Sextant – Apparent Altitude of the Sun:
1 sextant altitude (Hs)
2 Index error
3 non⎼adjustable error
(ε = 2 + 3)
4 DIP error (eye height)
On this self-instruction platform, you’ll discover the significance of sextant corrections through a series of eight identical exercises, commencing with the La Rochelle exercise.
What is the apparent altitude of the sun:
Having taken the sun sight with the sextant (Hs), we then adjust for the instrument’s inherent errors.
Additionally, we correct for the error introduced by the height of the observer’s eye above the horizon (Dip error) after obtaining the sun sight with the sextant (Hs).
This process yields the apparent altitude (Ha).
Short description:
1 sextant altitude (Hs)
2 Index error
3 non⎼adjustable error
(ε = 2 + 3)
4 DIP error (eye height)
Apparent altitude of the sun:
Detailed description
1 sextant altitude (Hs)
2 Index error
3 non-ajustable error
(ε = 2 + 3)
4 DIP error (eye height)
Also, you will find examples for completing the EasySextant exercises.
1 sextant altitude (Hs):
Measure the sextant altitude of the sun above the horizon and take the precise time of the observation. See the use of the sextant
Begin by roughly aligning the lower edge of the sun with the horizon using the release-catch locking device.
Then, gently turn the sextant drum to finely adjust the precise alignment of the sun’s lower edge with the horizon.
Balancing the sextant so that the sun’s lower edge just touches the horizon, and note the exact time of this moment.
Move the wrist with the right hand while fine-tuning the drum with the left hand simultaneously.
Once you have recorded the time, you should proceed to read and record the reading from the sextant.
1 example : sextant altitude Hs = 74°06′
2 example : sextant altitude Hs = 52°18′
3 example : sextant altitude Hs = 46°07′,4′
2 index error
In practice, often we first measure the index error and then proceed to measure the altitude of the sun.
Although the index error often remains stable over time, the sextant must be handled with care, as it is a precision instrument. A knock against the chart table, or placing it carelessly in its case, can alter the index error.
Indeed, the index error is the zero-sextant offset. It can reach a few minutes that will have to be added or subtracted from the readings.
In conclusion, the measurement of this error can be done using the horizon.
Apparent altitude of the sun:
index error, case one (No index error)
Start zeroing the sextant
Indeed, aim for the horizon and if the direct image and the reflected image are at the same level there is no index error
No index error
Index error, case two (positive index error)
Start zeroing the sextant.
Firstly, aim for the horizon and if the direct image and the reflected image are not the same level there is an index error
Then, align the direct image and the reflected image by turning the drum.
In fact, If the reading is to the right of zero, the index error is positive.
The expression “off the arc” is used to describe a situation where an index error measurement made with a sextant is outside the graduation of the arc.
In fact, the true zero of the sextant is at ⎼ 0°02′. (Please refer to the image above).
If we measure the Sun’s altitude as 30° on the sextant, and the index error is +2′, then the apparent altitude is 30° + 2′ (minus dip, which will be explained later on this page).
Index error, case three (negative index error)
Aim at the horizon and align the direct image and the reflected image by turning the drum.
Then, If the reading is to the left of zero, the index error is negative.
Index error = ⎼ 6′
It is worth noting that we generally tend to write +6′.
Apparent altitude of the sun:
3 non-adjustable error:
Also called eccentricity error
This is due to the poor machining of the alidade’s (pivoting point) axis and it is a fixed error.
Therefore, each sextant has a different eccentricity error.
Also, it is measured and given by the manufacturer depending on the measured altitude.
It is sometimes indicated on a label inside the sextant’s box. For a quality sextant, it is generally very low.
example: measured sextant altitude = 62°24’,3 →
non adjustable error correction +1’,5
(ε = index error + non adjustable error)
Apparent altitude of the sun:
4 DIP ERROR
DIP : The horizon seems to dip away as the observer’s height of eye increases
In fact, the distance that marks the horizon line depends on the height of the observer’s eye above sea level. If your eye is 10 meters above this level, then the horizon line is approximately 11 kilometers away.
For example, if you are on a small boat with an eye height of only 2 meters, the horizon line is about 5 kilometers away.
DIP formula = ⎼ 1,76 √ eye height (m) (always negative)
Example: eye height = 6 meters
⎼ 1,76 √ 6 = ⎼ 4,311.
We usually do not calculate the DIP error directly, but instead use the altitude correction tables below, where the DIP error for 6 meters is ⎼4.3.
Altitude correction tables 10°-90°
Simplified nautical almanac layout
