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Get Moon Phase by Date, written in JavaScript
/*
* 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));
@charlag

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@charlag charlag commented Jan 7, 2018

Hi! Thanks for the snipped.
I find your code useful for my personal page but it is not licensed. Could you state the license please?

@imlinus

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@imlinus imlinus commented Aug 14, 2018

Thanks for sharing Endel!

I modified your script slightly to return the name of the phase and put it inside an object.

// Original Snippet: https://gist.github.com/endel/dfe6bb2fbe679781948c

var Moon = {
  phase: function (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

    switch (b) {
      case 0:
        return 'new-moon';
        break;
      case 1:
        return 'waxing-crescent-moon';
        break;
      case 2:
        return 'quarter-moon';
        break;
      case 3:
        return 'waxing-gibbous-moon';
        break;
      case 4:
        return 'full-moon';
        break;
      case 5:
        return 'waning-gibbous-moon';
        break;
      case 6:
        return 'last-quarter-moon';
        break;
      case 7:
        return 'waning-crescent-moon';
        break;
    }
  }
};

// Moon.phase('2018', '01', '19');
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@mrorigo mrorigo commented Jan 16, 2019

@imlinus I modified your script to not have a bloaty switch case, and actually return an object containing both the name and the number of the phase ;)

// Original Snippet: https://gist.github.com/endel/dfe6bb2fbe679781948c

var Moon = {
  phases: ['new-moon', 'waxing-crescent-moon', 'quarter-moon', 'waxing-gibbous-moon', 'full-moon', 'waning-gibbous-moon', 'last-quarter-moon', 'waning-crescent-moon'],
  phase: function (year, month, day) {
    let 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
    return {phase: b, name: Moon.phases[b]};
  }
};

// Moon.phase('2018', '01', '19');
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@imlinus imlinus commented Feb 4, 2019

@mrorigo, beautiful!

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@t1mwillis t1mwillis commented Mar 15, 2019

Thank you for this! Did just the trick for me :)

In case anyone is wondering, the following returns an object with today's date that you can then inject into this. Month is 0 indexed.

var today = new Date();
var phase = Moon.phase(today.getFullYear(), today.getMonth()+1, today.getDate());
@corporateanon

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@corporateanon corporateanon commented Mar 26, 2019

Please fix that:

let c = e = jd = b = 0;

It creates 3 global variables: e, jd and b

@satouriko

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@satouriko satouriko commented Nov 13, 2019

What timezone is this script intended for?

@shawenyao

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@shawenyao shawenyao commented Oct 16, 2020

Thanks for the script. I borrowed function to build an app that shows what the Moon looks like on a given day.

https://moon.shawenyao.com/?date=1980-07-31

(Note that if no date is passed, it defaults to today)

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@datatypevoid datatypevoid commented Nov 1, 2020

Here is a TypeScript version with a bit more polish: https://gist.github.com/datatypevoid/f4dd1f6439feaa588bb2aaf4f8f4361f

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@orionrush orionrush commented Mar 10, 2021

Is anyone else getting inconsistent results with this script?

with the following Moon.phase('2021', '03', '10') I get a result of waning-gibbous-moon, however, the moon phase calendars I've consulted have the current phase as waning-crescent. This is a pretty big discrepancy. . .

@archipoeta

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@archipoeta archipoeta commented Mar 11, 2021

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.

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@archipoeta archipoeta commented Mar 11, 2021

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

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@orionrush orionrush commented Mar 11, 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.

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@archipoeta archipoeta commented Mar 11, 2021

teamwork! 💪

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