The Role of the Noon Sight

While John Harrison developed a viable marine chronometer in 1759, for the next hundred years or so a chronometer could cost as much as the rest of the ship combined. Consequently, the noon sight — which does not require an accurate clock to execute — remained hugely popular. Even into the 20th century, getting accurate time at sea was something of a challenge, particularly for small boat sailors.

Sextants were perfected around 1920 with the development of the drum-micrometer instrument. But there were other pieces that needed to come into play before the St. Hilaire method — published in 1885 — could reach its potential.

Between them, Sony, WWV, the post-war computing boom, and Seiko ushered in the modern era of celestial navigation. In a way that seems rather foreign to us today, this was a period when any vessel that sailed out of the sight of land had a sextant on board, and somebody who knew how to use it. And even the captain of a 27 foot yacht (if he hadn't yet bought his quartz wristwatch) could tune into WWV twice a day to calibrate his on-board timepiece. The 70's and 80's became something of a golden age for celestial navigation as the St. Hilaire method of sight reduction finally came into its own.

In 1972, there were 654 active ships in the U.S. Navy. This probably means the Navy had at least two and a half thousand active celestial navigators at any one time. So the editors of the 1977 edition of Bowditch had an enormous resevoir of talent to draw on — both trainers and do-ers — as it sought to present "best practices" in celestial navigation. Here is what they said about the noon sight (p. 560):

As accurate time became available at sea, and then more convenient tables and more accurate almanacs appeared, the noon sight lost its importance. Since the modern inspection table has been available, the use of meridian altitudes has decreased rapidly, and reduction to the meridian has all but disappeared.

The solution of a meridian altitude is simple and quick, but this is more than offset by:

The practice of observing a body when a line of position is desired, and solving those which happen to have a meridian angle of 0° or 180° in the same manner as other observations, is a growing practice that eliminates the need for remembering a separate procedure for bodies on the celestial meridian.

The modern navigator thinks primarily in terms of lines of position, rather than of latitude and longitude observations.

Frank Reed is the moderator of NavList — the web "home" of many people with a passion for celestial navigation. He indicates here (retrieved 2018 Feb. 15) that our understanding about what was considered "best practices" are correct.

...In the late 20th century, navigators were taught that Noon Sun sights were pointless.... there's nothing useful in a Noon Sun sight. It's just another sight, they were taught. They all yield lines of position. So ignore the noon sight!

Frank has a continuing passion for the noon sight. But the history of how celestial navigation was instructed in the 20th century is clear.

In 1995, the GPS system became fully operational...and the number of celestial navigators in the world immediately began to plummet.

In our own century, when celestial navigation is more a hobby than a necessity, noon and Polaris sights seem to have made a sort of comeback. Oddly enough, in the Sail Canada celestial exam, 29% of the exam's points are related to noon and Polaris sights.

Whether Sail Canada has tapped into the 21st century nostalgia for pre-modern celestial navigation, or whether they were simply passed by when the best practices of the modern era became current is difficult to say.

It is certainly true that if you have an accurate timepiece aboard, and if you are willing to spend 30 minutes before your estimated local apparent noon (LAN) taking sextant shots, and continue shooting for 30 minutes after LAN, you can plot your shots on graph paper and get a pretty accurate idea of when LAN occurred. From this, you can come up with a fairly good idea of your longitude, to go along with your latitude.

Latitude determination is easy. See footnote below for the unexpected complexity of getting a good longitude with this method.

In this course, however, I have chosen to adopt the primary celestial strategy that was used by working navigators from 1966-95. Part of the reason for using an older textbook — Budlong, written in 1978 — is that it comes from the same navigational milieu as Bowditch '77.

I can take 4 rapid sun shots — so that I can do sight-averaging — enter data in the worksheet and plot an LOP in 18 minutes: less than a third the time required for a noon shot. If for no other reason than to reduce the risk of seasickness by reducing the amount of time I spend staring through my sextant's telescope, I prefer the St. Hilaire method even if I am taking my shots right at LAN.

And besides, who wants to be taking yet more time for sextant sights when there are clouds, dolphins, and flying fish to be watched?

Noon and Polaris sights are terrific in an emergency setting, when you have no idea of UT. But for routine celestial navigation, using Pub. 249 to develop an LOP is the more powerful and versatile navigational tool.

John Karl has written an interesting book, Celestial Navigation in the GPS Age (2011), where he echoes the pragmatism of the Bowditch of 34 years before (p. 159):

The use of the special sights discussed in Chapter 7 (e.g. Polaris, noon) depends on the navigator's objective. Broader experience...and satisfaction can be gained from learning and practicing special sights.

But if the only interest is a reliable and simple backup (to the GPS) system, it might be reasonable to forget about special sights. Particularly as a backup system for the part-time navigator, the advantage of having good proficiency in one method might trump the advantages of these additional sights. These special sights have lost their importance from the days when accurate sight timing was they offer little advantage....

I think there has been an irrelevant historical carryover on the importance of determining latitude rather than a regular LOP. After all, a known latitude is just an east-west LOP. And if a sun shot is taken anywhere near local apparent noon, without regard to how near, and worked up as a regular St. Hilaire sight, the LOP will be nearly east-west anyhow. And that's the real consideration, the LOP crossing angle at noon relative to the morning and afternoon sun lines, used for running fixes.

Footnote:    Frank Reed has indicated that if you attempt to get longitude from a noon sight without compensating for the movement of the ship over the course of the hour you are taking sights — or particularly near the equinoxes, the movement of the sun — your longitude can be out by many miles. Antoine Couëtte unpacks an equation which could help you make these adjustments.

It is good fun to try and extract all the navigational possibilities you can from a pre-20th century navigational method like the noon sight. In a similar way, I have found it enormously gratifying when I was able to fix my position within 15 nm using a 9th century Arab kamal (or in other words, a stick and a piece of string) rather than my sextant.

But for the next three or four hundred hours you spend on celestial navigation, I suggest you concentrate on mastery of the St. Hilaire method. Once you achieve mastery and start to get bored, NavList is a great place to go to gratify your curiosity and further advance your skills. Here is a puzzle that was just posted yesterday:

Latitude by two stars observed simultaneously
From: Frank Reed
Date: 2017 Dec 21, 09:32 -0800

Can you figure out your latitude? Latitude is easy, right? In the photo below, we're looking out to a sea horizon in the pre-dawn hours in northern hemisphere winter. The star clouds of the Milky Way look familiar... There are no traditional navigational stars visible, but with a good star chart (e.g.  Stellarium), you can identify a few third magnitude stars. Measure the altitudes of any pair of them in the photo from the sea horizon, maybe using star-to-star angular separations to set an angular scale. Then look up the declinations and the difference in SHA between them (click on the stars in  Stellarium), and you can work out your latitude. There are historical methods for this that ignored time entirely and it's possible to re-derive a version of those with some clever  spherical trigonometry, but if that seems alien to you, then pick some arbitrary date, GMT and longitude and work it as a standard two-body sight using your favorite sight reduction technique.  Here's the challenge: can you find the photographer's latitude within five nautical miles?