| Amplitude | Day | Ex Meridian | Exercises | Pole Star | |
| Naut. Almanac |
Celestial Navigation
Amplitude
Determination of the observed altitude of the sun when the true altitude is zero
The semi diameter is half the angular distance of a
body as viewed from the Earth.
The altitude of the body’s centre above the rational
horizon is the quantity required in nautical astronomy. Since we are unable to
exactly see the centre of the Sun we take the altitude of the edge of the Sun
nearest to the horizon and then the semi diameter as given in the Nautical
Almanac is added to get the altitude of the centre of the Sun.
However since the Sun and all bodies are affected by
the temperature and pressure when taking the refraction into account the
Nautical Almanac has simplified the balance of the corrections into Total
corrections. One page gives for 10˚-90˚ and the other page gives for
0˚-10˚.
Thus when we take an altitude as the sun is rising and
the lower edge is just touching the horizon the actual body is well below the
horizon, as shown below.
Date 31.10.1999
We assume the Height of Eye at 10 metres- Total
Correction 0˚15’ – (-) 15.1’ (lower Limb – October to March)
|
HOE |
10m |
|
|
|
|
Sext. Alt. |
20.7 |
|
IE |
0 |
|
Obs. Alt. |
20.7 |
|
Dip (-) |
5.6 |
|
App.
Alt. |
15.1 |
|
Total
Corr. (-) |
15.1 |
|
True
Alt. |
0 |
We assume the Height of Eye at NIL- Total Correction
0˚15’ – (-) 15.1’ (lower Limb – October to March)
|
HOE |
NIL |
|
|
|
|
Sext |
15.1 |
|
IE |
0 |
|
Obs. Alt. |
15.1 |
|
Dip (-) |
0 |
|
App.
Alt. |
15.1 |
|
Total
Corr. (-) |
15.1 |
|
True
Alt. |
0 |
From the above we see that to get the True Altitude as
0˚ we have to get the Observed Altitude as 15.1’ at sea level and in
addition the height of eye would add some more.
The fact that it does not agree with the SD for that
day is due to the refraction,
The Bearing
Amplitude of a body is the arc of the horizon contained between the
It is also defined as the angle at the zenith between
the prime vertical and the vertical circle of the body when on the horizon.
A body always rises and sets on a Northerly bearing
when its declination is North and on a Southerly bearing when its declination
is South,
However when the declination
is 0˚ then the bearing is true East at rising and true West when setting. The
amplitude therefore takes the name of the declination.
The azimuth at rising or setting is the complement of
the amplitude, thus the Azimuth = (90˚ - amplitude).
The True amplitude of the sun (since the star is
practically never visible while the exact moment of its rise) depends upon the
solution of a right-angled spherical triangle.
Thus the formula:
Sine Amplitude = sine declination x secant Latitude.
The latitude is from the ships position, and the
declination is got from the Nautical Almanac.
We can see therefore that an error in the Latitude
would give a totally wrong amplitude and therefore a
wrong compass error – since amplitude is an excellent means for obtaining the
compass error.
Calculation
of the Setting time of the Sun:
(same calculation also for
the rising and the calculation of Civil Twilight)
Extracts from the Nautical Almanac
(see following sheet)
