Celestial Navigation in a Nutshell

Rating:
Summary: flawed introduction
Review: A reasonable introduction to Celestial Navigation, with some big problems. Schlereth clearly has done some CN himself, but his grasp of why it works is shakey at best.

As has been pointed out below by Nathaniel Meyers (why is his detailed correction so poorly rated?), there is a huge conceptual gaffe in the early parts of the book. Renee (below) points out that this gaffe is corrected later. Unfortunately, Schlereth makes precisely the same mistake (saying that the Sun's apparent position changes because of the *Sun's* parallax) near the end of the book (on page 108.)

I should emphasise that this is *not* a minor mistake, nor is it a technical issue that only the anal would get in a froth about. It is not an "approximation", nor is it a learning aid. It is a major misunderstanding, repeated twice in the book with no explicit correction, and it is a misunderstanding that confuses the very different nature of CN by Sun versus CN by moon.

I'm not sure if it's because Schlereth has no idea what's going on (and has simply been navigating by rote), but I thus hestiate to recommend this book to anybody. Without Meyer's review below, I would have been very confused, and, like him, I am left in the dark about whether there are additional errors to be found (like the San Salvador problem below.)

An additional problem is the hastily drawn diagrams -- they're the kind of diagrams where you can tell they were drawn on a Mac Classic with QuickPaint. Angles are drawn weirdly, great circles look funny, and, in general, the diagrams are sometimes sufficiently badly done that, unless you already understand the concept, you'll often end up *more* confused that before.

People learn Celestial Navigation because they want to learn about how to navigate by the regular motions of the solar system, not because they want to blindly fill out algorithmic tables while having a fake and incorrect picture of why it works. Schlereth has done us a disservice with this book.

Rating:
Summary: Examples clearly explained
Review: Hewitt Schlereth wrote this book to explain the use of the HO249 tables to fix one's position on the earth using the known positions of the sun, moon, planets, and distant stars. I would also recommend reading this book if you intend to use another method of sight reduction, such as George G. Bennett's "The Complete On-Board Celestial Navigator" (ISBN: 0071396578). Schlereth gives a good introduction to the celestial bodies used for navigation and how to use them.

Schlereth's examples show the relevant tables, highlighting the values used. He makes it clear how to enter the tables correctly every time, and gives a few tips on how to avoid transcription errors when "taking out" the numbers you need.

He begins the book with a little trigonometric fiction -- that we are interested in the angles between the sun, the "position" of the sun on the surface of the earth, and the observer. Of course, this is a bit of nonsense, as another reviewer, Nathaniel Meyers, has pointed out. Schlereth corrects himself in a later chapter, and I think he could have better begun with the story of how the famous Greek philosopher calculated the circumference of the earth thousands of years ago.

The eager student should work through the examples thouroughly. Be on the lookout for the error in transcribing one of the sights in the example of the approach to San Salvador -- the first fully-plotted example.

This book is a good introduction to celestial navigation -- a useful place to start before going on to more advanced sources of information.

Rating:
Summary: Examples clearly explained
Review: Hewitt Schlereth wrote this book to explain the use of the HO249 tables to fix one's position on the earth using the known positions of the sun, moon, planets, and distant stars. I would also recommend reading this book if you intend to use another method of sight reduction, such as George G. Bennett's "The Complete On-Board Celestial Navigator" (ISBN: 0071396578). Schlereth gives a good introduction to the celestial bodies used for navigation and how to use them.

Schlereth's examples show the relevant tables, highlighting the values used. He makes it clear how to enter the tables correctly every time, and gives a few tips on how to avoid transcription errors when "taking out" the numbers you need.

He begins the book with a little trigonometric fiction -- that we are interested in the angles between the sun, the "position" of the sun on the surface of the earth, and the observer. Of course, this is a bit of nonsense, as another reviewer, Nathaniel Meyers, has pointed out. Schlereth corrects himself in a later chapter, and I think he could have better begun with the story of how the famous Greek philosopher calculated the circumference of the earth thousands of years ago.

The eager student should work through the examples thouroughly. Be on the lookout for the error in transcribing one of the sights in the example of the approach to San Salvador -- the first fully-plotted example.

This book is a good introduction to celestial navigation -- a useful place to start before going on to more advanced sources of information.

Rating:
Summary: Celestial Navigation in a Nutshell
Review: I found Celestial Navigation in a Nutshell easy to follow and the diagrams made a distinct contribution to my understanding. I also liked Schlereth's informal, personal style. It too made this scary subject more readily available to my teensy mathematical mind.

I found his use of the plotting sheet a unique and helpful addition. I never came across this gimmick before. I was also grateful for the large easy-to-read type.

Rating:
Summary: Easy to understand, easy to use!
Review: I'm a student pilot (hopefully for not much longer!) and navigation is very interesting to me, not to talk of very important as well. I've heard of celestial navigation as being the ultimate form of navigation and I was interested in finding out more about it by reading various books but each time I did, I had to give up because I did not have the necessary PhD in astrophysics to understand what the author was trying to say.

So, it was with a little trepidation that I approached reading this book by Schlereth and to my pleasant surprise, celestial navigation makes sense! His use of simplified theory may not be to some people's taste but at the end of the day, I'm not studying for an exam, I'm trying to understand what celestial navigation is and how to use it. This book teaches you how exactly to use it and that's all that matters to any good author and should to any good reader. I can now go outdoors with a sextant and the necessary tables and find out where on earth I am, correct theory or no!

Rating:
Summary: Basic celestial navigation
Review: I've been sailing the coast of Portugal for a few years now, and have always been curious about celestial navigation. I dicided to buy this book because all experienced sailors i've met always talk about this kind of navigation. I read this book and got the hole picture. It's really very well written for biginners. If you are thinking about starting CN this is the right book for you.

Rating:
Summary: But his theory was outright wrong.
Review: In Chapter 3 page 9, as far as I've gotten, for example he explains that if you take the angle measure of the sun (or any object) and subtract from 90 degrees you get the angle measurement of your distance from the point of the earth directly under the sun (the GP). This is very true and is the fundimental of celestial navigation. But his explanation why is patent nonsense and absolutely wrong. He says one can make a triangle between the sun, the GP of the sun, and the observer. This is wrong. The sun is of such a great distance from the earth that for the intents and purposes of celestial navigation the sun is infinitely far away and all rays (and views of the sun) are parrellel to each other. So rather than a triangle, you'd get two parellel lines connected by a curved arc (the surface of the earth) with the line at the GP going straight up (perpendicular) to the curve and the line at the observe going up at the measured angle but parellel to the other line. Then he says, referring to his triangle that as all triangle angle add up to 180 degrees and the GP angle is 90 degrees (it goes straight up) so the Apex (angle at the sun) is 90 - measured angle. Angles of a triangle add up to 180 only if the sides are straight lines. If the sides are allowed to curve the angles can be anything. In fact if (as his triangle does) two sides are straight and the third curves away the sum of the angles are always less than 180. Then he concludes that the angle of the sun is equal to the angle curve of the earth because that is the side opposite it. He calls this "knowledge of a little trig". I call it out right nonsense without knowing the height of the triangle (in this case the height of the sun) you can't conlude any relationship between the two. In fact if you did draw a big enough triangle to reach the sun the angle at the sun would be miniscule, extremely close to zero, and nothing at all close to 90 - measure of the sun on the surface.
The correct explanation would be to draw the two parrellel lines, Draw the angle at the center of the earth, draw a perpendicular (straight across) line crossing the two parrellel lines. THe result is a quadrangle. Quadrangle angles add to 360. The two angles by the crossing line are 90 each and don't mean anything but are just used to demonstrate that the remaining to angles, the center of the earth (which is equal to the angle measurement of your distance to the GP) and the angle from the center of the earth to you to the sun add up to 180 degrees. Since the angle from the center of the earth to you to the sun is the sum of the angle from the center of the earth to the horizon (90) and the angle from the horizon to the sun (the measured angle, then the measured angle and the angle measurement of your distance to the GP add up to 90. That's not a "little trig" but "a lot of forgotten geometry and hand waving" but at least it is true rather than utterly false.

So, if I can't trust the writer in the theory, I can't recomend the book even if all of his celestial navigation is correct.

Rating:
Summary: But his theory was outright wrong.
Review: In Chapter 3 page 9, as far as I've gotten, for example he explains that if you take the angle measure of the sun (or any object) and subtract from 90 degrees you get the angle measurement of your distance from the point of the earth directly under the sun (the GP). This is very true and is the fundimental of celestial navigation. But his explanation why is patent nonsense and absolutely wrong. He says one can make a triangle between the sun, the GP of the sun, and the observer. This is wrong. The sun is of such a great distance from the earth that for the intents and purposes of celestial navigation the sun is infinitely far away and all rays (and views of the sun) are parrellel to each other. So rather than a triangle, you'd get two parellel lines connected by a curved arc (the surface of the earth) with the line at the GP going straight up (perpendicular) to the curve and the line at the observe going up at the measured angle but parellel to the other line. Then he says, referring to his triangle that as all triangle angle add up to 180 degrees and the GP angle is 90 degrees (it goes straight up) so the Apex (angle at the sun) is 90 - measured angle. Angles of a triangle add up to 180 only if the sides are straight lines. If the sides are allowed to curve the angles can be anything. In fact if (as his triangle does) two sides are straight and the third curves away the sum of the angles are always less than 180. Then he concludes that the angle of the sun is equal to the angle curve of the earth because that is the side opposite it. He calls this "knowledge of a little trig". I call it out right nonsense without knowing the height of the triangle (in this case the height of the sun) you can't conlude any relationship between the two. In fact if you did draw a big enough triangle to reach the sun the angle at the sun would be miniscule, extremely close to zero, and nothing at all close to 90 - measure of the sun on the surface.
The correct explanation would be to draw the two parrellel lines, Draw the angle at the center of the earth, draw a perpendicular (straight across) line crossing the two parrellel lines. THe result is a quadrangle. Quadrangle angles add to 360. The two angles by the crossing line are 90 each and don't mean anything but are just used to demonstrate that the remaining to angles, the center of the earth (which is equal to the angle measurement of your distance to the GP) and the angle from the center of the earth to you to the sun add up to 180 degrees. Since the angle from the center of the earth to you to the sun is the sum of the angle from the center of the earth to the horizon (90) and the angle from the horizon to the sun (the measured angle, then the measured angle and the angle measurement of your distance to the GP add up to 90. That's not a "little trig" but "a lot of forgotten geometry and hand waving" but at least it is true rather than utterly false.

So, if I can't trust the writer in the theory, I can't recomend the book even if all of his celestial navigation is correct.