For the full article in PDF format, click here

---

DUMMIES GUIDE TO ASTRO NAVIGATION

The idea of this booklet is to give you the ‘nuts and bolts’ in the process of gaining a position on your chart through celestial navigation without a deep understanding of the underlying theory.

It will be useful for those of you who just want to be able to ‘do it’, and also as an aid memoir to those of you who haven’t done it for a while. 

To keep things simple I have only included the process of the sun, which is the main celestial object used by most Astro-Navigators, however, the principal is similar for all the other bodies.

I am grateful for any input, so please let me know of anything you might like to see in these notes, and of course any errors that you may find!

Best Regards

Alan Denham

Email: This email address is being protected from spambots. You need JavaScript enabled to view it.

Falmouth 2011

---

Basic Guide to Celestial Navigation

---

• What do I need?
• Accuracy 
• Relationships
• Basic Theory – GP & Position Line 
• Using the Sextant
• Taking the Time
• Sextant Corrections 
• Finding the position (GP) of the sun (GHA & Declination)
• Calculating Local Hour Angle (LHA)    
• Sight Reduction (Using the Air Tables) 
• Finding the Calculated Sextant angle and bearing of the Sun
• Calculating the Intercept and Azimuth
• Plotting
• Meridian Passage
• Sun-Run-Sun 

 

What do I need? 
Sextant                                                       £90-£1000
Watch (Digital preferable)                   £10
Nautical Almanac                                    £20 - £35 (Annual publication)
Sight Reduction Tables (Air Tables)   £25 each (2 volumes Lat 0°-38° & 38°-89°, lasts forever)

Accuracy                                  
Good metal Sextant                         3nm     New £400 / Second Hand £200
Plastic Sextant (EBCO etc )            10 nm     New £90

Relationships
To understand Astro navigation, you need to appreciate the relationship between angles and distance, as well as angles and time.

Angles and distance

A circle has 360°
Each degree is made up of 60’
1’ = 1nm

Angles and time

360° = 24 hours
15° = 1 hour
1° = 4 minutes
1’ = 4 seconds

 

Basic Theory – GP & Position Lines                                                                                                                                                

Suns GP (Geographical Position)
The sun’s GP (Geographical position) is its position on the earth’s surface in terms of Latitude and Longitude. Imagine a line drawn from the centre of the sun to the centre of the earth. Where this line crosses the earth’s surface is the GP. This point is continually moving as the suns moves across the sky. To avoid confusion with terrestrial co-ordinates (Lat/Long), we use different terms. The Latitude of the sun is called DECLINATION, and like Latitude, is measured north and south of the equator, 0 degrees at the equator and 90 degrees at the poles. The longitude of the sun’s position is called GREENWICH HOUR ANGLE (GHA) and like Longitude is measured from Greenwich (0). However, unlike Longitude that is measured West and East, GHA is always measured West, from 0 to 360.

Zenith Distance (ZD) (distance from suns GP)
By measuring the angle between the sun and the horizon, you can easily work out how far you are away from the Sun’s GP. The sextant allows you to measure this angle. Here is the simple formulae;

90° - Sextant Angle = how far you are away from the Sun's GP.

e.g.

You measure the angle of the sun with your sextant, it reads 60° degrees.

90° – 60° = 30° degrees

We know there are 60’ in a degree, so 30° x 60' = 1,800’

We are 1,800nm away from the sun’s GP.

 Finding the GP (based on time)
The position (GP) of the sun is based on time (Universal Time (UT)). When we take our sextant angle on the sun, we also take the time by our watch. Our watch needs to be pretty accurate, so within a few seconds of UT. We can use a special set of tables called a NAUTICAL ALMANAC, to gives us the GHA and DECLINATION of the sun at this precise time.

Circle of position
We can now mark the sun’s GP onto a chart (a very large ocean chart). We can now measure off the distance from the sun’s GP (e.g. 1,800nm), and scribe a large circle around the GP. Our position is somewhere on this circle.

To get a position, we could take the bearing of the sun with our hand-bearing compass, and plot this from the GP (just like we would with a lighthouse). We now have a position.

THE THEORY IS VERY SIMPLE, HOWEVER, IT IS ALSO VERY IMPRACTICAL. DRAWING A CIRCLE OF THOUSANDS OF MILES IS INACCURATE, AS IS MEASURING THE BEARING OF THE SUN WITH A HAND BEARING COMPASS. WE NEED TO APPROACH THE SOLUTION ANOTHER WAY.

---

For the full article in PDF format, click here