Astro navigation: Is it a ‘chocolate teapot’ or still a useful skill to have?

Stuart Greenfield

Oct 27

12 min read

An idea of what it looked like - a fictional creation or something we still need to understand?

In brief

Astro navigation remains a core part of the RYA Yachtmaster Ocean certificate: a written paper on sights and sight reduction, an oral exam, and sight records from a passage out of sight of land.
We have more navigation technology than ever and fewer independent ways of fixing a position. Decca went in 2000, European Loran-C at the end of 2015. Almost everything now comes from satellites.
Jamming and spoofing are no longer theoretical: nearly 978,000 GPS jamming events were recorded worldwide in the first quarter of 2026, most of them in the Gulf, with the Black Sea second.
A sextant and an app on your phone is not a back-up plan. If the electronics have gone, so has the app, and very often so has your source of accurate time.
Two free downloads go with this piece: an on-deck checklist for sun and star sights, and a set of worked examples taken from one day at sea and plotted.

A solid deck in 1978

I learned astro navigation in 1978, aged eighteen, aboard HMS Hermes in the Mediterranean. It was the textbook version. A carrier makes a superb platform: a flat, steady deck high above the water, a horizon as sharp as a ruled line, a chartroom with every almanac and table the Admiralty printed, and chronometers that someone else had checked. We practised until it became routine. Morning stars, a sun run, noon latitude, evening stars. The fixes came out tight and nobody thought it remarkable.

Twelve months later I was navigating my father’s Sigma 33 Cabadah in the 1979 Fastnet.

As a Midshipman onboard HMS Hermes in 1978 - Astro Navigation was a twice daily routine.

A wet cockpit in 1979

On the night of 13 to 14 August we were about twenty miles north-west of the Scillies. The instruments were dead. We were down to bare poles, had been rolled, the cooker was out of its gimbals and the crew were below with the washboards in. Whatever the wind was doing, and my figure of 30 to 40 knots is an estimate rather than a reading, nobody was going to stand in that cockpit and bring a star down to a horizon that kept disappearing behind the next sea.

We ran off before the westerly towards the gap between the islands and Land’s End. The position that mattered came from a bearing on the Seven Stones light vessel when we raised her. One bearing, one known mark, and a navigator who knew roughly where he was before he looked.

That is the difference between learning astro and using it. On Hermes it was a discipline. On a 33-foot boat in a gale it was irrelevant.

In 1979, the Sigma 33 was barely a year old, a fresh design from David Thomas with a relatively broad stern, fractional rig, cast-iron fin keel and spade rudder. She was small but surprisingly fast. I raced our family-owned Cabadah in that year’s Fastnet.

A handful of sights in forty years

I have raced offshore since I was eleven. From nineteen onwards I carried a sextant on and off (in fact my Father’s orginal before buying a modern plastic version which was surprisingly good), and in the early years we had paper charts, the almanac and the sight reduction tables aboard. If we had needed a position, we could have produced one.

We rarely needed to. I can count on one hand the sights I took in anger across the whole of my racing. Most RORC racing happens within reach of the Channel coasts, and before GPS we had other tools. Dead reckoning, worked properly with a decent log, a compass you trusted and an honest allowance for leeway and tidal stream, was far better than people now assume. Radio direction finding was crude, but the marine radiobeacons gave you a line. The echo sounder gave you a contour. Put those together with a few bearings and you had a good fix.

The sextant stayed in its box for a simple reason. Taking a sun sight from a small boat that is being raced hard is difficult. Taking star sights at twilight from a boat under 40 feet, heeled and pitching, with the horizon coming and going, is harder still. When there were quicker and nearly as good alternatives, nobody chose the sextant.

More technology, fewer answers

Here is the irony. Today we carry more navigation technology than any yacht of my youth, but we have, perhaps, fewer independent ways of knowing where we are.

The ground-based systems have gone. Irish Lights shut its marine radiobeacons on or around 1 February 1999. The General Lighthouse Authorities stopped providing the public Decca Navigator service on 31 March 2000. The last European Loran-C transmitters in Norway, France and Germany were switched off on 31 December 2015, and the UK’s eLoran service ended the same day. Even the lighthouse authorities’ DGPS correction service was discontinued from 31 March 2022.

What replaced them all comes from space. GPS is American; Galileo is European; Russia runs GLONASS and China BeiDou. A modern receiver may use several at once, which makes it more accurate and slightly more robust. It does not change the fact that every one of them is a weak signal from a satellite.

GPS Jamming

That is now happening at scale. According to figures from the maritime analytics firm Windward, reported by Cyprus Shipping News in June 2026, nearly 978,000 GPS jamming events were recorded worldwide in the first quarter of 2026. About 98 per cent were in the Gulf: the Strait of Hormuz, the Persian Gulf and the Gulf of Oman, where more than 1,100 vessels were directly affected. The Black Sea came second with 7,630 cases. Finland, Estonia and Lithuania have publicly blamed Russian sources for jamming across the eastern Baltic. In March 2026 ships in the Gulf reported positions that put them on Iranian soil.

Jamming you can usually see: the position freezes or the receiver reports no fix. Spoofing is worse. The plotter carries on showing a confident, plausible and wrong position. If you are sailing in the Black Sea, the Gulf or the eastern Baltic, the honest answer is that you cannot rely on GNSS alone.

Governments know this. In May 2026 the Ministry of Defence awarded a £6 million contract to a QinetiQ-led team to develop deployable eLoran, with a working system due by April 2028. That is for the armed forces, not for us. Warships have inertial navigation and other back-ups that no yacht will ever carry.

You may have read that Starlink could take over from GPS. Not yet. Researchers at Ohio State University led by Zak Kassas have shown that the signals from Starlink and OneWeb satellites can be used to estimate position, and in a trial off Greenland cut a ship’s navigation error without GPS from more than a kilometre to about 27 metres. It is impressive research. It is not a navigation service you can switch on, yet!

The honest counterweight

The fear of a cascade of satellite collisions taking out navigation altogether deserves some perspective. The crowded orbits, where Starlink and most of the debris live, are a few hundred to a couple of thousand kilometres up. The navigation satellites sit at around 20,000 kilometres, roughly ten times higher than the top of that band. A debris cascade low down would be a serious problem for many things; it is not the most likely way to lose GPS. Deliberate interference on the ground is far more probable, and it is already here.

Still on the RYA syllabus

Which is why astro has never left the RYA Yachtmaster Ocean scheme, and rightly so.

The certificate has three parts. There is a written examination covering sights, sight reduction and worldwide meteorology. There is an oral exam, for which you must send the examiner a narrative of your qualifying passage and your navigational records from a passage out of sight of land. The RYA’s wording on what those records must show is precise: navigation carried out “without the use of electronic navigational aids”. They must include at least the planning, reduction and plotting of a sun-run-meridian-altitude sight, and a compass check using the bearing of a heavenly body. The qualifying passage itself must be at least 600 miles and 96 hours, with at least 200 miles more than 50 miles from land.

The sensible route is the shorebased Ocean course. If you sit its final exam under invigilation at an RYA training centre, you are exempt from the written paper. You still sit the oral and you still present sights you have taken at sea, but the examiner already knows you can do the arithmetic. What remains is to show you can do it for real.

The app myth

A great many sailors believe the modern answer is a sextant and an app. Take the sight, type in the altitude and the time, and out comes a position.

Think about when you would need that. You reach for the sextant because the GPS is unavailable or untrustworthy. In most of those situations something larger has gone wrong: a lightning strike, a flooded nav station, a dead battery bank, a dismasting that took the antennas and the wiring with it. The phone may survive. It may not. The tablet with the navigation software almost certainly shares the boat’s charging system.

If the app is your only way of reducing a sight, your back-up depends on the same electricity as the thing it is backing up. A sextant without the Nautical Almanac and a set of sight reduction tables aboard is an expensive ornament.

The hidden dependency is time

Even navigators who work everything by hand often take their time from GPS. That matters more than any other number in the calculation. Every four seconds of error in your watch is a minute of longitude, and at the equator a minute of longitude is a mile.

Before satellites, a deck watch was checked daily against radio time signals and its rate written down, so that the navigator knew not just its error but how fast that error was growing. Very few yachts do that now. Ask yourself where your UT comes from if every screen aboard goes dark.

The skippers in the 2026 Golden Globe Race live with this. Sixteen of them left Les Sables-d’Olonne on 6 September to sail alone around the world in boats and with equipment that would have been familiar in 1968. As Yacht magazine summarised the rules, sextants, compasses and paper charts are allowed; GPS, computers and electronic charts are not. When Damien Guillou led the fleet through the Lanzarote Gate in his Rustler 36 Solarem, he did it the way I learned in 1978, without the steel deck.

Is it practical on a race boat?

Maybe, with honest expectations.

From a small boat at sea, a good star fix in reasonable conditions will usually land within one to three miles. In a seaway, treat anything under five as sound. A single sun sight gives you a line, not a fix; you need the run to the noon sight or a second line later in the day to cross it. The usable window at twilight is short, often twenty to thirty minutes, and you will get three or four stars if you are organised and none if you are not.

Three things make the difference.

Plan the sights. Work out twilight times and preset the sextant before you go on deck. AP 3270 Volume 1 gives you the altitude and bearing of the best stars for your latitude and time, so you can set the sextant and look in the right direction.

Shoot a series, not a sight. Take five to seven sights over two or three minutes, plot them against time and draw a line at the rate the altitude should be changing. Throw out the one taken on a wave top. One good sight on the line is worth more than an average of six that includes a bad one.

Use a form. There are a great many small numbers in a sight reduction, and most errors are clerical: the wrong column, the wrong date at UT, a sign reversed. A pre-printed form catches them. Fill it in the same way every time.

Free downloads

I have put together two companion PDFs, both A5 so they fit a chart table or a plastic sleeve.

The on-deck checklist covers sextant checks and index error, taking sun and star sights, averaging a series, a sun sight by NP 401, noon and Polaris latitude, a three-star fix by AP 3270 Volume 1, the commonest errors and what they look like on the chart, and blank forms for every method.

The worked examples follow one yacht through 14 May 2027 on passage towards the Azores: a morning sun series averaged, the sun line reduced and plotted, a noon latitude and running fix, a three-star fix with precession and nutation, and Polaris. Every example ends on a plotting sheet, and each fix is compared with GPS.

What to carry

The kit is short and every item matters.

The Nautical Almanac (NP 314) for the current year. The 2027 edition is already on sale; buy it now if any passage runs into January.
Sight reduction tables. NP 401 (the same as the American HO 229) in the volumes for your latitudes, or AP 3270 Volumes 2 and 3 for speed. AP 3270 Volume 1 for stars, in the current Epoch 2025 edition.
A sextant, serviced, and a plastic spare.
A watch whose error and daily rate you know and log.
Plotting sheets, a plotter or parallel rule, dividers and soft pencils.
Pre-printed forms for each kind of sight, and a waterproof notebook for the raw readings.
A red torch.

My father’s sextant

Commodore G G Greenfield’s sextant and still used by me, although I have light weight plastic alternative as this is designed for use on the deck of a ship

Purhased in 1947 and very accurate as shown on the certificate

The sextant I still keep belonged to my father, Gordon Greenfield. He skippered Volunteer in my first offshore race in 1971 and owned Cabadah, the Sigma 33 I navigated through the 1979 Fastnet. He was also a professional seaman and a navigator by training: a Fellow of the Royal Institute of Navigation, and from 1979 to 1982 Commodore of the Royal Naval Reserve, the most senior reserve officer of his day, based at HMS Wildfire.

With the sextant came his copy of Norie’s Nautical Tables. John William Norie first published his navigation tables in 1803, and they have been in print ever since; Imray Laurie Norie & Wilson still publish them. The Australian National Maritime Museum puts it plainly: “Norie’s tables were an essential item for ship masters in the mid-19th century.” They were still essential on a merchant ship’s bridge in my father’s day.

Everything he needed to cross an ocean fitted in that box and that book. It still does.

The apps, as back-up

Apps are excellent for checking hand-worked reductions and for planning, as long as you treat them as a check and not as the method. A few that are well regarded, as examples rather than endorsements:

StarPilot (Starpath, iOS, around US$60) reduces sights of every body and has a Sight Analyzer that fits a series of sights to the correct slope, which is the averaging method in the checklist, done for you.
Celestial Nav (Skytip, iOS) reduces sun sights free, with corrections for index error, height of eye, temperature and pressure; the paid Pro version adds stars and plots your lines.
Celestial Navigation by Dr Harald Merkel (iOS, a couple of pounds) shows every intermediate figure, which makes it ideal for checking your own working line by line.
Navigator by Omar Reis (Windows) carries a perpetual almanac and will print almanac pages, which is a useful insurance policy if your printed almanac goes over the side or gets soaked.

For learning, the RYA Astro Navigation Handbook is the course book for Ocean theory. Mary Blewitt’s Celestial Navigation for Yachtsmen, now in its 13th edition revised by Andy Du Port, remains the clearest short book on the subject; Blewitt raced Fastnets herself and was Secretary of the RORC.

Back to basics

Very few yacht navigators today could fix their position with no electronics at all: no app, no laptop, no GPS time. Fewer still carry everything they would need to try.

That is not an argument for taking sights on every passage. It is an argument for having the books aboard, a watch you trust, a form you know, and a set of sights you have taken in calm water so that the first time is not the time it matters. On Hermes astro was the routine. On Cabadah it was very nearly impossible. Somewhere between the two is the navigator who can still find the boat when the plotter is lying.

Stuart Greenfield FirstShift — Yacht Racing Intelligence From first shift to first place.

Offshore racing is never just about the boat, the forecast, or the routing software.

It is about decisions made under pressure, in changing conditions, by tired people trying to keep the boat moving fast and safely in the right direction.

That is where races are won and lost.

At FirstShift, my aim is simple: to help owners, skippers and crews race offshore with more clarity, more confidence, and better preparation whether that is a RORC Channel race, a Fastnet campaign, or your first serious step into offshore racing.