<p>The following is more a series of observations than a full answer, but I hope other people can fill in the details.</p>

<p>The <a href="https://en.wikipedia.org/wiki/Magnitude_(astronomy)" rel="nofollow noreferrer">apparent magnitude</a> of the full moon is -13. The apparent magnitude of the moon during a lunar eclipse will differ from eclipse to eclipse (correlated to the <a href="https://en.wikipedia.org/wiki/Danjon_scale" rel="nofollow noreferrer">Danjon scale</a> value), but let's suppose it is 0 for a certain eclipse. (Note that this magnitude is different from the (pen)umbral magnitudes mentioned on the <a href="https://eclipse.gsfc.nasa.gov/LEcat5/LE2001-2100.html" rel="nofollow noreferrer">NASA website</a>; these magnitudes can be computed and do not depend on the Earth's atmosphere.)</p>

<p>This means the full Moon is normally reflecting $100^{13/5}$ times more light than during this eclipse. I'm not sure if the <a href="https://en.wikipedia.org/wiki/Albedo" rel="nofollow noreferrer">albedo</a> of the Moon is different for red light than it is for normal sunlight, but if not, that means the answer is $\frac{1361}{100^{13/5}} \frac{W}{m^2} \approx 8.6 \frac{\text{m}W}{m^2}$.</p>

CC BY-SA 4.0
2018-09-09T09:59:41.977Z
2018-09-09T09:59:41.977Z
Glorfindel
<p>As we know, the total solar irradiance normally received by the Moon is about 1361 $W/m^2$ (the same as the top of the Earth's atmosphere). Of course, this changes during lunar eclipses, <em>but the Moon doesn't go completely dark</em> even during total eclipses (it's usually just a dim red). It still receives light through the Earth's atmosphere. My question is how much?</p>

<p>I've tried googling this, but all the search terms I could think of just gave me a bunch of unrelated astrology results. <em>Surely people have measured this.</em></p>

<p>The worst case irradiance for any part of the Moon during a total eclipse is fine for my purposes, but I wouldn't mind a paper with an intensity curve over time if there are any.</p>

<p>Edit:<br>
This is not a duplicate of "<a href="https://astronomy.stackexchange.com/questions/2166">Red Moon a Characteristic of all Total Lunar Eclipses?</a>" That question is asking if all lunar eclipses are red and/or if the amount of redness varies. I'm asking how much light hits the Moon (per second per area) during a total lunar eclipse. Furthermore, none of the answers there even mention irradiance.</p>

CC BY-SA 4.0
2018-09-07T18:10:57.393Z
Possible duplicate of [Red Moon a Characteristic of all Total Lunar Eclipses?](https://astronomy.stackexchange.com/questions/2166/red-moon-a-characteristic-of-all-total-lunar-eclipses)
Carl Witthoft
CC BY-SA 4.0
2018-09-08T00:44:48.050Z
@CarlWitthoft That question is about redness rather than irradiance. [Florin Andrei's answer](https://astronomy.stackexchange.com/a/2192) to that question provides a link to "a list of 20th century eclipses, with a measure of the magnitude of the umbra", which is partially relevant, but it's a further step to then calculate the irradiance.
Chappo Hasn't Forgotten
<the-moon><astrophysics><lunar-eclipse>
What irradiance does the Moon receive during a total lunar eclipse?
CircleSquared
# What irradiance does the Moon receive during a total lunar eclipse?

(Asked by: username_0 on 2018-09-07)

As we know, the total solar irradiance normally received by the Moon is about 1361 $W/m^2$ (the same as the top of the Earth's atmosphere). Of course, this changes during lunar eclipses, *but the Moon doesn't go completely dark* even during total eclipses (it's usually just a dim red). It still receives light through the Earth's atmosphere. My question is how much?

I've tried googling this, but all the search terms I could think of just gave me a bunch of unrelated astrology results. *Surely people have measured this.*

The worst case irradiance for any part of the Moon during a total eclipse is fine for my purposes, but I wouldn't mind a paper with an intensity curve over time if there are any.

Edit:  
This is not a duplicate of "[Red Moon a Characteristic of all Total Lunar Eclipses?](https://astronomy.stackexchange.com/questions/2166)" That question is asking if all lunar eclipses are red and/or if the amount of redness varies. I'm asking how much light hits the Moon (per second per area) during a total lunar eclipse. Furthermore, none of the answers there even mention irradiance.

**Tags:** the-moon, astrophysics, lunar-eclipse

> **Comments:**
> **username_1**: Possible duplicate of [Red Moon a Characteristic of all Total Lunar Eclipses?](https://astronomy.stackexchange.com/questions/2166/red-moon-a-characteristic-of-all-total-lunar-eclipses)  
> **username_2**: @username_1 That question is about redness rather than irradiance. [Florin Andrei's answer](https://astronomy.stackexchange.com/a/2192) to that question provides a link to "a list of 20th century eclipses, with a measure of the magnitude of the umbra", which is partially relevant, but it's a further step to then calculate the irradiance.  

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## 1 answers

### Answer by username_3 on 2018-09-09. Score: 3

The following is more a series of observations than a full answer, but I hope other people can fill in the details.

The [apparent magnitude](https://en.wikipedia.org/wiki/Magnitude_(astronomy)) of the full moon is -13. The apparent magnitude of the moon during a lunar eclipse will differ from eclipse to eclipse (correlated to the [Danjon scale](https://en.wikipedia.org/wiki/Danjon_scale) value), but let's suppose it is 0 for a certain eclipse. (Note that this magnitude is different from the (pen)umbral magnitudes mentioned on the [NASA website](https://eclipse.gsfc.nasa.gov/LEcat5/LE2001-2100.html); these magnitudes can be computed and do not depend on the Earth's atmosphere.)

This means the full Moon is normally reflecting $100^{13/5}$ times more light than during this eclipse. I'm not sure if the [albedo](https://en.wikipedia.org/wiki/Albedo) of the Moon is different for red light than it is for normal sunlight, but if not, that means the answer is $\frac{1361}{100^{13/5}} \frac{W}{m^2} \approx 8.6 \frac{\text{m}W}{m^2}$.


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