Apparent sextant height of the sun

Apparent sextant height of the sun: 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).

Allans Beach Road, Dunedin, New Zealand. Author image: Aleks Dahlberg

Short description:

1 sextant height (Hs)

2 Index error

3 non⎼adjustable error

4 DIP error (eye height)


Apparent sextant height of the sun: Detailed description:

1 sextant height (Hs)

2 Index error

3 non-ajustable error

4 DIP error (eye height)

Here, you will find examples for completing the EasySextant exercises.


1 sextant altitude (Hs):

Theory Hs to Ha: sextant altitude to apparent altitude image miss Maple
Mrs. Tapley sailed with her husband aboard the bark Saint James during the late 19th century.source National Parks Gallery.link: https://npgallery.nps.gov/

Mesure the sextant altitude of the sun above the horizon and take the precise time of the observation. See the use of the sextant

Apparent sextant height of the sun, image the return of direct and reflected image
Apparent sextant height of the sun : sextant altitude to apparent altitude direct and reflected image

Once you have recorded the time, you should proceed to read and record the reading from the sextant.

Theory apparent sextant height: sextant altitude to apparent altitude image sextant reading sextant altitude 74°06'

1 example : sextant altitude Hs = 74°06′

sextant height Galapagos

2 example : sextant altitude Hs = 52°18′

sextant sun sight height Rotterdam

3 example : sextant altitude Hs = 46°07′,4′


Aughacasla Strand, Tralee Bay, Co. Kerry, Ireland. Author image: Maoileann

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

Apparent sextant height of the sun, image sextant zero

Aim for the horizon and if the direct image and the reflected image are at the same level there is no index error

Apparent sextant height of the sun, sextant altitude to apparent altitude image of direct image and reflected image on same level

No index error

index error case two

Start zeroing the sextant again

Apparent sextant height of the sun, image direct image and reflected image not on same level
Apparent sextant height of the sun, image direct image and reflected image not on same level

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

Aim the horizon and align the direct image and the reflected image by turning the drum.

Apparent sextant height of the sun, image off the arc or on the arc

If the reading is to the right of zero, the index error is positive.

Apparent sextant height of the sun, index error +2
index error “off the arc”= + 2′

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.

If the reading is to the left of zero, the index error is negative.

Apparent sextant height of the sun, index error -6
index error ” on the arc” = – 6′

Index error = ⎽ 6′

It is worth noting, we generally tend to put +6.


End of the rainy winter day,  from Brisbane, Australia/ Author image: Andril Slonchak

Apparent sextant height of the sun:

3 non-adjustable error:

Also called eccentricity error

Apparent sextant height of the sun, image rotation point of the sextant

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.

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.

Apparent sextant height of the sun, image inspection certificat
NON-ADJUSTABLE ERROR

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

non adjustable error correction +1’,5


Rain, Ocean Beach, San Francisco. Author image: Brocken Inaglory

Apparent sextant height of the sun:

Apparent sextant height of the sun: 4 DIP (eye height)

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!

Apparent sextant height of the sun, image dip error

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

sun stars and planets

Theory apparent sextant height altitude correction table

Example Calculation of Apparent Sextant Height: