<p>I know how to use <a href="https://ssd.jpl.nasa.gov/horizons/app.html#/" rel="nofollow noreferrer">JPL's Horizons</a> to get <code>moon illumination percentage</code> but I noticed that even at new moon, the illumination never goes to zero:</p>
<pre><code>b0003-Sep-09 12:10 *m    0.07382  
b0003-Sep-09 12:11 *m    0.07382  
b0003-Sep-09 12:12 *m    0.07381  
b0003-Sep-09 12:13 *m    0.07381  
</code></pre>
<p>I <em>could</em> just normalize relative to the lowest illumination percentage but I would prefer to have the JPL site, or some Python package like <a href="https://rhodesmill.org/skyfield/" rel="nofollow noreferrer">Skyfield</a>, spit out: <strong>the moon phase percentage is 0% at date, time = x, y</strong></p>

CC BY-SA 4.0
2022-10-05T18:27:15.500Z
Naively, it seems like if the moon would only reach 0% illumination during a solar eclipse. Maybe you can find a software package to compute when solar eclipses occured?
Roy Smart
CC BY-SA 4.0
2022-10-05T19:05:02.087Z
Horizons gives you the correct illumination, which is based on the 3D angle between the Sun & Moon. As I said [here](https://astronomy.stackexchange.com/questions/50592/tool-to-calculate-moonrise-moonset-and-moon-phase-for-any-date-for-any-location#comment112661_50592), the principle lunar phases (New Moon, First Quarter, Full Moon, Last Quarter) use the angle between the Sun & Moon in the ecliptic plane.
PM 2Ring
CC BY-SA 4.0
2022-10-05T19:11:38.913Z
FWIW, the next Full Moon is on 2022-Oct-09 20:54:58.918 UTC. That's geocentric, so it ignores parallax & atmospheric refraction.
PM 2Ring
CC BY-SA 4.0
2022-10-05T22:26:25.333Z
You should probably clarify if you're looking for the phase % or the illumination %, they are different things.  Illumination uses 3D angles, while the phase uses only 2D angles (specifically, the angles in the ecliptic plane).
Greg Miller
CC BY-SA 4.0
2022-10-05T22:50:50.067Z
@GregMiller I thought I was clear: “moon phase percentage”. I’ll add “not illumination” to the question.
Bob
CC BY-SA 4.0
2022-10-05T23:53:52.423Z
@Bob looking at the [Horizons Manual](https://ssd.jpl.nasa.gov/horizons/manual.html) I see percent illuminated and phase angle, I don't see "moon phase percentage" and I've never heard of that term. You use it in the title of your question, but you say "moon illumination percentage" in the body of your question, so while *you thought you were clear* I think you still need to clarify, and if you want stick with  "moon phase percentage" you'll need to explain what that means either by defining it yourself or adding a link to a formal definition.
uhoh
CC BY-SA 4.0
2022-10-06T09:53:50.400Z
@uhoh I *think* Bob wants: New Moon=0%, First Quarter=25%, Full Moon=50%, Last Quarter=75%. Or maybe he wants Full Moon=100%...
PM 2Ring
CC BY-SA 4.0
2022-10-06T10:37:26.610Z
@PM2Ring oh I see. Ya since that's a 2D universe concept there's no perfect proxy in a real-world problem.
uhoh
CC BY-SA 4.0
2022-10-06T14:06:44.610Z
The phase percentage would just be the phase angle divided by 360deg.
Greg Miller
<the-moon><software><moon-phases>
JPL Ephemeris or Skyfield - how to get moon phase percentage (not illumination percentage)
Bob
# JPL Ephemeris or Skyfield - how to get moon phase percentage (not illumination percentage)

(Asked by: username_0 on 2022-10-05)

I know how to use JPL's Horizons to get moon illumination percentage but I noticed that even at new moon, the illumination never goes to zero:
b0003-Sep-09 12:10 *m    0.07382  
b0003-Sep-09 12:11 *m    0.07382  
b0003-Sep-09 12:12 *m    0.07381  
b0003-Sep-09 12:13 *m    0.07381  

I could just normalize relative to the lowest illumination percentage but I would prefer to have the JPL site, or some Python package like Skyfield, spit out: the moon phase percentage is 0% at date, time = x, y

**Tags:** the-moon, software, moon-phases

> **Comments:**
> **username_1**: Naively, it seems like if the moon would only reach 0% illumination during a solar eclipse. Maybe you can find a software package to compute when solar eclipses occured?  
> **username_2**: Horizons gives you the correct illumination, which is based on the 3D angle between the Sun & Moon. As I said [here](https://astronomy.stackexchange.com/questions/50592/tool-to-calculate-moonrise-moonset-and-moon-phase-for-any-date-for-any-location#comment112661_50592), the principle lunar phases (New Moon, First Quarter, Full Moon, Last Quarter) use the angle between the Sun & Moon in the ecliptic plane.  
> **username_2**: FWIW, the next Full Moon is on 2022-Oct-09 20:54:58.918 UTC. That's geocentric, so it ignores parallax & atmospheric refraction.  
> **username_3**: You should probably clarify if you're looking for the phase % or the illumination %, they are different things.  Illumination uses 3D angles, while the phase uses only 2D angles (specifically, the angles in the ecliptic plane).  
> **username_0**: @GregMiller I thought I was clear: “moon phase percentage”. I’ll add “not illumination” to the question.  
> **username_4**: @username_0 looking at the [Horizons Manual](https://ssd.jpl.nasa.gov/horizons/manual.html) I see percent illuminated and phase angle, I don't see "moon phase percentage" and I've never heard of that term. You use it in the title of your question, but you say "moon illumination percentage" in the body of your question, so while *you thought you were clear* I think you still need to clarify, and if you want stick with  "moon phase percentage" you'll need to explain what that means either by defining it yourself or adding a link to a formal definition.  
> **username_2**: @username_4 I *think* username_0 wants: New Moon=0%, First Quarter=25%, Full Moon=50%, Last Quarter=75%. Or maybe he wants Full Moon=100%...  
> **username_4**: @PM2Ring oh I see. Ya since that's a 2D universe concept there's no perfect proxy in a real-world problem.  
> **username_3**: The phase percentage would just be the phase angle divided by 360deg.  

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## 0 answers

