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Laboratory Three

COMPUTATION OF THE TIME OF RISING, SETTING, MERIDIAN

PASSAGE AND TWILIGHT

Daily celestial navigation routine is determined by the normal flow of celestial

phenomena:

- in the morning: nautical and civil twilight, the sunrise- at noon: Suns meridian passage

- in the evening: sunset, civil and nautical twilight

How are used these moments of the day?

- the Sun sights at sunrise and at the sunset are used to determine the compass errors

- the Sun sight at meridian passage is used to determine the true latitude

- the sights during the time of nautical twilight are used to determine the fix of the vessel

1. COMPUTATION OF THE TIME OF SUNRISE AND THE TIME OF SUNSET USING

NAUTICAL ALMANAC

1.1. Computation of t! D!"# Wat" Tim! of $i%i&'! Sun(i%!

!he table used for this computation is located on the right page of the "lmanac and it

contains the following columns:Lat, Twilight, Sunrise

#rom this table the time of sunrise can be extracted for different given latitude within

northern or southern hemisphere at the $reenwich meridian

!he following formula is used:% &ZT LMT z = +

A')o(itm*

- compute the time of sunrise at the D' (at by of interpolation !he computed time is the

sunrise time at the central meridian of the time )one %z&- compute the difference of longitude between the central meridian of the time )one and

the meridian of D' (on %z-& and convert it into time

- add the difference of longitude to the time of sunrise at the central meridian and obtainthe sunrise time at the local meridian which is the dec* watch time of sunrise

Exercise 1:

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2

Compute the deck watch time !"T# o$ the sunrise on %pril &', ())* at estimated

position+

+, -, .+, =

/,- -0.1"=

.

- + , % &

+/ /2 3- /2 ,4 /,/

,/ /2 ,+ % & 2 /,- -0 .1

-/ 5 /2 3, % & - -0 .1

, % & 2

+0,

-/

h m h m

h m m

m h m

m

m

LMT Compt !"T Compt $ Compt

LMT $

$

!"T $

/ $

/

LMT

= = =

= + = + =

= = +

= +

= =

= /2 3- , /2 ,4h m m h m

=

1.+. Computation of t! D!"# Wat" Tim! of $i%i&'! Sun%!t

!he table containing the following columns:Lat, Sunset, Twilightis used

#rom this table the time of sunset can be extracted for different given latitude within

northern or southern hemisphere at the $reenwich meridian

!he same formula is used:% &ZT LMT z = +

!he computation algorithm is the same as the algorithm for the sunrise

Exercise 2:

Compute the deck watch time !"T# o$ the sunrise on 0une ((', ())* at estimated

position+

-0 -33., =

/23 +50.1=

.

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4

21 /+5 25/ .

% & 1 /+4 +4.

2/ % & +0+ . 1

h m

m

h m m

LMT $

$

!"T $

= =

+ = =

= = =

. COMPUTATION OF THE TIME OF CI$IL AND NAUTICAL

TWILIGHT USING NAUTICAL ALMANAC

!he sights are ta*en as follows:

- between the beginning of morning nautical twilight and the beginning of morning civil

twilight ie within the mo(nin) nauti"a' t-i'i)t

between the end of evening civil twilight and the end of evening nautical twilight, ie

within the !/!nin) nauti"a' t-i'i)t

.1. Computation of t! Tim! of t! Mo(nin) Nauti"a' an0 Ci/i' T-i'i)t

Exercise 5:

Compute the deck watch time !"T# o$ the morning nautical and ci

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