How is the Moon Phase interpreted

[ISY4Me]

I have the Sun Nodeserver running, but the Moon Phase just reports a number... right now it is showing 18.

I have looked for some documentation for this NS and I can not find a valid link.  Any input appreciated.

[Geddy]

@ISY4Me not seeing anything in the GitHub reademe 

https://github.com/UniversalDevicesInc-PG3/udi-sun-poly/blob/master/README.md

Maybe @bpwwer might know…looks like he made the conversion to PG3.

[Ross]

@ISY4Me I just downloaded the NS.  My Moon Phase number is now 21.34 (at 7:10am Pacific on Jan 4,2023).  It should be the age of the moon (in days) since the last new moon.  However, I just searched online and the moon is currently 22.55 days old.  So, the number doesn't make sense to me.  The azimuth and elevation do appear reasonable; however, I don't understand Zenith.  I think we should monitor the moon phase until the next new moon on Jan 11 (3:57am Pacific) and see if it starts over.  I don't know enough about NodeServers to see what math it's using.

Here's an explanation of the Age of Moon (sorry if you already knew this): https://in-the-sky.org/article.php?term=age_of_moon.

If you don't mind: when I clicked on More Info, I just got a Not Found page.  Did you use meters or feet for your elevation?

Ross

[ISY4Me]

Thank you @Ross.   Your response helped me understand the basics… I was not even close to thinking age/time since last new moon.  

[chris.com]

@Ross

Thank you for your post. I'm experiencing the same thing you are with the moon phase being a bit off from what I think it should be.

Suppose we assume that the moon phase value in the plugin denotes the number of days since the last new moon. According to Wikipedia, a synodic month can be as short as 29.27 days and as long as 29.83 days, with the average being 29.53059 days. So let's just pretend that it's 29.53059 days every time. Right now, my plugin says the moon phase is 19.71. Would it be reasonable to calculate the moon's percentage of fullness this way?

Moon Phase Value in Sun Polyglot plugin x 100 ÷ 29.53   or....
19.71 x 100 ÷ 29.53 = 66.74568236%

Timeanddate.com calculates the moon percentage in my location at 62.3%. However, nineplanets.org says it should be 69.34%. Sky-tonight says 61.4%. Not sure who to believe.

I'm not entirely certain that this moon phase value really is the exact number of days since the last new moon. According to this post, the plugin calculates its values using Astral. And according to Astral's documentation:

Quote

The moon phase method returns an number describing the phase, where the value is between 0 and 27.99. The following lists the mapping of various values to the description of the phase of the moon.

Not sure how Astral decided that 27.99 would be the number. That's definitely not a synodic month. Are they using a sidereal month? According to Wikipedia, a sidereal month is 27.321661 days. That's close to 27.99, but not quite. Maybe they rounding up? Not sure. But let's try the calculation again:

19.71 x 100 ÷ 27.99 = 70.41800643%

That's off from timeanddate.com's 62.3% calculation by an even greater amount, but closer to nineplanets.org's 69.34%.

And then there's this in Astral's documentation:

Quote

The moon phase does not depend on your location. However what the moon actually looks like to you does depend on your location. If you’re in the southern hemisphere it looks different than if you were in the northern hemisphere.

If I'm understanding that correctly, it doesn't take your location into account when calculating moon phase. Of course your location does affect when the moon rises and sets. But, from what I've read, your location shouldn't make a noticeable difference in how full the moon looks. It can affect its orientation and its position relative to other stars however.

FWIW, in my opinion the moon phase being expressed in this way (a not-quite-accurate sidereal month) is probably the least useful way I can think of. I would prefer the addition of the following values:

• Percentage Full: 0 - 100
• Waning or Waxing: 0 or 1
• Next full moon date & time
• Next new moon date & time
• Last full moon date & time
• Last new moon date & time

Eisy allows us to schedule events based on sunrise and sunset. It would be nice if we could also include moonrise and moonset as well. (This does require latitude and longitude to calculate.)

Of course, in order for this to come to fruition, Astral would have to be updated to produce these values, and/or the Sun polyglot plugin would need to find a different way of procuring and calculating the data.

I guess to be fair to the authors of the plugin, it is called "Sun" and not "Sun and Moon."

Edited September 23 by chris.com
I completely rewrote the post after gathering more information.

[Ross]

@chris.com

Hi Chris.  I agree with all of what you wrote.  I tried every which way to make sense of the the data but I eventually gave up and deleted the plugin.  I have a nice view of the eastern horizon and would love to have a moonrise plugin.   I've been told that integration with Home Assistant for these types of things may be my best bet.  I would like a notification that says "Moonrise is in 5 minutes" and then go out watch it!

Ross