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| Naut. Almanac |
Celestial Navigation
Day
The Earth rotates on its axis, and due to this
rotation the sun appears over a particular meridian after each rotation of the
earth. The time interval between two successive passage of the sun over the
same meridian is called a SOLAR DAY.
The time period between two successive passes of the
Moon over the same meridian is called a LUNAR DAY.
And the time period between two successive passes of
the ‘First Point Aries’ is called a SIDEREAL DAY.
The earth’s motion is not uniform thus the motion of
the sun appears also to be erratic. The transits of the sun over the meridian
which bring about the time interval called day is thus not uniform. The TRUE
SUN therefore registers what is called the APPARENT SOLAR TIME or APPARENT
SOLAR DAY.
Since we cannot work with a non uniform rotation a
mean is used, thus the mean rotation of the Earth over the entire year is taken
and the time interval that is obtained is called the MEAN SOLAR DAY. This is
equal in length to the mean or average of all the apparent solar days in the
year.
Since the ‘First Point of Aries’, an imaginary point
is fixed, thus the Sidereal Day is constant. The Lunar Day is longer than the
Solar Day and the Solar Day is longer than the Sidereal Day.
The Local Hour Angle of a body is its angular distance
WEST of the observer’s meridian expressed in unit of time from 0h to 24h or in
the unit of angular measurement from 0˚ to 360˚.
The LHA is expressed either as an arc of the celestial
equator or by the angle at the pole conatained
between the observers meridian and the meridian of the body.
The LHA subtracted from 24h would give the EAST Hour
Angle.
NOTE: When only Hour Angle is mentioned it is meant as
WEST Hour Angle.
Right
Ascension of a body:
Is its angular distance EAST of the first point of
Aries expressed in hours from 0h to 24h, it is the arc of the celestial equator
between Aries and the meridian passing through the body.
Ship Mean Time: or Local Mean Time: is the local hour
angle of the mean sun (LHA mean sun) + 12 h, and is represented by the arc Mid
VI, QM on our figs, or simply, arc MQ+12h
Suppose PT were the meridian passing through the true
sun, then the arc QT would be the local hour angle true sun (LHA sun) and this
quantity which increases as the sun moves WESTWARD is found from direct sextant
and time observation on board ship.
SHIP’S APPARENT TIME (SAT) or LOCAL APPARENT TIME
(LAT):
Lat IS THE local Hour Angle of the true sun + 12 hours
Or LHA (sun) + 12 hours
In the above figure it is the arc MID – VI – QT
Equation of Time
This is a quantity, which connects the Mean and the
Apparent times. The true sun is sometimes ahead or sometimes behind the average
or the Mean sun, and the Equation is the time interval which separates them.
In the above figure,
Arc (QM+12h) – arc (QT+12h) = arc MT, the equation of
Time
It is subtracted from or added to, the apparent time
to get the corresponding mean time, as tabulated in the Almanac.
The EQ. Of Time would be plus to apparent time for the
relative position of M and T as shown in the fig. Because the mean sun is ahead
of the true sun.
