Local Noon in an earth-bound position, occurs if the Geographical Position (GP)
of the Sun is located on the local meridian of this position.
This event is called Culmination or Meridian Transit.
At the moment of Culmination, the Azimuth of the Sun at this position is either
0° or 180° (North or South).
These are the different steps for the Noon Sight procedure:
Consequently, a Line of Position obtained from the Sun's GP at this time,
will have a constant Latitude (it will be in an East-West direction).
The navigator can take advantage of this event to check the Latitude
of his position.
The Longitude can also be calculated, although with lower accuracy.
Determine the approximate time of Local Noon:
from the Nautical Almanac the exact time of Local Noon for the Prime Meridian
of Greenwich can be retrieved. To obtain the time of Local Noon,
the Estimated Longitude converted to time
(1 hour corresponds to 15° longitude) is added (Eastern Longitude) or
subtracted (Western Longitude) to the Greenwich Local Noon time.
For the conversion of Longitude to the corresponding time difference,
the Time - Hour-Angle Conversion Table
can be used.
About one hour before Local Noon, the time and Altitude of the Sun
(UT_1 and H1) is measured and recorded. The Altitude of the Sun will increase further until
About 5 to 10 minutes before the estimated Culmination Time,
the Sun is observed with the sextant and the maximum Altitude (Hmax) is recorded.
The time to start this observation, depends on how precise the Estimated Latitude
is and also on the distance travelled since the first measurement.
Around the moment of Culmination, the Sun will appear to "hang" at a constant Altitude
for about one or two minutes.
No time record is made for this observation, only the Altitude "Hmax" is recorded.
After Culmination, the Altitude of the Sun will start to decrease.
Next, the sextant is reset exactly to the Altitude of the
first measurement (H1).
About one hour after Local Noon, the Sun's Altitude (using the same limb
used for the first measurement) is observed with the sextant while the altitude setting remains at H1.
The time at which this pre-set Altitude is reached is recorded as UT_2.
The Culmination Time at the local position is the "average" of the two
times recorded with equal Altitude (before and after culmination):
UT_culmination = (UT_1 + UT_2) / 2
From the measured local Culmination Time and the Culmination Time for the
Prime Meridian of Greenwich found in the Nautical Almanac, the Longitude of the local
position can be determined.
The Latitude of the local position is determined from the maximum Altitude (Hmax) and the
Declination of the Sun for the time of local Culmination (Nautical Almanac).
Additionally, two Lines of Position can be obtained from the two
measurements with equal Altitude (before and after Culmination).
Notice, that this method does not take into account, the change in Declination
that occurs between the first and last measurement (about 2 hours).
There is a Worksheet available
containing the necessary record sections and calculation schemes for
this Noon Sight technique.
If the position between the three measurements is changed, the
UT_1 and UT_2 times will have to refer to the position at Local Noon!
Remember that 15° of Latitude corresponds to 1 hour of time or one
minute-of-arc of Latitude corresponds to 4 seconds of time.
Time - Hour-Angle Conversion Tables and the
Interpolation Tables for Celestial Navigation
can be used for converting longitude into time and vice-versa.
Especially in tropical regions where the Altitude of the Sun can reach 90°
and the Lines of Position all end up running almost North-South,
the determination of Latitude using the Noon Sight becomes important.