Since the sextant measures the angle between an object in the sky and the horizon, your reading will be less accurate if you’re not making a measurement from the level of the horizon (or sea level). You’ll need to correct that difference to find the true altitude of the object you’re observing. The difference between your elevation and the level of the horizon is called “dip. ” Even if you’re at sea level, you still need to make a dip correction to take into account the difference between the horizon level and the height of your eye.
The horizon line forms the baseline for the angle of elevation of 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. [5] X Research source
Moving the arm rotates the index mirror until light hitting the index mirror hits the reflective portion of the horizon mirror. This makes the object in the index mirror appear to rest on the horizon. [7] X Research source Sextants designed for looking at the sun include shade glasses to protect your eyes from the sun’s rays.
If your sextant has a clamp, use it to lock the index arm in place so it can’t swing around freely once you have the object in position. Some sextants have a pair of levers that you squeeze to make larger adjustments with the arm. Release the levers once you have the arm positioned roughly where you want it.
For example, if you were sighting the sun, you’d make fine adjustments with the micrometer knob until it appeared that the bottom curve (or “lower limb”) of the sun was just resting on the horizon.
For instance, if you’re making a measurement of the sun’s elevation at 16 seconds past 7:35 in the morning, you would write down “7:35:16 AM. ” Quickly recording the time is especially important if you’re using a sextant in marine navigation, since your position and the position of objects in the sky are both changing.
For example, you may get a reading of 25° 6. 40’, or 25 degrees and 6. 4 minutes. The index bar may have a small magnifying glass to help you read the graduations on the sextant arc. A minute is 1/60 of a degree, and a second is 1/60 of a minute. Most sextants can take a reading that’s accurate to within 10 seconds. [12] X Research source
This problem occurs when the index and horizon mirrors don’t line up correctly when the index arm and minute scale are set to 0. When this error occurs, the horizon won’t look like a perfectly straight line across the 2 mirrors when you look through the scope. Turn the micrometer until the line looks straight to find the angle of the index error. If it’s greater than 1. 5’, you will need to make adjustments to your sextant to fix it. Consult the user manual that came with your sextant, if you have it.
The ‘ in this formula represents minutes (60ths of a degree). 1. 7725’ is a mathematical constant that represents a small fraction of a degree. For example, if you’re 25 metres (82 ft) above sea level, and your eye level is 1. 7 metres (5. 6 ft), your height above sea level would be 26. 7 metres (88 ft). Your dip would be 1. 7725’ x √26. 7 = 9. 16’. If you got an altitude measurement of 38° 10. 60’ for the sun, after correcting for dip you would get 38° 10. 60’ - 9. 16’ = 38° 1. 44’.
You can purchase the latest Nautical Almanac from an online bookstore. Many of the tables in the almanac are also available for free on a variety of websites related to navigation and sailing. Refraction always makes objects appear higher than they actually are, which means that the correction is always negative. You must subtract it from the apparent altitude. The correct refraction reading will depend on your apparent altitude measurement.
Add the semi-diameter correction to your observed altitude after correcting for index error, dip, and refraction. For example, if you were taking a measurement of the sun’s altitude in April, the semi-diameter correction would be 15. 9’. [17] X Research source This correction only applies to relatively close objects that appear circular through the sextant scope, like the sun and moon. The semi-diameter correction adjusts your reading from the bottom curve of the circle (the lower limb) to its center. More distant stars and planets just look like points of light, so this adjustment isn’t necessary for them.
The parallax correction accounts for the difference between your vantage point on the surface of the Earth and what you would see from the center of the Earth. The combined correction for parallax, semi-diameter, and refraction is known as the “third correction. ” For example, if your observed altitude for the sun was 38° 10. 60’ and you made your observation in April, you might make your corrections as follows: 38° 10. 60’ + 1. 2’ (index error) = 38° 11. 8’ 38° 11. 8’ - 9. 16’ (dip) = 38° 2. 64’ 38° 2. 64’ - 1. 1’ (refraction) = 38° 1. 54’ 38° 1. 54’ + 15. 9’ (semi-diameter) = 38° 17. 44’ 38° 17. 44’ + 0. 1’ (parallax) = 38° 17. 45’ (true altitude)
For example, if you were at the equator and taking your measurement during the spring or fall equinox, the sun would be exactly 90° overhead at noon. If you were at one of the poles, it would be at exactly 0° (on the horizon).
Apply the corrections as follows: Observed altitude of the sun +/- index error (+ if the difference is negative, - if it’s positive) - dip - refraction + semi-diameter + parallax = true altitude of the sun
For example, if you got a reading of 82° 17. 3’, subtract this from 90° to get 7° 42. 7’.
For example, on February 1st, the declination is 17° 12’ south of the equator. The declination of the sun is at its highest (23° north of the equator) on the June solstice and lowest (23° south) on the winter solstice.
For example, if you got a measurement of 7° 42. 7’ after subtracting the sun’s altitude from 90°, and it’s currently April 13, the sun is north of the equator and its declination is 8° 54’. Add 7° 42. 7’ + 8° 54’ to get 16° 36. 57’. This is your latitude!
