move the sextant’s index arm until the object you’re trying to find the position of is also visible on the horizon.A second mirror, the index mirror, is mounted on the moving arm. Moving the arm rotates the disk the index mirror is on until light hitting the index mirror hits the reflective portion of the horizon mirror, making the object the light comes from appear to rest on the horizon.
- Sextants designed for looking at the sun include shade glasses to protect your eyes from the sun’s rays.
Clamp the index arm in place.The clamp is a flip-lock that prevents the arm from moving freely.
Fine-tune the position of the arm by turning the micrometer knob until the object rests on the horizon.Make the adjustments gradually while swaying the sextant from side to side until the object just touches the horizon.
Record the time at which you made your sighting.You’ll need to record the time in hours, minutes, and seconds, starting with the seconds first to avoid errors.
- Quickly recording the time is especially important if you’re using a sextant in marine navigation.
Record the angle measure.You can read the angle of elevation for the object as follows:
- The degrees of elevation will be at the center of the index bar (the part of the index arm the clamp and micrometer knob are attached to) in a window over the sextant arc. The index bar may have a small magnifying glass to help you read the graduations on the sextant arc.
- The minutes and seconds can be read from the graduations on the micrometer knob.
Correct the angle measure according to your position and the object you’re sighting.The angle measure you found with the sextant needs to be corrected for each of the following things:
- Index error. This error is caused when your sextant doesn’t read the horizon as 0 degrees, but as more or less than 0. If your sextant reads the horizon angle as greater than 0 (a positive number), subtract the horizon angle from the angle measure of the object. If your sextant reads the horizon angle as less than 0 (a negative number), add the number of degrees difference to the angle measure of the object.
- Dip. This correction adjusts for your position above sea level. Find your elevation in feet (if in meters, multiply by 3.28), then multiply the square root of your elevation by 0.98 to find the amount to correct the angle measure by.
- Refraction. Light rays bend when passing through a substance; this bending is called refraction. The thicker the atmosphere, the greater the refraction. You can get the correct refraction correction for where you are by consulting the Nautical Almanac.
- Parallax. You need to correct for parallax if you’re observing the sun, moon, or a planet with your sextant. This correction factor is available from the Nautical Almanac.
- Semi-diameter. If you’re observing an object with a significant apparent diameter (the sun or moon), you need to find the apparent distance from its edge to its center. This correction factor is available from the Nautical Almanac.
Finding Your Latitude With a Sextant (Daytime)
Find the sun’s angle of elevation at its highest point.This occurs at noon, local time (standard time).
- Follow the instructions in “Part One: Finding Angle of Elevation for an Observed Object.”
Consult reference tables to find the latitude the sun should be directly over on the day you’re observing.The sun appears directly overhead (at zenith, 90 degrees elevation) at the equator (0 degrees latitude) on the vernal and autumnal equinoxes (the first days of spring and fall).
- From the March equinox, the place where the sun appears directly overhead moves northward until the June solstice, then it moves back toward the equator until the September equinox. The latitude where the sun is directly overhead on the June solstice is the Tropic of Cancer, 23.5 degrees North latitude.
- From the September equinox, the place where the sun appears directly overhead moves southward until the December solstice, then it moves back toward the equator until the March equinox. The latitude where the sun is directly overhead on the December solstice is the Tropic of Capricorn, 23.5 degrees South latitude.
- If you are north of the Tropic of Cancer, the sun will always appear south of you at its highest point. If you are south of the Tropic of Capricorn, the sun will always appear north of you at its highest point. If you are between the tropics, the sun may appear either to your north or south at its highest point, or directly overhead, given the time of year.
Find the difference between the elevation angle of the sun and the zenith.If the sun appears south of you at an elevation angle of 49 degrees, subtract 49 from 90 to produce a difference of 41.
- If you are making this observation on either the June or September equinox, this difference is your latitude, in this case 41 degrees North latitude. If the sun had appeared north of you at this same elevation on either of the equinoxes, your latitude would be 41 degrees South latitude. If it isn’t one of the equinoxes, you have more work to do.
- If the latitude at which the sun is directly overhead is north of the equator and the sun appears to your south at its highest point, add this latitude (the solar declination) to the remaining angle to get your latitude. If the sun appeared overhead at a latitude of 20 degrees North latitude when you saw it at an elevation of 49 degrees from your position, your latitude would be 61 degrees North latitude (90 – 49 + 20). Likewise, if the latitude at which the sun is directly overhead is south of the equator and the sun appears to your north at its highest point, you would add the latitude to the remaining angle to get your latitude.
- If the latitude at which the sun is directly overhead is south of the equator and the sun appears to your south at its highest point, subtract this latitude from the remaining angle to get your latitude. If the sun appeared overhead at a latitude of 20 degrees South latitude when you saw it at an elevation of 49 degrees from your position, your latitude would be 21 degrees North latitude (90 – 49 – 20). Likewise, if the latitude at which the sun is directly overhead is north of the equator and the sun appears to your north at its highest point, you would subtract the latitude from the remaining angle to get your latitude.
Finding Your Latitude With a Sextant (Nighttime)
Find Polaris, the North Star.Polaris is the brightest star in the constellation Ursa Minor (the Little Bear, Little Dipper). It is located at the end of the Little Bear’s tail/Little Dipper’s handle. If you have trouble spotting it, there are two ways to find it.
- Sight along the two stars at the outer end of the bowl in the Big Dipper in the direction the bowl opens. These pointer stars will lead your eye to Polaris.
- Sight from the Great Square in Pegasus through the constellation Cassiopeia (resembling an “M” or “W,” depending on its position in the sky). When the Big Dipper is below the horizon, this is a substitute method to find Polaris.
Find Polaris’ altitude with your sextant.See “Part One: Finding Angle of Elevation for an Observed Object” for instructions for how to do this. The angle of elevation for Polaris will be the same as your latitude.
- This method works only for locations in the Northern Hemisphere, as Polaris is not visible for locations south of the equator.
QuestionHow do I make a sextant?wikiHow ContributorCommunity AnswerTry YouTube or Google for tutorials on how to do this.Thanks!
- Devices related to the sextant include the quadrant, quintant, and octant. These items are so named because their arcs describe a quarter circle, fifth of a circle, and eighth of a circle, respectively. All are graduated so the angle measures on their arcs are double the angle measures their arcs physically describe; that is, a sextant’s arc describes a 60-degree angle but is marked with a range of 120 degrees.
- Modern sextants feature larger mirrors than antique sextants, with mirror diameters of 2 inches (5cm), as opposed to the 1-inch (2.5cm) mirrors of antique sextants. Some also feature artificial horizons for use in conditions when a natural horizon cannot be seen.
- Related to the navigational sextant is the astronomer’s sextant. It differs from the navigational sextant in that it is a much larger instrument and does not use mirrors to measure angles, thus meaning that it cannot measure angles any greater than its 60-degree arc.
- The sextant’s telescope must be set parallel to the plane of the sextant. To check for this, look at two stars more than 90 degrees apart and use the sextant to make them appear next to each other. Move the sextant so the stars move to one side of your field of view. If the two stars separate when you do this, you have a collimation error and must adjust the sextant’s telescope. This is a problem only with antique sextants; modern ones use adjustable telescopes.
- The horizon mirror also must be perpendicular to the plane of the sextant. You can check for this by moving the index arm to 0 degrees and looking through the horizon mirror. Rotate the micrometer knob (tangent screw) back and forth so that you see both the star and its reflected image. If the reflected image passes directly through the actual image, your horizon mirror is aligned correctly. If it passes to one side, you have side error and must adjust the horizon mirror until the images pass through each other.
- In addition to index error, sextants are vulnerable to other problems that must be corrected if they are found. If the index mirror is not perpendicular to the sextant’s frame, it can cause the sextant’s arc to appear broken when viewed through the index mirror. This is called perpendicularity error. You can check for this by locking the index arm at 60 degrees and looking through the index mirror.
- These three errors must be checked for and corrected in the order listed above: perpendicularity error, side error, and collimation error.
Video: How to Navigate by the Sun
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