How do we get ship’s position by star sight ? Written by Capt Rajeev Jassal on Jassal on March 21, 21!
"hese days, there is an enor#o$s a#o$nt o% in%or#ation available on celestial navigation& Which #eans there is also a lot l ot o% noise to si%t thro$gh& 'nd when yo$ do si%t thro$gh it, yo$ still have to answer one ($estion)
How do yo$ apply all o% this *nowledge practically on board the ship& "al*ing speci%ically abo$t star sight, there are tons o% reso$rces that deals with the calc$lation part& +e it ongit$de by chrono#eter or intercept #ethod calc$lations& Most o% $s *now all these calc$lations b$t only that #$ch& +$t how can we $se these to calc$late the sight ? -n this post, - will show yo$ step wise proced$re to $se the star sight to calc$late yo$r position on board& et $s start&
+asics o% celestial sight - have covered the basics o% the celestial navigation in a di%%erent article& .o$ can read this article by clic*ing on here&& here 'nd i% yo$ have read it yo$ wo$ld *now that celestial navigation is based on calc$lation o% /enith distance and a/i#$th o% the celestial body&
0enith distance and altit$de o% the celestial body are inter related& Here is the relation& 0enith distance "r$e 'ltit$de 3 4 5egrees& 5 egrees&
'll we want is "r$e altit$de alti t$de o% the celestial body which can be calc$lated by #eas$ring the altit$de o% the celestial body by a se6tant and applying %ew corrections to the #eas$red altit$de by altit$de by se6tant& - won’t go #ore deep into the calc$lations part& - ass$#e yo$ already *now that&
7e6tant altit$de is #eas$red by bringing the celestial body on the $ser’s visible hori/on and reading the altit$de %ro# the se6tant& Here is a video that e6plains the $se o% se6tant&
7tep 1) Calc$late the twilight ti#e 7o %ar so good& 8or #eas$ring the se6tant altit$de two things sho$ld be visible&
Celestial body Hori/on +$t the iss$e with the star sight is that, stars are visible at night and hori/on is visible when there is daylight& 9r, when the hori/on is clearly visible, there aren’t any stars in the s*y and when the stars are visible, the hori/on isn’t visible& "hen how do we #eas$re the se6tant altit$de o% the stars?
:a$tical twilight is the ti#e when so#e stars #ay be visible and d$ring the ho$rs o% twilight there is still so#e daylight that hori/on is visible too& "he na#e ;:a$tical twilight< is given beca$se this is the ti#e when #ariners can see both the hori/on and stars and is ideal %or the star sight&
+$t how to *now the ti#e %or the :a$tical twilight? "he twilight ti#e is given in the na$tical al#anac& -n the #orning, the period %ro# co#plete dar*ness to the s$nrise is divided into three twilights&
We are interested in the period o% :a$tical twilight& With respect to ti#es #entioned in the na$tical al#anac, this wo$ld be the period between the ti#e o% ;na$tical twilight< and ;Civil twilight<& 7o %or #orning star sights, we need to calc$late the na$tical twilight ti#e %ro# the al#anac& "hat wo$ld be the ti#e we need to start loo*ing %or the visible stars& 8or the evening twilight, here is how the period %ro# s$nset to co#plete dar*ness is divided&
7o %or evening twilight, can yo$ g$ess what ti#e yo$ sho$ld start loo*ing %or the stars %or star sight? 9r in other words, what is the ti#e %or start o% na$tical twilight? .es, yo$ got it right === Civil twilight ti#e #entioned in the al#anac is the ti#e o% ;end o% civil twilight< and ;start o% the na$tical twilight<&
8or evening star sights, this is the ti#e we wo$ld be interested in&
"he twilight ti#e given in the 'l#anac are M"& "his need to be converted to the ship’s ti#e so that we *now at what ti#e >ship’s ti#e will be na$tical twilight& et $s say the 5R position is
atit$de) 2@ degrees Min$tes >:orth
ongit$de) A 5egrees Min$tes >Bast 7hip’s "i#e is) M" @ Ho$rs 5ate) 14th Jan$ary 21!, Bvening star sight 8ro# the :a$tical 'l#anac, get the ti#e o% evening civil twilight >start o% :a$tical twilight %or 2@ degrees :orth&
.o$ will need to interpolate as the ti#es are %or 2 degrees :orth and D degrees north& Here is the calc$lation to calc$late civil twilight ti#e as per the ship’s ti#e&
7o in this case we need to be ready with the se6tant at 1A2 Hrs ship’s ti#e >1D2 Hrs M" loo*ing %or the stars&
7tep 2) 8ind the in%or#ation on available stars -% yo$ see the ti#e period o% the na$tical twilight >start and end o% na$tical twilight %ro# the na$tical al#anac, yo$ wo$ld note it to be less than D #in$tes in #ost cases&
-deally we wo$ld have aro$nd 1 #in$tes o% the ti#es to ta*e the se6tant altit$de o% the stars& -% we don’t *now where to loo* %or in the s*y, or which star we are planning to #eas$re the se6tant altit$de %or, we will never be able to ta*e the star sight& -n other words, period o% na$tical twilight is never eno$gh to loo* %or the stars, identi%y the star and #eas$re it’s se6tant altit$de& Re#e#ber that we need identi%y at least three stars separated per%ectly %ro# each other >close to 12 degrees di%%erence in a/i#$th %or a per%ect star sight& 7o what do we do? We need to *now be%ore hand abo$t
Which stars wo$ld be available %or star sight "he stars we plan to $se %or the star sight 'ppro6 '/i#$th o% these stars so that we *now which direction to loo* %or these stars
'ppro6 altit$de o% these stars so that we *now how high in the s*y to loo* %or these stars
"his is where star %inder >:E D2D is $sed to %ind this in%or#ation& :E D2D >7tar %inder consists o% one sheet %or the layo$t o% the stars& "his sheet has two sides, one %or :orthern he#isphere and other side %or so$thern he#isphere&
-t also consists o% transparent te#plates %or di%%erent latit$des o% the observer&
"he instr$ctions %or $se o% star %inder are clearly given on the star %inder sheet&
-n a n$tshell, here is what we need to do& Calc$late the H' 'ries %or the ti#e o% observation& 's we disc$ssed we need to ta*e the star sight at start o% :a$tical twilight& 8or evening sights this is Civil twilight ti#e in the 'l#anac& Choose the transparent te#plate nearest to the 5R latit$de& 7o i% o$r 5R latit$de is 2A degrees, we need to choose D degrees te#plate& We need to place the transparent te#plate on the star %inder sheet as per the instr$ctions above& "he stars inside the web o% lines are the stars that wo$ld be visible to yo$ at the ti#e o% na$tical twilight&
:ow we need to choose the best D stars a#ong these stars& What do we #ean the best stars %or star sight? "he criteria is
"he star need to a bright star so that it is easily visible d$ring the twilight, and
"he a/i#$th o% the three chosen stars sho$ld %or# as close e($ilateral triangle as possible et #e e6plain these two points& "he brightness o% the stars is denoted by di%%erent sy#bols o% the stars in the star %inder&
'll these stars are visible to the h$#an eye b$t considering that there isn’t co#plete dar*ness d$ring twilight, we #$st try to choose the stars with #agnit$de 2& or less >pre%erably with #agnit$de less than 1&&
'part %ro# brightness o% the stars, we also need to ta*e into acco$nt the a/i#$th o% the stars& We do not want to select stars with a/i#$th line parallel to the other selected stars& "his is beca$se then the position lines that we will get %ro# the star sight will be close to parallel to each other& 'nd that is not good %or getting the position o% the ship&
8or e6a#ple, in the below con%ig$ration star Fega and star 'ltair both are bright stars b$t we cannot choose both o% these&
"his is beca$se the a/i#$th o% the Fega is 22 degrees and a/i#$th o% 'ltair is 2D degrees& +oth o% these stars are separated by only 1 degrees& -% we choose these two stars the position lines that we will get %ro# these stars will be separted by only 1 degrees& 7o i% - have to pic* three stars %ro# the below, - wo$ld pic*
'rct$r$s with '/i#$th aro$nd 1@ degrees
'lioth with '/i#$th aro$nd 1G degrees Fega with '/i#$th aro$nd 22 degrees
'part %ro# a/i#$th, we also need to note down the appro6 altit$de o% the selected star& 8or e6a#ple in below pict$re, the appro6 altit$de o% Fega is @@ degrees&
7o with the help o% star %inder >:E D2D, we will be able to choose three stars that we need %or star sight& 'lso we will have the appro6 a/i#$th and altit$de o% these stars&
7tep D) 8ind the selected stars in the s*y :ow we *now the stars we need to star sight and its location in the s*y& 7o when the na$tical twilight starts, be ready with the se6tant to #eas$re the altit$de o% the selected stars as these beco#e visible&
eep on loo*ing in the direction the a/i#$th o% the selected star& 8or e6a#ple %ro# the bridge wing gyro repeater, loo* where is 1@ degrees& "he star 'rct$r$s will be visible in this direction& +$t then the ($estion is, how high in the s*y to loo* %or this star ? .o$ can get this in%or#ation by #eas$ring the s*y with yo$r %ist& Height o% one %ist is e($al to 1 degrees o% altit$de&
7o %or star 'rct$r$s >appro6 altit$de G degrees, start %ro# the hori/on >in the direction o% 1@ degrees bearing and #eas$re height o% G %ist& "his is where this star wo$ld appear& 5o sa#e %or other two stars and get to *now the appro6i#ate position where yo$ wo$ld e6pect the star to appear& :ow *eep on loo*ing closely in these three locations in the s*y and #eas$re the se6tant altit$de o% the star as soon as the star appear&
7tep I) Meas$re the se6tant altit$de +e%ore yo$ get ready %or #eas$ring the se6tant altit$de, get to *now the ;-nde6 error< o% the se6tant& >- a# not eleborating on the se6tant errors in this post& :ow as yo$ see yo$r selected stars in the s*y, #eas$re its altit$de by se6tant& 5o not %orget to note down the e6act ti#e o% #eas$ring the se6tant altit$de& -nacc$racy in noting down the ti#e can ca$se error in %inal position o% the ship&
7tep @) Calc$late the position line and position thro$gh which to draw it -t’s calc$lation ti#e now& 8or each observation, calc$late the position line& 8or star sights, intercept #ethodis pre%erred %or calc$lations& With the intercept #ethod, we get by calc$lations
the intercept >towards a/i#$th or away %ro# a/i#$th
'/i#$th Eosition line >4 degrees %ro# the a/i#$th We already *now the 5R position o% the ship& With these val$es we can plot the position line on the chart& et $s say that %or star 'rct$r$s, we got
-ntercept ) 1& :M towards a/i#$th '/i#$th ) 1! degrees Eosition line will be 1! degrees 14! degrees
We j$st need to *now %ro# where to draw this position line& 8ro# the 5R position we need to draw a/i#$th line and c$t the intercept o% 1 :M& -% the intercept was 1& :M away we wo$ld need to draw the a/i#$th line in the opposite direction and c$t 1:M #ile on this line& "he position line will be 4 degrees %ro# the a/i#$th at the point o% intercept that we c$t& -t wo$ld loo* so#ething li*e this&
"his is how the position line loo*s in theory& +$t in reality when ta*ing star sights on board, we need to plot this on the chart with all the lines >a/i#$th, intercept etc to the #eas$re#ent& "his is how we need to plot the position line on the chart&
7i#ilarly, we need to draw the position lines obstained %ro# the celestial observation o% other two stars on the chart& "he position where all the three position lines wo$ld #eet is the position o% the ship obtained %ro# the star sight& "his is how this star sight plotted on the chart #ay loo* li*e&
J$st $se the parallel r$ler %or reading the position yo$ j$st obtained %ro# the star sight& 'nd congrad$lations === .o$ are a tr$e navigator now& What’s #ore, we have developed a sight calc$lator %or yo$ to easily calc$late and $nderstand the calc$lation part&
J$st inp$t all the val$es and clic* on the ;Calc$late sight< and it will calc$late the sight instantly&
Concl$sion
"he irony with celestial navigation is that everyone teaches the easiest part which is the sight calc$lations& +$t how to $se all those calc$lations practically on borad to get the ship’s position re#ains $nanswered& With regard to star sight, we need to *now the stars that we wo$ld $se %or the sight #$ch bo%ore the act$al sight& "his can be done by $sing the star %inder :E D2D and getting the pict$re o% the available stars at the ti#e o% star o% na$tical twilight& 8ro# the available stars, we need to choose the best three stars& +est stars %or star sight does not always #ean brighter stars& 'part %ro# the brightness, the position lines obtained %ro# these three stars sho$ld be separated by so#e angle >ideally by G degrees& We can *now this by *nowing the appro6i#ate a/i#$th o% the choosen stars %ro# the star %inder& 9nce we have choosen the stars %or star sight, we need to #eas$re the se6tant altit$de o% these stars once these appear on the s*y&
Rest is the calc$lation part with which we get the position line and intercept val$es& 8inally we need to plot these position lines on the chart to get the celestial %i6 o% the position& Share this:
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'bo$t Capt Rajeev Jassal Capt. Rajeev a!!al "a! !ailed #or over $% &ear! 'ainl& on cr(de oil) prod(ct and c"e'ical tanker!. He "old! *A in !"ipping , Logi!tic! degree #ro' London. He "a! done e-ten!ive re!earc" on (antitativel& 'ea!(ring Sa#et& c(lt(re onboard and !a#et& cli'ate a!"ore w"ic" "e believe! i! t"e 'o!t i'portant ele'ent #or !a#er !"ipping.
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14 Co##ents
'#it chandel Mar 21, 21!
7ir, K #ade it in so si#ple lang$age that there is no #argin %or do$bt&& - donLt thin* there wo$ld be any easy e6planation li*e this&& -t really clari%ies the do$bt ia# having and hope so %or other also it will be help%$l&& "han6 again sir %or $r e%%ort&& +lessings %or $&&& Reply
Rajeev Jassal Mar 21, 21!
lad yo$ %o$nd it $se%$l '#it&&& Reply
'bhishe* $#ar Mar 22, 21!
"han*s a lot sir& Reply
Rajeev Jassal Mar 2I, 21!
"han*s 'bhishe*&&& Reply
-van Mar 2G, 21!
Capt& Rajeev& May od bless yo$ %or all yo$r *indness in sharing yo$r easy to $nderstand articles& .o$Lre the best 7ir& Waiting %or #ore articles %ro# yo$& Reply
Rajeev Jassal Mar D1, 21!
lad yo$ li*ed it -van&&& Reply
bobja#es Mar 2A, 21!
"han* yo$ capt %or sharing yo$r *nowledge with $s& -t is s$ch a nice blog& "hese are the *eys which we can get %ro# yo$& https)yachtcharter$ae&wordpress&co#21A11G#a*eitgrand celebrateyo$rbirthdayonayachtthisyear Reply
Rajeev Jassal Mar D1, 21!
"han*s +ob&&& Reply
Bric Mar D, 21!
"his is what i have been waiting %or& 7o#e other says this now not so i#portant d$e to high technology on ships nowadays& +$t %or #e it is good also to *now have a *nowledge in celestial navigation& Reply
Rajeev Jassal Mar D1, 21!
"han*s Bric&&&Celestial navigation is as i#portant as other aspects o% navigation&&& Reply
5#itry Mar D1, 21!
"han*s Capt== yo$r articles are very $se%$l and in si#ple lang$age= Reply
Rajeev Jassal 'pr D, 21!
lad to hear that 5#itry&&& Reply
7hahbaa/ 'h#ad 'pr D, 21!
"hn6 alott sir&&$r e6planation is si#ply s$perb&&&by reading $r blogs it helps #e alot&&please #a*e a blog on topic gyroco#pass n #agnetic co#pass& these two topics are really di%%ic$lt to $nderstand& eagerly waiting sir& Reply
Rajeev Jassal 'pr D, 21!
lad yo$ %o$nd it $se%$l 7hahbaa/&&& - will write on yroco#pass and #agnetic co#pass&&& Reply
oren/ +& Calvo 'pr 2@, 21!
Fivid e6planation& -# also reading yo$r article basics o% dra%t s$rvey& "han* yo$ Capt& Reply
Finay $#ar singh 'pr 2G, 21!
7ir than*s %or e6plaining se6tant and it’s $se and calc$lation in very si#ple lang$age&&nobody can e6plain better than this && Reply
'sh$tosh $#ar 'pr D, 21!
-t was wonder%$l article, sel% e6planatory& Can yo$ please elaborate on why do we nor#ally $se intercept #ethod %or star sight calc$lation and what are the #ethod o% %inding stars i% star %inder is $navailable onboard& "han*s in advance sir& Reply
Manjeet
