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copied!<p>I ported some code to Python for this a while back. I was going to just link to it, but it turns out that it fell off the web in the meantime, so I had to go dust it off and upload it again. See <a href="http://bazaar.launchpad.net/~keturn/py-moon-phase/trunk/annotate/head:/moon.py" rel="noreferrer">moon.py</a> which is derived from <a href="http://www.fourmilab.ch/moontool/" rel="noreferrer">John Walker's moontool</a>.</p> <p>I can't find a reference for this for what time spans it's accurate for either, but seems like the authors were pretty rigorous. Which means yes, it does use trig, but I can't imagine what the heck you would be using this for that would make it computationally prohibitive. Python function call overhead is probably more than the cost of the trig operations. Computers are pretty fast at computing.</p> <p>The algorithms used in the code are drawn from the following sources:</p> <p><strong>Meeus, Jean. Astronomical Algorithms. Richmond: Willmann-Bell, 1991. ISBN 0-943396-35-2.</strong></p> <p>A must-have; if you only buy one book, make sure it's this one. Algorithms are presented mathematically, not as computer programs, but source code implementing many of the algorithms in the book can be ordered separately from the publisher in either QuickBasic, Turbo Pascal, or C. Meeus provides many worked examples of calculations which are essential to debugging your code, and frequently presents several algorithms with different tradeoffs among accuracy, speed, complexity, and long-term (century and millennia) validity.</p> <p><strong>Duffett-Smith, Peter. Practical Astronomy With Your Calculator. 3rd ed. Cambridge: Cambridge University Press, 1981. ISBN 0-521-28411-2.</strong></p> <p>Despite the word Calculator in the title; this is a valuable reference if you're interested in developing software which calculates planetary positions, orbits, eclipses, and the like. More background information is given than in Meeus, which helps those not already versed in astronomy learn the often-confusing terminology. The algorithms given are simpler and less accurate than those provided by Meeus, but are suitable for most practical work.</p>

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