Show HN: Tiny Moon – Swift library to calculate the moon phase

Show HN: Tiny Moon – Swift library to calculate the moon phase(github.com)

106 points by mannylopez 1 year ago | 35 comments

Tiny Moon is a tiny Swift library to calculate the moon phase for any given date, works super fast, and works completely offline.

All of this started when I realized that we only have 12, sometimes 13, full moon's in a year. That doesn’t seem like that many.

I set out to build a MacOS app to remind me when a full moon occurs, so that I could take a moment and step outside to appreciate it.

The MacOS app I ended up creating can be found at https://apps.apple.com/us/app/tiny-moon/id6502374344 along with the source code [0], all powered by the Tiny Moon library.

I knew that I wanted the app to work offline, so working with a network request was out of the picture. Taking inspiration from SunCalc [1] and Moontool for Windows [2], I decided to create my own library and wrote Tiny Moon as a Swift Package to power my app.

The app tries to be as minimal as possible, does what it does very fast, and works completely offline.

[0] https://github.com/mannylopez/TinyMoonApp

[1] https://github.com/mourner/suncalc

[2] https://www.fourmilab.ch/moontoolw/

adityapurwa 1 year ago |

If granular accuracy is not an important factor, you can also use Islamic calendar to calculate the moon phase. Muslims uses lunar cycles for some religious events (e.g. the Ayyamul Bidh or 3 days fasting during full moon). So when its 15 of an Islamic calendar, it’ll be a full moon.

I used this approach because most platforms supports islamic calendar.

I really like the offline first approach, and would definitely use the library when there’s a need.

Thanks for sharing!

throwup238 1 year ago | |

I wonder, how is Allah's tolerance towards technotheological errors?

What if there's a bug in the calendar for Ramadan or Salah?

adityapurwa 1 year ago | | |

In Islam, err is forgiven. If you’ve done your best and there’s a bug you missed, then its forgiven, and when you discover it, just fix it.

That’s why if a scholar based with intensive research and data, has concluded that X is true. If in the end its really true, they’ll receive two rewards; when its debunked and it was actually false, they receive one reward. The sin is to be ignorant and without any research and data concluded that X is true.

Specifically for Ramadan, the guideline is to use Moon sighting. If you see a new moon, then you begin fasting. If not, then you postpone for a day before starting Ramadan. Usually there’s a committee that does this and they will announce the result. But it doesn’t prevent anyone with the ability to observe to decide when to start Ramadan.

Salah guideline is the sun, e.g the dawn prayer is when there’s thin strike of the sun on the horizon. So to calculate the prayer time you use the sun position relative to your position on earth. If there’s a bug that somehow err the prayer time to ~5 minutes. We can always observe the sun first.

So there’s always this second factor that you can use to validate the first method. Time seems off? Look at the sun. Sun not visible? Estimate with time. Both seems off? Estimate with the variables that you can observe.

“ God does not burden any soul with more than it can bear “ (Al Baqarah 286)

——- So to answer the question, the tolerance depends on your effort on trying to reduce the err.

God knows best

rubiquity 1 year ago |

I know a lot of people into fishing that would love this as an app to quickly check the moon phase on a given date when planning a multi-day fishing trip or deciding when to go out. Apex predators are typically less hungry around and during a full moon due to the extra light making hunting at night easy.

nektro 1 year ago | |

the ios weather app has moon info for the current, previous, and next month

ChrisMarshallNY 1 year ago |

I don't really have use for this, but I must compliment the author on a job well-done. The code is well-structured, well-documented, well-tested, and well-designed.

mannylopez 1 year ago | |

Thank you! I was a technical writer before learning how to program, so it's important for me to write well-documented and well-tested code.

Doing so also allowed me to completely remove and replace my first attempt at implementing the moon phase algorithm when it turned out to be not-accurate enough (the values were off by about 4 hours).

shainvs 1 year ago |

Is there a plan to add this for linux / windows? Either way, super cool project

robsh 1 year ago | |

The following formula will return the closest moon phase emoji in Excel:

=LET(phase,MOD(ROUND(MOD(NOW(),29.5275)/3.691,0)-2,8)+1,UNICHAR(127760+phase)))

mannylopez 1 year ago | |

Good news! According to Swift Package Index [0], Tiny Moon is already compatible with Linux environments.

0. https://swiftpackageindex.com/mannylopez/TinyMoon/

ginko 1 year ago | |

  $ curl wttr.in/Moon

jheriko 1 year ago |

this looks incredibly bad from the implementation. there is a much simpler algorithm that works for hundreds of years in either direction of the present...

by comparison this is an absolute mountain of code. here is a good one - but note that it is only so accurate for each quarter phase (...and not sure why it destroys the formatting):

function approximateMoonPhase(julianDay)

{

    // from Meeus p.319

    // JDE = 2451 550.09765 + 29.530 588 853 k

    // + 0.000 1337 T2

    // - 0.000 000 150 T3

    // + 0.000 000 000 73 T4

    const t = toJulianCenturiesSinceJ2000(julianDay);

    const lhs = julianDay - 2451550.09765 + (-0.0001337 + (0.00000015 - 0.00000000073 * t) * t) * t * t;

    return lhs / 29.530588853;

}

enjoy. its from the same source as your work appears to use...

which, as someone who as implemented a ton of this stuff. its kind of a damning sentiment that we still refer to such an old book instead of learning the problem space and making better solutions... i have a whole book in me about this at some point.

jes5199 1 year ago |

nice! I have needed this before. I think you're using the Meeus algorithm from Astronomical Algorithms? it's a classic, great choice

mannylopez 1 year ago | |

Yes, pretty much every good library I came across referenced Meeus' Astronomical Algorithms [0] book and formulas. Those were a bit dense for me to fully parse, but Dr. Louis Strous` page [1] on finding the position of the moon helped simplify it for me (although that implementation is less accurate, I think).

The algorithm in Moontool for Windows [2], which Tiny Moon is based off of, is based off of Meeus' algorithm.

0. https://www.agopax.it/Libri_astronomia/pdf/Astronomical%20Al...

1. https://aa.quae.nl/en/reken/hemelpositie.html#4

2. https://www.fourmilab.ch/moontoolw/

8mobile 1 year ago |

Hi,

Great library, super documentation and the code seems well written with lots of tests. Kudos, I will definitely check it out for my next ios project.

Thanks Otto

ks2048 1 year ago |

Nice work. Just FYI for anybody else - it adds an icon to status bar (if that's what it's called? - top right icons). I thought it was going to open an app window and didn't notice the icon, so thought it wasn't working. (In hindsight, it's clear from screenshots in app store).

asrael_io 1 year ago | |

I think it's called the menu bar, and apps that don't open windows (with an icon in the menu bar) are known as "menu bar apps". Until recently, these were a PITA to implement with Apple's SDK. Thankfully they provided API support sometime I think starting with macOS 13... before that it was pretty kludgy.

rgovostes 1 year ago | | |

The menus themselves are called "menu bar extras": https://developer.apple.com/design/human-interface-guideline...

whartung 1 year ago |

And then, there was an anecdote!

I bought my friends daughter a watch for her birthday. She was, like, 12 or something. I got it at Target.

And it had a moon phase dial! How about that?!

So I was determined to set it accurately for her.

To wit I started dredging through the internet. And to be completely honest, I'm not quite sure how I was doing that, having nothing but a Netcom.com shell account available to me. Perhaps I was using Lynx, or using some mail gateway, or who knows what. It wasn't no 10 seconds on Google, I'll tell you that.

In the end, I stumbled upon some code for my HP-48. Aha! This should do nicely!

Somehow, I got the code into the calculator. Did I type it in? Did I download it over Kermit? Who knows. Times were moving fast back then. Somehow, someway, however, I got that code in and calculated away.

Finally! ACCURATE results. Done with precision and math and engineering. Done Right!

To wit, I then proceeded to get the watch set properly.

And...it was a fashion watch for a 12 year old girl from Target. This may come as a shock to some, I was obviously taken aback by it, however -- the moon phase did not work. It was just a moon on a dial that moved, though some undetermined mechanism. It could not be set independently.

Oh.

gmiller123456 1 year ago |

I have a site with a lot of these stand alone "snippets" so that you don't have to include/port an entire astronomy library just to get the Sun rise/set times, etc. https://www.celestialprogramming.com/

Most of them are written in JavaScript only, but specifically written to be easy to port to other languages.

082349872349872 1 year ago |

Here's a tiny calculator for moon phase that I kludged up ca. 2018 but (at least as of this month) is still tracking the full moon (which makes sense given that the algo came out of a dead tree source from over half a century ago?):

  _=min
  lphase = lambda y,m,d: _( lkp(ph,p)
        for lkp in [lambda t,x: _(v for (kl,v),(kh,w)
                        in zip(t,t[1:]) if kl <= x < kh)]
        for daynum  in [lambda y,m,d: daynum(y-1,m+12,d) if m<3 else
                         y*365 + y//4 - y//100 + y//400 + (153*m+3)//5 + d]
        for o   in [daynum(y,m,d)-daynum(2000,1,6)]
        for p   in [o%29.53] # mean; varies significantly!
        for ph  in [[(0,"new"),(0.5,"waxing crescent"),(7,"first quarter"),
                (8,"waxing gibbous"),(14.5,"full"),(15.5,"waning gibbous"),
                (21.7,"last quarter"),(22.7,"waning crescent"),(29,"new"),
                (30,None)]] )

madcaptenor 1 year ago | |

So this is just saying that the phase of the moon is the day mod 29.53, starting from a new moon on 2000-01-06. That's about a minute off the actual mean length of a lunation - I guess the error from that approximation is less than the approximation in your "varies significantly"?

(For what it's worth - 29.53 * 300 = 8859 exactly, and 2000-01-06 + 8859 days = 2024-04-08, and I am quite sure there was a new moon on the latter date because of the solar eclipse.)

082349872349872 1 year ago | | |

Yes. (any cultures that have a purely lunar calendar could do away with all the date fiddling? but then again, I guess they'd just know when the next full moon would be...)

[If that constant is only a minute off, it ought to be good for a few centuries more, and I'm not planning on being around longer than decades, so probably good enough for my purposes.]

kickingvegas 1 year ago |

Side note, if you use Emacs you can get the phases of the Moon by pressing “M” in either calendar or Org Agenda.

eesmith 1 year ago | |

With M-x lunar-phases you get the next three lunar months; something like:

  Thursday, June 6, 2024: New Moon 8:40am (EDT)
  Friday, June 14, 2024: First Quarter Moon 1:20am (EDT)
     .. I removed a few lines ...
  Monday, August 19, 2024: Full Moon 2:24pm (EDT)
  Monday, August 26, 2024: Last Quarter Moon 5:34am (EDT

Also, M-x sunrise-sunset computes sunrise and sunset after you give it lat/long.

// from Meeus p.319 // JDE = 2451 550.09765 + 29.530 588 853 k // + 0.000 1337 T2 // - 0.000 000 150 T3 // + 0.000 000 000 73 T4 const t = toJulianCenturiesSinceJ2000(julianDay); const lhs = julianDay - 2451550.09765 + (-0.0001337 + (0.00000015 - 0.00000000073 * t) * t) * t * t; return lhs / 29.530588853;

_=min lphase = lambda y,m,d: _( lkp(ph,p) for lkp in [lambda t,x: _(v for (kl,v),(kh,w) in zip(t,t[1:]) if kl <= x < kh)] for daynum in [lambda y,m,d: daynum(y-1,m+12,d) if m<3 else y*365 + y//4 - y//100 + y//400 + (153*m+3)//5 + d] for o in [daynum(y,m,d)-daynum(2000,1,6)] for p in [o%29.53] # mean; varies significantly! for ph in [[(0,"new"),(0.5,"waxing crescent"),(7,"first quarter"), (8,"waxing gibbous"),(14.5,"full"),(15.5,"waning gibbous"), (21.7,"last quarter"),(22.7,"waning crescent"),(29,"new"), (30,None)]] )

Thursday, June 6, 2024: New Moon 8:40am (EDT) Friday, June 14, 2024: First Quarter Moon 1:20am (EDT) .. I removed a few lines ... Monday, August 19, 2024: Full Moon 2:24pm (EDT) Monday, August 26, 2024: Last Quarter Moon 5:34am (EDT