Understanding PhaseQuarter of Moon calculation

[mivion]

Understanding PhaseQuarter of Moon calculation

[0xStarcat]

Potential Solution ?

This looks like it can be resolved by removing this line in moon.js:

i = (i+1) & 3;

Here's what results, and these are much closer aligned to expectations:

Day: 1  || illuminatedFraction: 0.2718262029195282 || phaseQuarter: 1
Day: 2  || illuminatedFraction: 0.1913535738047783 || phaseQuarter: 1
Day: 3  || illuminatedFraction: 0.12275534591473419 || phaseQuarter: 1
Day: 4  || illuminatedFraction: 0.06789228968867916 || phaseQuarter: 1
Day: 5  || illuminatedFraction: 0.028402658847401396 || phaseQuarter: 1
Day: 6  || illuminatedFraction: 0.005654730897217575 || phaseQuarter: 1
Day: 7  || illuminatedFraction: 0.0006617445257049992 || phaseQuarter: 2
Day: 8  || illuminatedFraction: 0.01398865166284069 || phaseQuarter: 2
Day: 9  || illuminatedFraction: 0.04567593153994548 || phaseQuarter: 2
Day: 10 || illuminatedFraction: 0.09519380617986456 || phaseQuarter: 2
Day: 11 || illuminatedFraction: 0.16142284737506968 || phaseQuarter: 2
Day: 12 || illuminatedFraction: 0.2426405680532956 || phaseQuarter: 2
Day: 13 || illuminatedFraction: 0.3364851635748891 || phaseQuarter: 2
Day: 14 || illuminatedFraction: 0.43987279594989304 || phaseQuarter: 2
Day: 15 || illuminatedFraction: 0.5488689342271671 || phaseQuarter: 3
Day: 16 || illuminatedFraction: 0.6585597621788225 || phaseQuarter: 3
Day: 17 || illuminatedFraction: 0.763029581554865 || phaseQuarter: 3
Day: 18 || illuminatedFraction: 0.8555981913616235 || phaseQuarter: 3
Day: 19 || illuminatedFraction: 0.9294546722062444 || phaseQuarter: 3
Day: 20 || illuminatedFraction: 0.9786814411700813 || phaseQuarter: 3
Day: 21 || illuminatedFraction: 0.9994095420276781 || phaseQuarter: 3
Day: 22 || illuminatedFraction: 0.9906443297921943 || phaseQuarter: 0
Day: 23 || illuminatedFraction: 0.9543665946579276 || phaseQuarter: 0
Day: 24 || illuminatedFraction: 0.8948694419357917 || phaseQuarter: 0
Day: 25 || illuminatedFraction: 0.8176725929146917 || phaseQuarter: 0
Day: 26 || illuminatedFraction: 0.728477536842415 || phaseQuarter: 0
Day: 27 || illuminatedFraction: 0.6324682679459483 || phaseQuarter: 0
Day: 28 || illuminatedFraction: 0.5340178279576334 || phaseQuarter: 0
Day: 29 || illuminatedFraction: 0.4367005906508668 || phaseQuarter: 1
Day: 30 || illuminatedFraction: 0.3434676777124016 || phaseQuarter: 1
Day: 31 || illuminatedFraction: 0.2568689766546564 || phaseQuarter: 1

I'm running this on a fork with tests (https://github.com/0xStarcat/Moshier-Ephemeris-JS), so I can say confidently removing this line does NOT break anything.

Does this line need to be in there? Was the & bitwise operator intended as a % modulo maybe?

Honestly, if you changed the line to:

i = (i+2) % 4;

and moved it right above $moshier.body.moon.position.phaseQuarter = i; so it ran every iteration, it makes the calculated value make much more sense.

New Moon = 0
First quarter = 1
Full moon = 2
last quarter = 3

i can add a PR for this if you'd like. What do you think?

Thank you! :)

[0xStarcat]

I'm beginning to think this line

i = (i+1) & 3;

was maybe meant to serve as an "approximator" or an orb in astrological terms, for when to designate the start and end of a quarter. In this case, from checking the phaseDaysBefore and phaseDaysAfter values, it seems like the quarters are approximated within 3 days on either side of the true quarter event.

I suppose this has a useful purpose, and based on this, there's probably something that can be done to make the output of these fields more useful like

1. letting user define the "approximation" or "orb" amount,
2. returning Waxing, Waning, Crescent and Gibbous strings for shape and direction, etc

In any case, I've done a little bit of this in my fork 0xStarcat@135ca2b

[0xStarcat]

Closing this down because I think I've answered this sufficiently for myself