HIỆU CHỈNH ĐỘ CAO ĐO ĐƯỢC BẰNG SEXTANT(Sextant Altitude Corrections )

THE PURPOSE OF SEXTANT ALTITUDESThe entire purpose of taking a sextant altitude is to get a “position line”, and that is all we get A position line runs at right angles i.e.. @ 900 to th

An “Octant”

THE PURPOSE OF SEXTANT ALTITUDES

The entire purpose of taking a sextant altitude is to get a “position line”, and that is all we get

A position line runs at right angles (i.e @ 900) to

A position line runs at right angles (i.e @ 900) to the bearing of a celestial body at the time of taking a sextant altitude

The time at which we take this sextant altitude will

THE PURPOSE OF SEXTANT ALTITUDES

A sextant altitude at Meridian Passage will give us our latitude, but not a corresponding longitude

A sextant altitude taken at any other time will give

a position line onlyNonetheless, when we take a sextant altitude there are a number of corrections which we must

apply in order to obtain the True Altitude of the body

Corrections are necessary as the observed altitude is taken from the surface of the earth instead of at the centre and….

In the case of the sun, the altitude of the

edge i.e upper or lower limb of the sun is taken rather than it’s centre

H of E

Centre of Earth

H of E Visible Horizon

Visible Horizon

Centre of Earth

Visible Horizon

Zenith

H of E

Visible Horizon

Rational Horizon

Zenith

H of E Visible Horizon

Sensible Horizon

Rational Horizon

H of E Visible Horizon

Sextant Altitude

Observed Observed Altitude Altitude

::
Is the angle measured between the body and the visible horizon after the Sextant Index Error has been applied

Apparent Apparent Altitude Altitude

::
Is the angle between the limb of the body and the sensible horizon

H of E Visible Horizon

Apparent Altitude

Sensible Horizon

Centre of Earth

True True Altitude Altitude

::
Is the angle between the centre of the body and the rational horizon at the centre of the earth

H of E Visible Horizon

Sensible Horizon

Centre of Earth

Rational Horizon

True Altitude

Index Error:Index

Error: Is an instrument error causing the sextant to

Is an instrument error causing the sextant to read too high or too low It may be given as +

or

or , or as “on the arc” or “off the arc”., or as “on the arc” or “off the arc”

Dip: The observer is usually well above the surface of

the earth and is therefore looking downwards to the visible horizon

up to the plane of the sensible horizon

This correction is obtained from the very front

pages of the Nautical Almanac as follows

Altitude Correction Table A 2

at the front of the

Nautical Almanac

• Corrections are given for HoE in metres & feet

• There are three columns i.e HoE

in metres; HoE in feet; the dip correction

• The table is entered with the HoE (feet or metres)

• Please note the range of the

tables; also interpolation will be required in certain cases

• If the HoE matches a value in the tables then the preceeding

correction is to be taken

Terms & Definitions



Refraction:-Rays of light from the body to the observer are “bent

downwards” as they pass thru’ the earth’s

atmosphere This makes the altitude seem greater than it actually is and so a negative correction is

applied

Altitudes of 100 or less should be avoided due to

abnormal refraction

Terms & Definitions

Semi-Diameter:-Since the sun appears as a disc rather than a point of light the observer must measure to the edge, or limb of the sun for an accurate measurement

The altitude angle is taken as ‘upper limb’ or ‘lower

limb’ (the top or bottom of the sun’s disc) and the

Terms & Definitions

Semi-Diameter

(continued):-The values for semi-diameter are given in the daily pages

of the Nautical Almanac and must be subtracted from

Upper Limb observations and added to Lower Limb

observations.

HOWEVER! Corrections for refraction and semi-diameter are combined into one correction which we call the Total Correction which is obtained from the Nautical Almanac

• Total Correction is given for the Sun’s Upper & Lower limbs for the

periods October ~ March and April ~ September

• Also for the Stars and Planets

• Please also take note of the additional correction for the planets Venus and Mars

Enter the tables with App.Alt.

Terms & Definitions

 Stars and planets, being so far away, appear as points of light and do not have a correction for semi-diameter.

 Mars and Venus however, may have an additional correction due to their varying proximity to the earth and their altitude.

 These will be found in the Altitude Correction Tables at the front of the N.A.

Terms & Definitions

Semi-Diameter

(continued):-The value for the sun changes as the distance of the earth from the parent body changes during it’s yearly orbit around it.

Earth at Perihelion (closest) January 2nd.

SD = 00 16.3’

Earth at Aphelion (farthest) July 4th.

Procedure for Altitude Correction

(Sun Lower Limb; November)

• Sextant Altitude(Sun) : 45 0 25.5’

• Index Error (I.E.) : 00.2’

• Observed Altitude : 45 0 25.7’ was the IE on or off the arc?