Apparent sextant height of the sun: On this page

1 sextant height (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 height 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 height (Ha).

### Short description:

1 sextant height (Hs)

2 Index error

3 non⎼adjustable error

(**ε** = 2 + 3)

4 DIP error (eye height)

### Apparent sextant height of the sun:

### Detailed description

1 sextant height (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):

Mesure 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 and the horizon with 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 noting the exact time of this moment.

Moving the wrist with the right hand and fine-tuning the drum with the left hand are done 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 height of the sun. Certainly, the index error remains unchanged, but it’s essential to handle the sextant with care as it is a precision instrument.

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 sextant height of the sun:

#### index error case one

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

Start zeroing the sextant again

Firstly, aim for the horizon and if the direct image and the reflected image are not the same level there is an index error

#### index error case two

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.

Also, the expression “off of arc” is used to describe a situation where a 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).

As a result, imagine that I measure the height of the sun at 30°, we must add these 2′ to find the true height.

conclusion: Index error +2′

#### index error case three

Aim 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, we generally tend to put + 6′.

Apparent sextant height 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 height.

It is sometimes displayed in the sextant’s box. For a quality sextant, it is generally very low.

example: measured sextant height = 62°24’,3 **→ **

non adjustable error correction +1’,5

(**ε** = index error + non adjustable error)

Apparent sextant height of the sun:

### Apparent sextant height of the sun: DIP (eye height)

#### 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.

In conclusion, if you’re on a small boat and your eye is only 2 meters above the water’s surface, the horizon line is located at about 5 kilometers!

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 but use the altitude correction tables below, and there we find the DIP error for 6 meters = ⎼ 4,3

##### Altitude correction tables 10°-90°

Simplified nautical almanac layout