Get the 8 phases instead of quarters

[costular]

Hi!

First of all, @cosinekitty thanks a lot for this library, an enjoyer of the Kotlin library here! 🙋🏼

Now, I have a question (probably silly one) regarding the moon phases. I can see that moonPhase returns the Moon's phase as an angle, then it can be transformed into the different quarters (new moon, first quarter, full moon, third quarter).

However, is there any way to obtain the 8 moon phases (New Moon, Waxing Crescent, First Quarter, Waxing Gibbous, Full Moon, Waning Gibbous, Last Quarter and Waning Crescent)? 🤔

Thank you in advance 🙇🏼

[cosinekitty]

Hi @costular and welcome to the project!

Are you trying to determine which of the 8 phases the Moon is closest to? Or are you trying to find the time when the Moon reaches an exact phase?

If it is the first one, you can take the ecliptic longitude angle, divide by (360/8) = 45 degrees, and round to the nearest integer. You would have to watch out for the special case of rounding up to 8 or rounding down to 0 both indicating a new moon. Or maybe instead of finding the nearest phase, you want to find the most recent phase that has been reached. In that case you would take the floor of the ratio, rather than rounding.

If it is the second case (searching), take a look at searchMoonPhase. If you pass that function 45 degrees as the phase angle, it will find the exact moment of Waxing Crescent.

I hope this helps!

[cosinekitty]

Can you try printing $time instead of $day to check that it has the value you think? I don't see anything obviously wrong, but it would be good to check. It would also be good to print out $moonAngle.

[costular]

Of course, I added both time and moonAngle. Here the results:

Time 2023-04-05T00:00:00.000Z, phase: FULL_MOON, angle 166.16086912956342, ratio 3
Time 2023-04-06T00:00:00.000Z, phase: FULL_MOON, angle 177.7589423017851, ratio 3
Time 2023-04-07T00:00:00.000Z, phase: WANING_GIBBOUS, angle 189.57710406332762, ratio 4
Time 2023-04-08T00:00:00.000Z, phase: WANING_GIBBOUS, angle 201.61800046343592, ratio 4
Time 2023-04-09T00:00:00.000Z, phase: WANING_GIBBOUS, angle 213.87867169348166, ratio 4
Time 2023-04-10T00:00:00.000Z, phase: LAST_QUARTER, angle 226.35274489880675, ratio 5
Time 2023-04-11T00:00:00.000Z, phase: LAST_QUARTER, angle 239.03166389230609, ratio 5
Time 2023-04-12T00:00:00.000Z, phase: LAST_QUARTER, angle 251.9043281623089, ratio 5
Time 2023-04-13T00:00:00.000Z, phase: LAST_QUARTER, angle 264.9549427207888, ratio 5
Time 2023-04-14T00:00:00.000Z, phase: WANING_CRESCENT, angle 278.15959524241936, ratio 6
Time 2023-04-15T00:00:00.000Z, phase: WANING_CRESCENT, angle 291.48268770465484, ratio 6
Time 2023-04-16T00:00:00.000Z, phase: WANING_CRESCENT, angle 304.8746001715859, ratio 6
Time 2023-04-17T00:00:00.000Z, phase: NEW_MOON, angle 318.2718610420979, ratio 7
Time 2023-04-18T00:00:00.000Z, phase: NEW_MOON, angle 331.60064869026274, ratio 7
Time 2023-04-19T00:00:00.000Z, phase: NEW_MOON, angle 344.7835620115358, ratio 7
Time 2023-04-20T00:00:00.000Z, phase: NEW_MOON, angle 357.7484144042802, ratio 7
Time 2023-04-21T00:00:00.000Z, phase: WAXING_CRESCENT, angle 10.436933694059807, ratio 0
Time 2023-04-22T00:00:00.000Z, phase: WAXING_CRESCENT, angle 22.811326481617026, ratio 0
Time 2023-04-23T00:00:00.000Z, phase: WAXING_CRESCENT, angle 34.85765140908565, ratio 0
Time 2023-04-24T00:00:00.000Z, phase: FIRST_QUARTER, angle 46.58608712089196, ratio 1
Time 2023-04-25T00:00:00.000Z, phase: FIRST_QUARTER, angle 58.02878110028093, ratio 1
Time 2023-04-26T00:00:00.000Z, phase: FIRST_QUARTER, angle 69.2360346251144, ratio 1
Time 2023-04-27T00:00:00.000Z, phase: FIRST_QUARTER, angle 80.27156177843175, ratio 1
Time 2023-04-28T00:00:00.000Z, phase: WAXING_GIBBOUS, angle 91.20760718480335, ratio 2
Time 2023-04-29T00:00:00.000Z, phase: WAXING_GIBBOUS, angle 102.12053066150557, ratio 2
Time 2023-04-30T00:00:00.000Z, phase: WAXING_GIBBOUS, angle 113.08693186374859, ratio 2
Time 2023-05-01T00:00:00.000Z, phase: WAXING_GIBBOUS, angle 124.17989556362605, ratio 2
Time 2023-05-02T00:00:00.000Z, phase: FULL_MOON, angle 135.46497770820034, ratio 3
Time 2023-05-03T00:00:00.000Z, phase: FULL_MOON, angle 146.99603822556594, ratio 3
Time 2023-05-04T00:00:00.000Z, phase: FULL_MOON, angle 158.8113959726682, ratio 3

[cosinekitty]

I think I see it now. I think 0 should be new moon, not waxing crescent. 1 should be waxing crescent, and so on.

[costular]

If I make that change the result is the next:

Time 2023-04-05T00:00:00.000Z, phase: WAXING_GIBBOUS, angle 166.16086912956342, ratio 3
Time 2023-04-06T00:00:00.000Z, phase: WAXING_GIBBOUS, angle 177.7589423017851, ratio 3
Time 2023-04-07T00:00:00.000Z, phase: FULL_MOON, angle 189.57710406332762, ratio 4
Time 2023-04-08T00:00:00.000Z, phase: FULL_MOON, angle 201.61800046343592, ratio 4
Time 2023-04-09T00:00:00.000Z, phase: FULL_MOON, angle 213.87867169348166, ratio 4
Time 2023-04-10T00:00:00.000Z, phase: WANING_GIBBOUS, angle 226.35274489880675, ratio 5
Time 2023-04-11T00:00:00.000Z, phase: WANING_GIBBOUS, angle 239.03166389230609, ratio 5
Time 2023-04-12T00:00:00.000Z, phase: WANING_GIBBOUS, angle 251.9043281623089, ratio 5
Time 2023-04-13T00:00:00.000Z, phase: WANING_GIBBOUS, angle 264.9549427207888, ratio 5
Time 2023-04-14T00:00:00.000Z, phase: LAST_QUARTER, angle 278.15959524241936, ratio 6
Time 2023-04-15T00:00:00.000Z, phase: LAST_QUARTER, angle 291.48268770465484, ratio 6
Time 2023-04-16T00:00:00.000Z, phase: LAST_QUARTER, angle 304.8746001715859, ratio 6
Time 2023-04-17T00:00:00.000Z, phase: WANING_CRESCENT, angle 318.2718610420979, ratio 7
Time 2023-04-18T00:00:00.000Z, phase: WANING_CRESCENT, angle 331.60064869026274, ratio 7
Time 2023-04-19T00:00:00.000Z, phase: WANING_CRESCENT, angle 344.7835620115358, ratio 7
Time 2023-04-20T00:00:00.000Z, phase: WANING_CRESCENT, angle 357.7484144042802, ratio 7
Time 2023-04-21T00:00:00.000Z, phase: NEW_MOON, angle 10.436933694059807, ratio 0
Time 2023-04-22T00:00:00.000Z, phase: NEW_MOON, angle 22.811326481617026, ratio 0
Time 2023-04-23T00:00:00.000Z, phase: NEW_MOON, angle 34.85765140908565, ratio 0
Time 2023-04-24T00:00:00.000Z, phase: WAXING_CRESCENT, angle 46.58608712089196, ratio 1
Time 2023-04-25T00:00:00.000Z, phase: WAXING_CRESCENT, angle 58.02878110028093, ratio 1
Time 2023-04-26T00:00:00.000Z, phase: WAXING_CRESCENT, angle 69.2360346251144, ratio 1
Time 2023-04-27T00:00:00.000Z, phase: WAXING_CRESCENT, angle 80.27156177843175, ratio 1
Time 2023-04-28T00:00:00.000Z, phase: FIRST_QUARTER, angle 91.20760718480335, ratio 2
Time 2023-04-29T00:00:00.000Z, phase: FIRST_QUARTER, angle 102.12053066150557, ratio 2
Time 2023-04-30T00:00:00.000Z, phase: FIRST_QUARTER, angle 113.08693186374859, ratio 2
Time 2023-05-01T00:00:00.000Z, phase: FIRST_QUARTER, angle 124.17989556362605, ratio 2
Time 2023-05-02T00:00:00.000Z, phase: WAXING_GIBBOUS, angle 135.46497770820034, ratio 3
Time 2023-05-03T00:00:00.000Z, phase: WAXING_GIBBOUS, angle 146.99603822556594, ratio 3
Time 2023-05-04T00:00:00.000Z, phase: WAXING_GIBBOUS, angle 158.8113959726682, ratio 3

However, according to the NASA calendar the full moon is today and tomorrow, not 7 Apr nor 8 Apr. So, the phases were kinda close with the previous code, but the dates don't match for all the phases 🤔

[cosinekitty]

These calculations look correct, but they may not be answering the question you are trying to ask. You are taking a snapshot of the Moon's position every 24 hours. The Moon moves quite a bit around the Earth every 24 hours. Yes, the full moon is tomorrow, or more precisely in UTC:
2023-04-06 time 04:35. (Month 04 is April, not May). Your algorithm skips over that time and lands on April 7, which is the first time the ecliptic longitude is past full moon. So everything looks correct to me. I think what you really want is to use searchMoonPhase to find exactly when each phase change occurs.