JS function to calculate moon phase

moon_phase.js

function moon_phase(date) { // ported from http://www.voidware.com/moon_phase.htm
    var year = date.getYear(),
        month = date.getMonth(),
        day = date.getDay();

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

    ++month;

    jd = 365.25 * year + 30.6 * month + day - 694039.09; // jd is total days elapsed
    jd /= 29.53; // divide by the moon cycle (29.53 days)
    phase = parseInt(jd, 10); // int(jd) -> phase, take integer part of jd
    jd -= phase; // subtract integer part to leave fractional part of original jd
    phase = Math.ceil(jd * 8); // scale fraction from 0-8 and round by adding 0.5
    phase = phase & 7; // 0 and 8 are the same so turn 8 into 0

    switch (phase) {
        case 0: phase = "New Moon"; break;
        case 1: phase = "Waxing Crescent Moon"; break;
        case 2: phase = "Quarter Moon"; break;
        case 3: phase = "Waxing Gibbous Moon"; break;
        case 4: phase = "Full Moon"; break;
        case 5: phase = "Waning Gibbous Moon"; break;
        case 6: phase = "Last Quarter Moon";
        case 7: phase = "Waning Crescent Moon"; break;
    }
    return phase;
}
//document.write(moon_phase(new Date(Date.now())))

Worked amazingly. Thanks so much.

This seems to be bugging out on me.

function moonphase() {
  var date = new Date(Date.now());
  var year = date.getYear();
  var month = date.getMonth();
  var day = date.getDay();
...
return phase + " - "+year+"/"+month+"/"+day;
}

moonphase() returns:
"3 - Quarter Moon - 121/9/1"

Today, which is 20th Sept 2021 - and almost a full moon.

Any ideas? Is it a date parsing or index issue?

Right, the problem is the methods are either wrong/old or have changed since this was first written.

They need to be changed to

var year = date.getFullYear();
var month = date.getMonth(); //index 0-11 check 
var day = date.getDate();

Today 27/09/2021 it returns:
8 - Waning Crescent Moon

when it is actually a Waning Gibbous.

There should be a break after the sixth case; this will never return 'Last Quarter Moon'.

Doesn't work for me