Get Moon Phase by Date, written in JavaScript

getMoonPhase.js

/* 
 * modified from http://www.voidware.com/moon_phase.htm 
 */

function getMoonPhase(year, month, day)
{
    var c = e = jd = b = 0;

    if (month < 3) {
        year--;
        month += 12;
    }

    ++month;

    c = 365.25 * year;

    e = 30.6 * month;

    jd = c + e + day - 694039.09; //jd is total days elapsed

    jd /= 29.5305882; //divide by the moon cycle

    b = parseInt(jd); //int(jd) -> b, take integer part of jd

    jd -= b; //subtract integer part to leave fractional part of original jd

    b = Math.round(jd * 8); //scale fraction from 0-8 and round

    if (b >= 8 ) {
        b = 0; //0 and 8 are the same so turn 8 into 0
    }

    // 0 => New Moon
    // 1 => Waxing Crescent Moon
    // 2 => Quarter Moon
    // 3 => Waxing Gibbous Moon
    // 4 => Full Moon
    // 5 => Waning Gibbous Moon
    // 6 => Last Quarter Moon
    // 7 => Waning Crescent Moon
    
    return b;
}

console.log(getMoonPhase(2015, 3, 29));

pretty sure the month is zero-indexed, as i am getting waning crescent with '02'.

update: oh, no disregard-- Feb 10, 2021 was also waning crescent.
update: i am able to repro the issue, something is weird here.

Figured it out, the params need to be integers.
Otherwise it breaks down on this line:
jd = c + e + day - 694039.09; // jd is total days elapsed
because it ends up being "int + int + str - int" and comes up n-days short depending on the day parameter.

Now I get waning crescent for 3/10/2021

@archipoeta that's magic - thank you!

So to make this solution more robust, it would be best to parseInt and make sure that day and month are two digits, and year four digits.

It's perhaps less likely that year would get less than two digits. I guess a sensible fallback might be the current century if say it were provided only two digits.

teamwork! 💪

Date 16 may 2022 actualy is the day start of fullmoon.

but this script shows that fullmoon start at 13 may 2022..

Is it something wrong?

Full moon was last night here, there are regional differences, but not by that much. You can check the exact time of the full moon for your area with an ephemeris such as this one: https://in-the-sky.org/ephemeris.php?ird=1&irs=1&ima=1&iph=1&iob=1&objtype=1&objpl=Moon&objtxt=the+Moon&tz=0&startday=1&startmonth=6&startyear=2022&interval=1&rows=100

And then cross-reference with the scripts results.