Preamble
This is a concise explanation to help you determine your geographical position using a sextant and the Almicantarat app on your Android or iPhone.
The Sextant
The main purpose of a sextant is to measure the angle between a celestial body, such as the Sun, a planet, or a star, and the horizon. It has two mirrors: the index mirror, which moves with the index arm, and the horizon mirror, which is half see-through and half reflective.
When using the sextant, you look through the telescope (C) and aim at a celestial body. The image of the celestial body reflects off the index mirror (A), then onto the horizon mirror (B), and back into the telescope. This allows you to see both the horizon and the celestial body in the telescope simultaneously. You can then read the angle between the celestial body and the horizon on the sextant’s scale (E).
The Shot
Looking directly at the Sun, whether through the telescope or mirrors, is dangerous and can cause blindness. Always use filters to protect your eyes.
To shoot a celestial body, set the index arm to zero, hold the sextant in one hand, and look at the celestial body through the telescope. With your other hand, slide the index arm and follow the reflected celestial body until it reaches the horizon. Then, use the micrometer drum to align it with the horizon. If it’s the Sun, align the base of the disk with the horizon. For the Moon, alignment depends on its phase.
If it’s twilight, stars may be visible in the telescope before they are visible to the naked eye. In this case, set the estimated altitude of the celestial body on your sextant and aim at the horizon in the estimated azimuth. The celestial body should appear in the telescope near the horizon.
The sextant must be perfectly vertical when taking the measurement. One technique is to place the celestial body above the horizon if it is to the west and below if it is to the east, then gently swing the sextant left and right. The celestial body will describe a small arc while descending if to the west or ascending if to the east. Note the time (click the “Update Time Now” button in Almicantarat) when the base of the arc touches the horizon.
The Corrections
The sextant altitude slightly differs from the angle between the observer’s theoretical horizon and the direction of the celestial body. To obtain the true altitude, several corrections must be applied to the sextant altitude in the following order: Index Error, Dip, Refraction, Semidiameter, and Parallax. Fortunately, Almicantarat handles most of this for you. All you need to do is measure the Index Error and set your eye height above sea level in the Almicantarat “Settings” page.
How to measure the Index Error
- Set the index arm to zero.
- Hold the sextant vertically and look at the horizon or a star through the telescope.
- If the true object and its reflected image appear vertically displaced, the index error is not zero.
- Adjust the screw until the true object and its reflection are horizontally aligned.
- Read the index error: if it is above zero, the correction is negative.
The Celestial Fix
Taking three shots allows you to plot the ground points of three celestial bodies on Earth, forming three circles of position around them. You are located at the intersection of these three circles.
Since the shots are taken at different times, the initial circles must be advanced along the dead reckoning track to the final dead reckoning position. This process is called a running fix, which Almicantarat will calculate for you.