A collection of snippets that get the phase of the moon, currently, the implementations are:

Systems Level Languages:

• C/C++
• Rust
• Zig
• Nim

Scripting Languages:

• Arturo
• Lua
• Janet
• JavaScript
• Python
• Raku
• Ruby

DSLs:

• awk
• bc

These functions take a time as an input (usually in unix epoch seconds or the languages official way of doing time), and return the "age" of the moon in radians, such that (1-cos(x))/2 returns the illuminated fraction of the moons surface, this indirection is needed because across a full cycle, the same illuminated percent appears more than once, the first and third quarter are a good example.

All implementations contain test cases showing how to get the illuminated fraction and percent using the code.

When the age of the moon is converted into a range between [0,1]. You can then get the age of the moon in days by multiplying it by ~29.5.

You can also get the "index" of the phase, required for the phase name and emoji, with the illuminated fraction and the angle. As an example, here's some rust code for a different tool using the same algorithm:

fn phaseidx(ilumfrac: f64, ang: time::Angle) -> usize {
    match (ilumfrac, ang.degrees() > 90.0) {
        (0.00..0.04, _) => 0,
        (0.96..1.00, _) => 4,
        (0.46..0.54, true) => 6,
        (0.46..0.54, false) => 2,
        (0.54..0.96, true) => 5,
        (0.54..0.96, false) => 3,
        (_, true) => 7,
        (_, false) => 1,
    }
}

(Note that your angle has to be converted to a positive angle with a-360.0*(floor(a/360)) or some similar modulo (not remainder) operation)

All these snippets are based off the algorithm in moontool, a GUI program made in the 80s by John Walker, which based its algorithms off the book Practical Astronomy With Your Calculator.

These rules may be bent for microlanguages and DSLs.

Your function must be self-contained, such that someone could copy-paste it into their code and not get an error. It also must not effect the environment outside it if at all possible (no #define or mutating globals). It must be as "pure" and self-contained as the language will allow.