Bareback Latitude

[HenryC]

I've written here before about how celestial navigation is losing its justification in this age of GPS and other electronic navigation systems.  However, there is one situation where CELNAV techniques can be used for emergency navigation when all other systems are disabled.  The stars will always be there, and latitude sailing is a method that's been around for centuries; its how Columbus discovered the New World and found his way home, and how Magellan and Drake circumnavigated the planet.  It works, its easy, and requires  no skills or special equipment. All you need is a cheap plastic sextant, a magnetic compass and a nautical almanac.  You don't need sight reduction tables, a calculator  or any means of doing the math.  You don't even need a timepiece.

Latitude sailing, which is how sailors found their way at sea prior to the introduction of the chronometer in mid-18th century, involves sailing north or south until you reach the latitude of your destination, then sailing east or west until you reach its longitude.  It is impossible to determine longitude without knowing the time in Greenwich, UK, but latitude is very easy.  The Nautical Almanac tabulates the declination of every navigational body throughout the year, and declination is just another word for latitude.  So if you can determine (with a sextant) how far away a celestial body is from you in degrees measured along the earth's surface, it is a simple addition or subtraction to determine your latitude.

If you measure the altitude of a celestial body above the horizon with a sextant, subtracting that number from 90 degrees tells you how far the body is from the point directly overhead, your zenith, the "zenith distance" (ZD).  And the ZD is how far away the spot on earth that body was over at the moment of observation is from you.  So, for example, if you measure a celestial body to be 50 degrees above the horizon, then it is 90-50 = 40 degrees away from you.  And a degree is exactly 60 nautical miles.  That is all there is to it. 

Of course, knowing how far away the geographical position of a body is from you still doesn't tell you exactly where you are.  But if you take your sighting when the body is due north or south from you, you will know how far N or S of the body's Declination (or latitude) you are. 

Imagine an imaginary line drawn in the sky, from pole to pole, going through the spot directly overhead (your zenith).  This line is called the meridian, specifically,  it is YOUR meridian, because every spot on earth has one.  Your local meridian sweeps across the celestial sphere from W to E as the Earth rotates. If you observe the body when it is on the meridian, it is directly N or S of you, and as high in the sky as it is going to get that day.  You can calculate exactly what time this is going to happen, but if you don't have a clock, it doesn't matter.  You just observe it with a sextant and when it is as far from the horizon as it can get, it is on the meridian.  When this happens with the sun, we call it "noon".

So when the body starts approaching the meridian, you start taking readings with your sextant, writing them down one after another.  As the body reaches the top of its arc, the change in altitude slows down to a crawl, becoming very gradual, until it crosses the meridian and starts coming back down.  At that moment, when the sextant reading is at its maximum, and the body is due N or S of you, you take that value, subtract it from 90, and that's how far you are to the N or S of the body in degrees.  You look up the declination for the body for that day and time in the almanac(nearest hour is good enough) and calculate your latitude.  No celestial body changes declination faster than 1 minute of arc per hour, and the Almanac gives you the declination for every even hour of Greenwich time.  So you don't need to know the exact time.  As long as you know roughly  what time zone you're in, you can make a very good guess as to a body's declination at the time of observation.  You'll be off by at most 1 nautical mile N or S for every hour of time you're off, and then only for the Sun when its near the equinoxes in late fall and spring.

For example, lets say you are sailing in the N hemisphere, and the noon Sun crosses the meridian to your S at a sextant elevation of 49d 15'.  You are therefore 90d - 49d 15' = 40d 45' degrees N of the Sun.  The Sun's declination at that moment (from the Almanac) is -15d 10' (the minus sign indicates degrees S of the Equator).  Your latitude is then 40d 45' -15d 10' = 25d 35' N. 

Let's do an example with the exact same sextant reading, but for a solar declination of +15d 10' instead.  You are still 40d 45' N of the sun, so your latitude is 40d 45' + 15d 10' = 55d 55' N.  The Sun can be N or S of the Equator, and you can be in either the N or S hemisphere, but you can sketch a little diagram for yourself to keep straight whether you have to add or subtract. 

Most navigators do a noon sun because its easy to identify in the sky, but you have to correct for index, refraction, semidiameter and dip (pages A2 and A3 of the Almanac).  For the Sun, Refraction and semidiameter corrections are combined in the tables on pages A2-A3, but you still need to correct for index and dip. Stars and planets are harder to identify, but they do not require the semidiameter correction.   Index correction is the error in your sextant, which you determine by shooting the horizon.  Dip is the correction for how high your sextant is above sea level.

The Moon can also be used, especially since it can be observed day or night,  but the corrections are a bit more complex and I won't go into them here.  Still it would be wise to read up on them in the Almanac.

Case study:  When I lived on the W Coast, I read about a solo yachtsman who was knocked down and dismasted in a Pacific storm.  He lost his rig and all his electronics but managed to rig a jury mast and bent a jib to it as a makeshift lateener.  He was able to determine latitude with noon sunlines and was able to sail within sight of the Golden Gate.  Those of you familiar with that coast know that except for San Francisco, there are few safe harbors there.  Its a lee shore, frequently shrouded by fog,  and you can get in a lot of trouble in a real hurry.

[Elk River]

Henry:

     Another article written very nicely and simply.  You make it sound so easy, I hope when I get around to practicing it, I can get it to make sense to me without too much undue frustration.  Like everything else, I guess, practice, practice, practice.

     Thanks again,

     Elk River

[HenryC]

Thanks, Elk River

That article is a simplified version of my chapter on the noon sun, "Celnav Seven - Noon Sun", which you can read here:

http://cpyoa.geekworkshosting.com/forum/index.php?topic=3643.0

The "nooner" is the basic celestial navigation exercise everyone should have accomplished successfully at least once before going offshore. Even if you never use it, its a real confidence builder.  Even if you're sitting at the chart table with water up to your waist and your rig is carried away, you can always tell yourself "If the sun comes out tomorrow, I can at least get a latitude."

With a latitude and a compass, you can at least point the boat in the right direction.

Its the bare minimum.