Classes in celestial navigation and related topics

A nice old vernier sextant

GPS Anti Spoof App...

Get my new app. Defeats GPS Spoofing and great for sextant-training, too.

GPS Anti Spoof app screenshot

Instant feedback with no paperwork at all. There's a standard version (free on Android, inexpensive on iPhone/iPad) that works with Sun, Venus, and Polaris sights and can function for three to seven days without network access. If you're serious about your navigation, then get the PRO version which includes the Sun, Moon, Venus, Mars, Jupiter, Saturn, Polaris and all 57 standard "navigational stars" and works offshore and off-network for months at a time.

What does it do?
The app calculates a live sextant altitude, exactly as it should appear on the instrument, from the current GPS position. The app has two basic functions.
1: TRUST THE SKY: If you're a reasonably skilled celestial navigator, and your position is mission-critical, then the app is a sensitive, accurate test of the GPS position. If your position is being spoofed by criminals, pirates, military or government intelligence forces, you can detect that with an easy, quick sight of your favorite celestial body. An observed celestial altitude will not match the app's calculated altitude if you're being spoofed. You can detect GPS Spoofing at sea using this app within normal limits.
2: TRUST THE GPS: Turn it around. If you're learning or improving your sextant skills in circumstances where your position is not yet mission-critical, and you have a reasonable expectation that the GPS position is perfect as usual, then the app is a sight-trainer, an instant check on your instrument and your skill using it. Shoot the Sun and compare. No paperwork at all. When learning, this also cleanly separates the two sides of the process of learning celestial navigation: the skill of sextant sights on one side, and clearing and plotting sights on the other. You can quickly ramp up your skills using a sextant with the instant feedback from this app. Save the math for another day.

Some frequently asked questions:
What is T adjust?
The app uses your device's built-in system clock. It periodically checks that clock against the time derived by the GPS/GNSS chip. These will be very close in modern devices on most networks, but differences up to a few seconds are not unusual, and differences may grow substantially when off-network The displayed "UTC" at the top of the app's display running off the system clock adjusted by that amount "T adjust". That is, the UTC is the correct time.
Why is there a delay of 10 seconds?
If you have an assistant to press pause on the app, set the delay to zero and use a "ready... now!" call-out. If you're working alone, call out (aloud or in your head) "zero" and then start counting seconds until you hit 10. Then at that instant, tap pause. The time will be displayed with the notation "-10s". If you're recording sights on paper, you can do that arithmetic easily. The app of course does it automatically. The displayed altitude is correct for the moment when you called out "zero" at the start of your ten-second count.
What is Net Wx?
Wx is a common shorthand for weather (dating from early telegraphy). If you have internet access, and "Net Wx" is ticked, the app will periodically check online for weather data. Otherwise you can manually enter temperature and pressure.
What is SL pressure?
Barometric pressures inland on high mountains, e.g., are normally reported as an equivalent sea level pressure, backing out the decrease in pressure due "purely" to the thinner atmosphere at altitude. It is also possible to measure pressure directly, in which case SL pressure should be de-selected. Usually this should be left selected.
What is DOV?
This is the gravitational deflection of the vertical which is caused by gravitational anomalies, like the attraction from large volcanic peaks, and can amount to one or even two minutes of arc in geologically "young" regions. The app includes a global database of DOV values. In some areas, e.g. the US northeast and nearby coastal waters, the deflection is very nearly zero. When a signifiicant gravitational deflection exists, it is also displayed on the main page of the app. You may see this pop up when sailing in the Caribbean, for example. Normally this should be lefft selected.
Does the app work at sea?
Absolutely. You do not need internet access.The app optionally uses network access for weather data, but otherwise it receives GPS/GNSS signals off-network in the middle of the ocean. One note: when your device is off-network a position fix from GPS may take two to three minutes. Be patient. This is normal. The receiver in your device when it is on-network benefits by downloading initialization data from nearby ground stations which allows nearly instant fixes.
Will it work without built-in GPS?
If you're only using the device for sight training, you may find that you get good results without built-in GPS. The device has alternate means of determining an approximate position fix, for example, triangulating off mapped WiFi networks, and may well provide a position fix accurate enough for the app. Take note of the implied accuracy of the fix as displayed. Note for iPad users: Apple long ago decided that iPads should not normally have GPS chips. Why? We can only speculate. If you want to use an iPad for this app or any other navigation apps that access a GPS/GNSS position, you will need a "cellular-capable" model of iPad (you do not need to purchase service).
Can it detect all GPS Spoofing?
No. This celestial process can only detect spoofing greater than about half a nautical mile in scale, and of course it will not detect spoofing in a direction perpendicular to the observed celestial body. If the Sun is due east of you, and your position is being spoofed ten miles directly to the south, the actual and spoofed positions lie nearly along the same celestial line of position for the Sun. The altitude is unchanged. If you can see two celestial bodies, then you can detect spoofing in any direction. Celestial sights also cannot detect spoofing when skies are overcast (!). Celestial altitudes also have low reliability in the middle of the night.
How often should I check?
If you have any reason to suspect GPS Spoofing, take a sight of the Sun or other celestial body once every 15 minutes. The sight itself and comparison with the app should take 30-60 seconds at most. Look at the app before picking up the sextant to preset to the expected altitude. Sights are quick and easy.
When should I worry?
You should always worry. However, if differences between observations and displayed altitudes in the app are smaller than half a mile to a mile and vary randomly from one observation to the next, then you're probably seeing nothing more than observational "noise". If you see differences above a mile, increasing as time passes, then you should strongly consider the possibility that your GPS position is, in fact, wrong. Your actual position is towards the celestial body's azimuth if the observed altitude is higher than the displayed altitude, bearing in mind that any single celestial sight determines a line of position (perpendicular to the Sun's azimuth) and not a single-point fix.


Doug MacPherson wrote: 5/16/2020
I recently took online versions of Frank Reed's "Celestial Navigation in the Age of Sail", and "Lunars - Finding Longitude by Lunar Distances". I couldn't have been more happy with them. Having originally learned post WWII celestial methods as an officer in the United States Navy, and taken it up as a hobby, I was quite familiar with that era's procedures. However, I was intrigued by how they managed prior to then. Frank's two classes filled that void. His vast knowledge of the subject, both the technical aspects of the work as well as the historical significance were perfectly balanced. These are classes that can be thoroughly enjoyed by both the novice as well as the well versed practitioner. Recipe's for doing the work, the science behind those recipes, and actual voyages by the sailors that practiced the art were all presented with wonderful clarity. If "time sights", "cleared lunar distances" or "apparent time" have ever roused an interest, you owe it to yourself to take one of Frank's classes.

Doug MacPherson
Lieutenant, USN sep.
Samuel S Lyness wrote: 4/29/2019
Frank, a wonderful course in Lunars. I learned a lot. I admire your teaching skills and your astounding fund of knowledge. I would wish to emulate your style of instruction. Best regards, hope to sign up for your course in Cel. Nav. in Age of Sailing.
Sam Lyness
Captain Richard D. Buchanan wrote: 11/12/2019
I have taken Frank's Modern Celestial Navigation class twice. I am always inspired and I always come away with a few practical techniques and more than a few insights into the history and beauty of celestial navigation. You owe it to yourself to enjoy this class, whether or not you are a mariner.
John Workman wrote: 11/20/2018
I just took one of Franks classes and it was awesome!!

Frank taught an incredible class on celestial navigation that brought me from novice to some solid understanding of sextants, their history and most importantly their use as a aid to seeing the sea...and knowing where you are on this planet!

Hands on, wealth of knowledge, great resources at Mystic Seaport, he really covered a lot of ground! There was a lot of math but unlike in my youth, I was on the edge of my seat to soak up knowledge!! Frank made it relatable and real. The sextant which is such an iconic tool of the sea, was demystified. By the end of class i felt comfortable with it. I had mastered how it worked, how to read it and how to adjust it to insure its accuracy.

I came away with all of the cheat sheets and understandings of equations and concepts that breathe the life into what you capture through your sextant sightings.

I would highly recommend Frank and believe the Mystic Seaport with its planetarium, an ideal setting for my class with him discovering this timeless tool of the sea.

Frank did a great job keeping the class interesting with visual aids, both on screen and out on the seaport grounds. Frank had also noticed i was interested in the Draken. This is the Viking ship which had made its voyage across the Atlantic and up and down the east coast, resting for winter in Mystic as the troops regroup, gathering resources for another ocean voyage. He took extra time to talk about and show with polarized film the concept of the "Viking Sun Stone" which is a suspected navigational aide the Vikings may have used to traverse the globe as they had.

All in all i would highly recommend this class to any and all folks interested in learning about navigation and sextants. Informative and digestible, but most of all useful to the point where i am comfortable with the instrument and have the formulas needed to continuing to set my sights on the horizon!!

I look forward to more classes to learn more from Frank and strengthen my understandings of celestial navigation!

Thank you!!!
Mark Coady wrote: 6/6/2017
I have now done every course I think that has been offered so far at Mystic Seaport taught by Frank Reed in the last two years. I found the courses to all be extremely rewarding.

Several things stand out. The course material is presented in a balanced way, with a well thought mixture of detailed calculation, broken up by historical, factual, and hands-on aspects. This type of teaching is well suited to most, as it provides periods of more intense reasoning with relaxation and humor. Anyone can walk away with new-found knowledge. I also feel that the approach of understanding historical context and a simple practical approach is unique. It has gone a great way toward clearing up a lot of my preconceived ideas and confusions resulting from the many contradictory or esoteric approaches found in various volumes or on the internet.

Very simply, I learned a lot and it went a long way toward clearing up a mess. I was fascinated the whole time. The courses and NavList provide the tools to keep learning even after the course is over. I left able to measure what I see with a more calibrated eye for real world application, and a greater appreciation of human history. I can strongly recommend these classes for the curious, the fascinated, the historian, the hardcore navigator, or the armchair one. There is something in them for all.

I also found the NavList community to be helpful and encouraging as my journey continues. I hope I can undertake even more material in additional courses in the future.

"There is nothing - absolutely nothing - half so much worth doing as simply messing about in boats" (Kenneth Grahame, from the "Wind in the Willows")

Capt. Mark

Dr. Russell D. Sampson wrote: 6/22/2017
I took Frank's 19th Century Celestial Navigation class in April 2013 and really enjoyed it. Not only was the class interesting but my fellow classmates were too; a retired skipper of a ballistic missile sub, the son of the fellow who invented GPS, a teacher, a captain of a Panamax container ship and a fellow who crossed the Atlantic solo - twice!

The class was also a great resource for my teaching and my own research interests such as the visibility of celestial objects in the daytime (Jupiter and Venus) and the effects of astronomical refraction near the horizon. I hope to take more workshops with Frank.

Dr. Russell D. Sampson
Wickware Planetarium
Eastern Connecticut State University
Philip M. Sadler wrote: 6/22/2017
What a joyful and stimulating experience to enroll in Frank Reed's class, Celestial Navigation: 19th Century Methods. Frank is a skillful and engaging teacher, able to draw students into this fascinating subject, whether they be novice or experienced. His depth of knowledge is tremendous. Participants get a real taste of what it was like to be aboard a sailing ship of the day. I learned much to enliven my own teaching and decode 19th century ship's logs. It is a rare experience, indeed, to have so much thoughtfulness, enthusiasm, and fun packed into two days. This is the way to learn!

Philip M. Sadler, Ed.D.
F.W. Wright Senior Lecturer in Celestial Navigation
Harvard University Astronomy Department
Cambridge, MA

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