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I was browsing through a Wikipedia article about the trigonometric identities, when I came across something that caught my attention, namely forgotten trigonometric functions.

The versine (arguably the most basic of the functions), coversine, haversine and exsecant formulas had once been utilised for navigational purposes, prior to GPS tracking systems. However, recently, they have become less common in modern mathematics and beyond. Why is that?

Here is a link to a PDF file describing all of these now-obsolete trig functions:

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Those functions are much less used than before for one reason: the advent of electronic computers.

Before that, one had to rely either on tables or on slide rules. Tables were usually table of logarithms, and they included the logarithms of trigonometric functions as well. The trigonometric functions were then useful not only for geometric applications, but also to simplify algebraic calculations with logarithms.

For instance, to compute $\log\sqrt{a^2+b^2}$ when $\log a$ and $\log b$ are known, you could find $\theta$ such that $\log\tan\theta=\log\frac ba=\log b-\log a$, then $\log\sqrt{a^2+b^2}=\log a+\log\sqrt{1+\tan^2\theta}$ and $\log\sqrt{1+\tan^2\theta}=\log\frac{1}{\cos \theta}=-\log\cos\theta$. There are many similar formulas.

For geometric applications, sometimes versine and similar functions allow computing with greater precision while not adding too much computation. See for instance the haversine formula used to compute great circle distance (useful in navigation). The straightforward formula with arccosine has poor accuracy when the angle is small (the most common case), due to the fact that cosine is flat at $0$. However the formula with haversine is more accurate. To achieve the same, you would have to use $\sin^2(\theta/2)$ everywhere, which require more computations (but it's still reasonable with logarithms). Therefore, navigation table like Nories's nautical tables have an haversine table.

Note that tables of logarithms are usually accurate up to 5 digits (some larger tables had 7 digits, some very special ones had better precision but were difficult to use : more digits = much more space on paper). Slide rules have roughly 3 digits of precision.

All of this is rendered pretty useless with calculators, which have usually around 15 digits of precision and compute fast enough that we don't have to worry about speeding things up with extra functions.

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    $\begingroup$ The log tables we used at school were four figure. I repeatedly got marked down (indirectly) for using five figure tables: I gave my answers to four figure precision, rather than the three I should have done if I were using four figure tables $\endgroup$ – JCRM Apr 17 at 12:28
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    $\begingroup$ Oddly enough, I bought a log book 5 years back. I have no idea what my 11 year old self was thinking, but it was pretty cool. $\endgroup$ – Mohammad Zuhair Khan Apr 17 at 13:49
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    $\begingroup$ @Keith No evidence, but I have a hunch that the additional functions, as well as the tables, were never meant for higher mathematics, but were intended for practical applications in which one does have to evaluate expressions. $\endgroup$ – David K Apr 18 at 0:51
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    $\begingroup$ Completely unrelated, but physical log tables have actually lead to at least one important discovery in mathematics: Benford's Law. As someone who used to work as an archaeologist, I sometimes morn the loss of our physical implements. :\ $\endgroup$ – Xander Henderson Apr 18 at 3:32
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    $\begingroup$ @MohammadZuhairKhan: When you want to challenge the "no calculators" rule ... $\endgroup$ – Joshua Apr 18 at 3:47
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Historically, those oddball functions were used primarily in navigation to reduce sextant readings and times to latitude and longitude. With the advent of radio, the need for that was greatly reduced, as we have radio direction finding, loran, and, more recently, GPS.

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