Display 13: Take Your Own Latitude

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With these four navigation instruments you must try to 'shoot' the Pole Star in the constellation Ursa Minor

So you need to measure the angle between the Pole Star and the horizon.

The Navy Quadrant

Look through the two bronze sights or peep-holes at the side of the quadrant to the Pole star. Let the lead hang still for a moment and block it with your finger against the quadrant. Then read the angle. Subtract that figure from 90° to calculate your latitude; record the tens on your form.

The Marine Astrolabe

Look through the two peep-holes in the alidade to Pole Star.

Read your latitude from the left scale. Note the tens on your form again.

The Jacob's staff

With your eye just off the staff, look towards both Pole Star and the horizon. Slide the wing away from or towards you while keeping Pole Star visible above the sliding wing and the horizon below. If necessary, secure the wing with the screw and read the angle on the flat side of the wing.

You can read your latitude from the lower scale.

If using the upper scale then subtract your measured angle from 90° to determine the latitude. Record the tens digit on your form again.

The Sextant

Release the index arm with two fingers. Set it at 0° at the scale, and turn the micrometer until it also reads 0.

Look through the telescopic sight at the horizon line, which you can see through the horizon mirror. The horizon mirror is only partially silvered, allowing you to look through the mirror and telescopic sight to the horizon beyond. Move the arm until you see Pole Star reflected on the horizon mirror of the index mirror. Once you see Pole Star reflected on the horizon mirror, move the arm slightly back and forth to adjust the position of the index mirror. Move the arm first with the clamp and then with the micrometer until Pole Star is level with the horizon when you look through the telescope. Read the latirude in grades from the vernier scale. ( and the minutes from the Microadjustment)

To determine your latitude at night using the North Star, you need to apply a few more corrections:

The index correction of your sextant
the Refraction
the height or dip you are standing at relative to the sea,
the Parallax

If you were to use the sun, you would still need to subtract the corrected value from 90° and adjust for the sun's declination, which varies with the seasons. (add or subtract)

All these corrections and the declination can be found in the Nautical Almanac.

https://en.wikihow.com/Using-a-Sextant