Here are the basics for finding your position using only
the noonsite and how to to use the tables you can print from our on-line
site.
Legal Disclaimer: Use this information at
your own risk. If you have comments or corrections, contact us
here.
If you like these
tools and find them useful, feel free
to make a secure donation to help us keep on cruising!
Basics This page assumes you have some basic understanding of calibrating a
sextant and taking sites. If not, try reading this
short
primer about sextants then come back to this page to learn how to do
quick noon sites without the Nautical Almanac.
Tools
A watch that is accurate to the
seconds of the universal GMT clock or a radio capable of picking up the
time signal.
A way to measure the angle between
the horizon and the sun. A sextant is ideal, but there are other
improvised ways. See the "Emergency
Navigation" on the previous
page.
Also you'll need a set of tables which you
can generate using our Java application, or you can buy a
Nautical
Almanac or some other tool. The tables you need have the following
information:
Observation Corrections:Refraction, Dip, and Parallax
Correction
tables (they can be ignored in an emergency, but
Refraction
can not
be
ignored if the observed angle is
less than 10°).
To print out your own
correction tables click here.
Celestial Information: Sun's time
of meridian passage (MP), the semi-diameter (SD) size (can be set to 16.0' in an emergency),
Declination of the Sun (DEC) at the time of meridian passage. To make your own tables click here.
Click here for 2006 data,
or 2007 data, or
2008 data.
Shooting the Noonsite
Using your sextant at about 11:30am local time, start
measuring the angle between the bottom edge of the sun (called the lower
limb) and the horizon. Note the time in minutes and the angle of
observation.
Keep repeating this step until the angle reaches its biggest value and
starts to decrease for about 30 more minutes.
Using your information, you can make a plot of the sun's
altitude versus time as shown in the following figure from American
Practical Navigator.
To find the time of your local apparent noon (LAN), you can
average the times where the sun is at the same height on the ascending and
descending sides. Thus the first average is Ta=(t1+t7)/2, then
Tb=(t2+t5)/2 and Tc=(t3+t4)/2. The total average becomes LAN = (Ta+Tb+Tc)/3.
After you've done a bunch of noonsites, you'll get pretty good at gauging
the time of noonsite without all this extra work. You can also cheat a
little by looking at the time of meridian passage at GMT and start taking
sights about 15 minutes before your estimated LAN.
Do the Math
Now the fun part! Using the time of your noonsite
(LAN), we can calculate your longitude and using the angle you measured at
noon (Hs or Sextant Altitude), we can find the latitude.
Longitude Procedure:
From the tables, look up the time of the Meridian
Passage (MP or Mer. Pass.).
Take the difference in time of your local time in GMT (Local
Hour Angle) LHA minus MP.
Next, convert the time
difference into arc by remembering Hours*15 = degrees, Minutes/4 =
degrees (convert reminder to seconds), Seconds/4= minutes.
The result is your Longitude.
Latitude Procedure:
Take your observed altitude,
Hs, and correct for dip, refraction, parallax, and any sextant errors.
This yields H, which we need to correct for the sun.
Look up the sun's
semi-diameter (SD) correction (for Lower Limb) for your date and add that correction as
well. This is Ho, the corrected altitude.
Take 90° - Ho to compute the
zenith distance (Zd). If the sun is to your North, call your
zenith distance South. If the sun is to your South, call the
zenith distance North.
Look up the sun's declination
(DEC).
If the DEC and the Zd (from
Step 3) have the same N or S direction, add the two angles. If
they are the opposite, subtract the smaller angle from the larger, and
take the N or S label from the larger angle.
This result is your latitude!
Example
After careful observation on
November 27, 2004 the LAN was measured 33°28.7' to the South and occurred at 14h 11m 30s GMT.
Eye height 10 ft, Sextant errors = 0.
Finding the difference is
14:11.30 - 11:47.45
13:71.30 - 11:47.45 (manipulate the hours to minutes for subtraction)
13:70.90 - 11:47.45 (manipulate the minutes to seconds for subtraction)
13:70.90 - 11:47.45 = 2h 23m 45s (resulting difference)
Convert 2 hours to degrees
2*15 = 30°
Convert 23 minutes 23/4 = 5.75 (take the whole part of 5.75 as 5°)
Convert 0.75 *4 = 3 (this is the remainder degrees changed to minutes)
3 * 60 = 180 (changed to seconds)
Now add this 180s remainder to
the 45s from the difference in Step 2
180+45 = 225 seconds (next divide by 4 to get back to minutes of arc).
225/4 = 56.25 (converted back to minutes for the solution)
Now put it all together
30° + 5° + 56.25' = E
35°56.25'
This result is your Longitude. Note that it is
East because your local apparent noon (LAN) is later than noon at MP. Also keep in mind if you are on the other side of the International Date
line to make sure you look up the correct day for GMT noon.
Latitude:
Take Hs = 33°28.7', (from the
tables) correct
for dip (eye height) = -3.1', refraction = -1.4' and parallax = 0.12,
sextant errors =0. H = Hs - 3.1' - 1.4' + 0.12 = 33°24.3' (note
we are working in degrees, minutes and fraction of minutes only).
The sun's SD on Nov. 27, 2004 is 16.2 (see table below).
To get the full corrected altitude:
Ho = H + SD or Ho = 33°24.3'
+ 16.2 = 33°39.5'
Now compute zenith distance, Zd=90°
- Ho = 90° - 33.39.5' = N 56°20.5'. (Note change 90° to
89°60.0' then subtract and it's North based on the given information of
the sun being to the South.)
The sun's DEC from the tables
is S 21°14.15' as given from the tables,
so
DEC=S 21°14.2'.
DEC and Zd have the same
direction, so subtract them. 56°20.5' - 21°14.2' = 35°06.3'.
Since the North measurement is
the larger number, the latitude is
N 35°06.3'.
