NavList:

Steve G, you mentioned that you have recently purchased "Astronomical Algorithms" by Jean Meeus. You'll find that many members of the NavList community own a copy. We're sort of a club... Would you like a t-shirt or a cap? ;)

There's lots of intriguing material in that book, and you may also want to find a copy of his earlier little book "Astronomical Formulae for Calculators" which dates back to the 1980s (pdf of an early edition). And by the way, in the title here, does "calculators" refer to handheld electronic devices or does it refer to humans as "calculators" --meaning people who enjoy working their own astronomical computations? I can see it both ways, but the latter seems to make more sense here. :)

In "Astronomical Algorithms" (pdf at archive.org, too), you'll find many pages filled with numbers that are intended to be incorporated into code in software/apps. Naturally, since this book and the other "...for Calculators" have been around a while, this has been done many times over. Count on it! So before your start trying to key it all in, you'll want to go looking for pre-constructed app components or code snippets.

There is a tool on the S&T website that calculates Galilean satellite events here. Nice, right? It's a simple web app, created 20 years ago, using javascript to make it go, and we can get inside of it easily and read the code here. I'm attaching a little screen capture from this behind-the-scenes code... See anything familiar? I highlighted one specific number in the screenshot. I can search for that specific number and see where it leads: https://www.google.com/search?q="203.4058646"... For some curious variants, which tell us interesting things about the recent history of this computational exercise, try this, too: https://www.google.com/search?q="203.405863".

But here's a question that anyone interested in these things should consider. Suppose I use the world's best algorithms and the world's fastest computer to calculate the time when the moon Europa leaves the shadow of Jupiter on some date in January of 2025. After that result is published, why would apps (and humans building apps) ever calculate that number again? Of course, some tiny adjustments might be made, but these things are not like predicting the weather. Orbital motions are highly deterministic for centuries. Europa will emerge when it emerges... and right on schedule. Yet every time I click on that app on the S&T website (linked above) or any of numerous other tools, they calculate it fresh for next month as if the running code is doing something new, useful, important... There's a bit of "lunacy" in this (intentional pun). We know how to code, but we cannot stop.

Frank Reed

PS: We don't really have "Astronomical Algorithms" shirts... but maybe we should. :)

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