take the time and trouble to
learn about this ancient art / science?
one could ask why do
people go to sea in small boats, climb mountains, etc?
we all have an inbuilt need to connect to our environment or
even an aspiration to the divine.
satisfaction from being able to truly place ourselves, not just on the
surface of our little planet but within the breath-taking beauty of the heavens.
Looking for a Sextant?
to the GPS you know where you are, you can check your position against
the sextant, see the difference, note any little errors, gradually
correct them and become almost as good s the GPS itself.
the purposes of celestial navigation on the surface of our planet you
don't need to know about any of those crazy Copernican ideas of the
As far as we are concerned
pre-Copernican, Ptolemaic notions of what we see up there are
Of course it is always good
know what really is going on, but for now all we need is to be aware of
what we can see.
And what we see up there
Sun, Moon, Stars, and Planets marching in step across the
They all rise to our east
set in the west.
imagine that all these celestial bodies are on the inside surface of an
immense crystal globe which encloses the earth.
imagine that this crystal globe has an equator, north and south poles,
latitude and longitude which reflect those of the earth.
Our celestial sphere now
grid system which we can use, just like
the lat and long on earth, to locate the position of celestial objects
in relation to us.
So, now we earth bound
navigators have two spheres to consider.
first is the surface of the earth, the second is the inside surface of
that imaginary crystal globe surrounding the earth.
On both spheres we use the
same imaginary sets of coordinates, latitude and longitude.
We use this grid system to
pinpoint the location of anything on either sphere.
The grid on the celestial
sphere is a projection of the grid on the terrestrial sphere.
So if you were stood at the
Earth's North Pole directly above your head would be the Celestial
And by the same token the
Celestial equator would be directly above someone stood on the
OK, so far so good, now it
starts to get a bit complicated.
The equivalents of the
terrestrial latitude lines are called lines of Declination on the
While longitude line
equivalents become Hour Circles on the celestial sphere.
where longitude, on the terrestrial sphere is measured through 180
degrees east or west of the Greenwich meridian, on the celestial sphere
it is numbered from 0° all the way around to 360°.
And the 0° hour circle
known as the Celestial Meridian is known as the First Point of
There is also quite a bit of strange archaic language used
Navigation but to begin with there are a few terms which are worth
These are to do with how
you or I relate to the celestial bodies.
First is the point directly
over your or my head this is known as your or my Zenith, and the bit
of the surface you
are standing on is your body's Geographical
Position or GP.
The rate of change is
approximately 1° every four days.
Now don't rush out to buy a
sextant, you can make a rough measurement of the suns angle using your
Stretch your arm out
towards the horizon, use your clenched fist as a rough measuring
Your fist will give you a
rough angle of 10° this can be subdivided by each finger being the
equivalent of 2°.
if you want to find due east on the morning of the 1st March, (20 days
before the equinox) you know that the sun will be rising 5° south of
east, so you need to measure half a fist to the north, of where it
rises, to find due east.
OK, so that tells you where
east and therefore all the other compass directions are but it
tell you where you are.
But what we have learned so
far is that the GP of the sun and therefore is Declination changes
throughout the year.
And, that this information
is predicted for us in the Nautical Almanac.
So how, using astro navigation, do you
let's stay with the sun for now as it is the easiest celestial body
find and consider the ocean navigators bedrock, the Noon Sight.
The reason for taking
the Noon Sight is to determine the exact time when the sun has risen to
its highest point in the sky.
This is known as your Local
At your Local Noon the
sun is directly north or south of you depending on where you are on the
This means that its
Celestial Meridian will correspond with your Longitude at that
particular moment in time.
Now longitude is
measured in degrees around the globe but one circuit of the globe is
also equal to one solar day.
if you have an accurate watch (chronometer) set to UT (Universal Time)
or GMT as it used to be called, the difference between your Local Noon
and UT Noon can be converted into degrees and distance west or east of
the Greenwich meridian.
The distance of a body's
GP from the Greenwich Meridian is its Greenwich Hour Angle (GHA).
for every hour your local noon is after UT noon you are 15° further
west of the Greenwich meridian and conversely every hour your local
noon is before noon UT you 15° further east.
Unfortunately the sun
isn't a very good time keeper, it can be as much as 20 minutes slower
or faster than UT.
And any error in time
will give us an error in longitude.
is why we need that Almanac, it will give us the UT or Greenwich Mean
Time that the sun reaches its highest point at Greenwich, for every
hour of the day and for every day of the year.
This is the sun's
Greenwich Hour Angle (GHA).
We can then use that
time as our starting point.
And the need for
accuracy is also why the sextant and an accurate watch are so important
to celestial navigation.
practice there are minor corrections which need to be applied but these
only require some simple addition and subtraction and there are simple
forms which can be used to make it even easier.
For now, it's the
concept we are concerned with.
regardless how accurate you are or how simple it is, having only one
coordinate won't tell us where we are, we could be anywhere along
meridian line between the north and south poles.
The lack of an accurate
way of telling the time is why sailors in days of yore had problems
determining their longitude.
What they were able to
do, with considerable accuracy, was work out their latitude.
So to plot our position on that line of longitude we need
to know our latitude.
remember, is the celestial spheres equivalent of our terrestrial
The almanac will tell us
the declination of the sun for our particular date and time.
Now if the sun were
directly overhead our declinations would be the same and we would have
However, that is
unlikely, usually we are going to be either north or south of the sun.
So to arrive at our
latitude, we begin by measuring the angle between our horizon and the
object, the sun, with our sextant.
This angle is the sun's Altitude.
However this angle is
one we are measuring on the surface of the earth.
The suns declination is
measured from the center of the earth.
To get from altitude to
declination we need what's known as the Zenith Distance,
this is simply 90° minus the Altitude.
we now add or subtract the sun's declination, which we got from the
almanac, to or from that zenith distance and we have our Latitude!
Why did I say add or
Well, that will depend
on whether declination is north or south.
are also other corrections, which in practice have to be taken into
consideration but for now we are just looking at the concept.
to find your latitude is, like
the Noon Sun Sights relatively simple.
However, when using a sextant you will need to be able to see
horizon as well as the star.
So sights are taken at dawn or dusk, when both are visible.
Finding your latitude and longitude using Polaris requires a
similar process to that of the Noon Sun sight, except that it does
require a few more simple corrections because Polaris is never
precisely at the North Celestial Pole.
In fact Polaris makes a very small circuit around North Celestial Pole.
despite that tiny discrepancy Polaris is always more or less on the
observers Meridian whereas the sun is only on your Meridian at noon.
All the other stars circle around the Celestial Poles, how
they appear to circle and whether or not they set will depend on your
Travelers from time immemorial have been using the stars to help them
find their way.
The Pacific Islanders were especially adept at using star patterns for
And they are just as useful for today's navigator, fortunately
for us we do not have to keep all the relevant information in our heads
as did those Polynesians, all we need is in the Almanac.
However, if the Almanac were to give us the GHA of every star, the book
would be too large and heavy for the average cruiser.
So, what the Almanac
gives us is its Sidereal
Hour Angle (SHA).
The SHA is the star's
distance from the First
Point of Aries,
(represented by the sign of the ram) which using simple
worksheets and some simple maths we can then convert.
So, the almanac only needs
to tabulate the movement of this one
position then by giving us the angular distance to all the navigational
stars, we can easily calculate the GHA for any star at any time.
So, what is this
First Point of Aries?
is a point on the celestial sphere which is used as a reference point,
it is in fact where the 'Ecliptic' and the Celestial Equator cross
the relevance for celestial navigation is that, as far as we
are concerned, all the stars remain in the same relative positions to
it and each other through out the year.
advantage of using the stars for navigation is that as they revolve
around the celestial sphere they remain in the same relationship to
each other, which is why they used to be known as the 'Fixed Stars'.
planets however move, but their movements are predicted in the almanacs
so we can tell precisely where each body is for any second throughout
year, these used to be called the 'wandering objects'.
the astronomers using their telescopes can count innumerable planets
the ones we are interested in are those which are easily visible to the
Mars, Jupiter, and Saturn plus of course the Sun are the only ones used
for navigation unless we include the Moon with this group.
method used to reduce an observation of a planet is similar to taking a
sun sight, except that as you also need to see the horizon to measure
the altitude this must be done at twilight.
The Moon, if I can be
forgiven for including it here under Planets, can also be used for
because it is so much closer to the earth than the other object we use,
many more 'corrections' have to be made when reducing the sight.
who has read Joshua Slocum's account of his sail around the world
know that he used Moon sights as his main way of telling the time, his
only clock was an old wind up alarm clock with no minute hand.
A reasonable quality sextant
such as the one I bought years ago will
cost about the same as four or five hand held GPS sets.
There are some much
cheaper plastic sextants on the market which I am led to believe are
just as accurate.
am also led to believe (never having used one) that care must be taken
over the storage of the plastic ones as they are much more susceptible
to developing errors, so might need adjusting more often.
The sextant as I'm sure
you are aware by now is used to measure the angle between the horizon
and a celestial body.
can also be used to measure the any sighted angle, such as the angular
distance between two landmarks or the height of a lighthouse, when
Like any good magic
trick it's all done with mirrors.
is called a sextant because the calibrated arc is one sixth of a circle
60° , however because of the mirrors it can actually measure angles
to 120° .
The frame of the sextant
looks a bit like a slice of pizza.
I am perfectly aware that the majority of Wooden Boat aficionados are sensible folk. However, I need to point out that I am an amateur wooden boat enthusiast simply writing in order to try to help other amateur wooden boat enthusiasts. And while I take every care to ensure that the information in DIY Wood Boat.com is correct, anyone acting on the information on this website does so at their own risk.