sextant apparent to observed altitude

Calculation: The conversion from sextant apparent altitude (Ha) to observed altitude (Ho) requires correction for atmospheric refraction and compensation for the varying diameters and distances of both the Sun and the Earth throughout the year. Moreover, these adjustments collectively result in the final observed height, measured from the center of the Earth to the center of the Sun. (Altitude = Height)

Firstly, we examine the method for calculating the observed height of the sun.

If you’ve been following the course, you’re already familiar with the underlying principles of observed height theory.

sextant apparent to observed altitude, errors due to external factors and period of the year

This page contributes to the primary objective of offering guidance and interactivity on the calculation and understanding of the Line of Position (LOP) using only a sextant and the sun, and then tracing this line on a plotting sheet. See recommendations.

Klein River meets the Atlantic Ocean (Walker Bay, South Africa): Author Olga Emst

sextant apparent to observed altitude:

Ha to Ho on this page (Red section):

sextant apparent to observed altitude: example calculation apparent altitude to observed altitude

 North coast of Faial (Azores). Author image: Schnobby

sextant apparent to observed altitude:

Three entries:

-1 Ha = 32°14′,5 (Apparent altitude.)

In fact, this apparent height was obtained in the blue section (Hs to Ha)

-2 Date: Observation somewhere in JULY

-3 lower limb of the sun

sextant apparent to observed altitude image upper and lower limb
description lower limb and upper limb
Altitude correction table from The nautical almanac
sextant apparent to observed altitude correction table
Altitude correction table from nautical almanac (sun only)

In fact, App altitude is about 32°, the observation between april and september and we took the lower limb → correction = 14′,5

Sextant apparent to observed altitude: solution example

Hs …………………. = _ _° _ _’, _

index error…….. = _ _° _ _’, _

Dip………………… = _ _° _ _’, _

 _________________________ +

Ha………………….. = 32° 14′, 5

sun correct LL …= 00° 14′, 5

__________________________ +

Ho…………………….= 32° 29′, 0

sextant apparent to observed altitude, image observed altitude from center earth

Observed height = 32° 29′,0


Hambantota District in southern Sri Lanka. Author image 2: Sara Weerarathne