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

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
the bearing of a celestial body at the time of
taking a sextant altitude
The time at which we take this sextant altitude will
dictate the type of position line we obtain


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 only
Nonetheless, 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

Zenith

H of E

Centre of Earth



Visible Horizon
Zenith

H of E
Visible Horizon

Centre of Earth


Visible Horizon
Zenith

H of E

Visible Horizon

Centre of
Earth

Sensible Horizon
Zenith

900
Sensible Horizon
H of E

Visible Horizon

Centre of
Earth



Rational Horizon
Zenith

Sensible Horizon
H of E
Visible Horizon

Rational Horizon
Centre of
Earth



Zenith

Sextant

Altitude

H of E
Visible Horizon

Centre of Earth


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


Apparent Altitude:Is the angle between the limb of the body
and the sensible horizon.


Zenith

Apparent
Altitude

Sensible Horizon
Visible Horizon

H of E

Centre of Earth

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


Zenith

Sensible Horizon
Visible Horizon

H of E

True
Altitude

Rational Horizon

Centre of Earth


Index Error:Error:Is an instrument error causing the sextant to
read too high or too low. It may be given as +
or - , or as “on the arc” or “off the arc”.


Dip:Dip:The observer is usually well above the surface of
the earth and is therefore looking downwards to
the visible horizon.
A negative correction is applied to bring the angle
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 A2

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


Diagram
Zenith

Sensible Horizon
H of E

Visible Horizon

Rational Horizon
Centre of
Earth

DIP


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.