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Night-blooming cereus are plants that only bloom at night. Their light-sensitive cells must be able to tell the difference between sunlight, and sunlight reflected off the moon.

What is the physical difference between sunlight, and sunlight that has been reflected? Can we say the difference is in a distinct property (like irradiance or wavelength)?

Edit: for anyone interested in the interdisciplinary biological aspect of this question, I think you might be interested to know about circadian factors in cells. My initial assumption was that circadian biology might have the answer to the mechanism to which the cereus distinguishes between night and day.

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    $\begingroup$ The intensity of moonlight is way smaller than that of sunlight, what can be observed by going outside on a full moon and comparing the light to that of an ordinary day. $\endgroup$
    – Lucas Baldo
    Jun 12, 2021 at 23:13
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    $\begingroup$ The Wikipedia article that you cited doesn't say anything about the Moon. Why do you think that moonlight has anything to do with it? I don't know how the cacti know, but I don't need to see the Moon to know when it's night time. $\endgroup$
    – Solomon Slow
    Jun 12, 2021 at 23:28
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    $\begingroup$ The premise of this question is flawed: there is no correlation between moon being up and the nighttime. $\endgroup$
    – void_ptr
    Jun 12, 2021 at 23:54
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    $\begingroup$ This is actually a very insightful correction $\endgroup$
    – PineappleThursday
    Jun 13, 2021 at 3:43
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    $\begingroup$ @void_ptr Thus subsitute "presence of moonlight" with "absence of sunlight"? $\endgroup$
    – David Tonhofer
    Jun 13, 2021 at 13:03

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By far the primary physical difference is intensity- moonlight is much less bright than sunlight.

Such flowers bloom when the light intensity is low- it does not matter if they are in moonlight or a greenhouse.

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Moonlight is quite reddened compared to Sunlight. The lunar albedo is roughly 0.07 at 400 nm and double that or 0.14 at 700 nm.

It doesn't look terribly red because our blue and red receptors aren't pinned at those extremes, but for any given plant you would have to look at the specific receptor molecules at play and what their sensitivities are. See for example Are two colors (red + blue) necessary for LED grow lights, or would either color be sufficient? in Biology SE and some of the plots there.

From this answer to Why doesn't a full/gibbous moon high in the sky ever seem to look orange? Shouldn't it? and also this answer to Why is this moon red?

enter image description here

above: "Figure 8: Averaged geometrical moon albedos measured by GOME from July 1995, November 1995, and September 1996." From ESA's GOME moon measurements, including instrument characterisation and moon albedo. below: From this answer and obviously the EPIC camera of the DSCOVR satellite; one of the famous "Moon photobombs Earth" photos.

Moon photobombs Earth

Buzz Aldrin carries the EASEP

above: "Buzz Aldrin carries the EASEP." from here

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    $\begingroup$ The "Moon photobombs Earth" picture is jaw-dropping. Thanks. $\endgroup$
    – Eric Duminil
    Jun 14, 2021 at 13:20
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Night-blooming cereus are plants that only bloom at night. Their light-sensitive cells must be able to tell the difference between sunlight, and sunlight reflected off the moon.

No, they don't. If you had to design a "night-detector", what would be easier:

  • detect $\frac{0.001\mathrm{W}}{\mathrm{m}²}$ of possibly silvery white, possibly polarized moonlight,
  • or detect that you received $\approx \frac{1000\mathrm{W}}{\mathrm{m}²}$ of irradiance a few hours ago, and now you don't get anything over $\frac{0.01\mathrm{W}}{\mathrm{m}²}$? You could also check that the temperature is lower and humidity is higher than before, just to be sure.

Finally, your moonlight-detector would be broken half of the time. As mentioned by @void_ptr: "there is no correlation between moon being up and the nighttime". The new moon is below the horizon all night long.

Just for fun, here's a plot of moonlight visibility in Arizona in 2021:

enter image description here

By the way, biology can be really strange and surprising sometimes, so it's actually possible for those plants to use the full-moon as a night-time indicator. I just wanted to make it clear that they don't necessarily have to.

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In addition to the other answers, an important physical difference is the perceivable inconsistency between the apparent intensity of the light source (moon vs. sun), and the spectral (color) distribution.

Of course, this requires the (obviously wrong) conjecture, that both are thermal light sources, which behave approximately according to the black body radiation profile (Planck's law).

In colloquial terms: an object as big as the moon, if it were a thermal light source, that emits only such a small amount of light in the direction of the observer, would be much cooler than the sun (see Stefan-Boltzmann law), and hence, would have a lot more reddish tint to the color of its light. Within certain limits we are able to perceive different color temperatures, and possibly a plant could do that too, given appropriate cells or chemistry. Hence, as our brain would naively expect the moonlight to be much redder, if it were a thermal light source, this is why we can subjectively tell that the moon is not a thermal light source.

Whether the biological species you mention is capable of comparing intensity with color, this I do not know. And the big question is, what is the benefit of evolving such a complicated mechanism, when probably it is for the plant much biologically easier to compare intensity with a given threshold value.

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    $\begingroup$ I don't perceive it as pale yellow. The moon to be looks to be "silvery white", perhaps even a slightly cool white. $\endgroup$
    – James K
    Jun 13, 2021 at 15:27
  • $\begingroup$ @JamesK: Well, I do, but that's why I weakened the sentence to "possibly", because there seems to be so much going on: subjective color perception, Rayleigh scattering in the earth's atmo, absorption by the moon's surface. If somebody provides proof that the subjective bias I have alleged points into the wrong direction, I will correct it. $\endgroup$
    – oliver
    Jun 13, 2021 at 15:33
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    $\begingroup$ @oliver: Yes, unless it's low in the sky and/or there's a lot of smoke/haze in the air, it appears silvery white. (The very rare blue moon - which is a real physical phenomenon, not the widespread calendar myth - being a notable exception.) Perhaps you live in an area where smog &c is the norm? $\endgroup$
    – jamesqf
    Jun 13, 2021 at 15:50
  • $\begingroup$ @JamesK I've seen the moon as both colours. And obviously the red harvest moon. $\endgroup$
    – DKNguyen
    Jun 13, 2021 at 15:58
  • $\begingroup$ @jamesqf: luckily not :-) But as I said, I am more targeting to the perceptual causes of the moon's color, and that may be partly a personality trait. $\endgroup$
    – oliver
    Jun 13, 2021 at 15:59
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I have already remarked in the comments that the blooming of this species is actually correlated with the moon cycle, and not symply to the absence of daylight (as usual circadian rhythms).

A bit more literature search shows that, indeed, it is the intensity of the light which is the key factor here:

Circa-monthly rhythms triggered or synchronized by the 29.5 day lunar cycle of nighttime light intensity, or specifically the light of the full moon, although explored in waterborne and certain other species, have received far less study, perhaps because of associations with ancient mythology and/or an attitude naturalistic studies are of lesser merit than ones that entail molecular mechanisms.

A relevant post in biology SE

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  • $\begingroup$ If it's indeed the case, it's fascinating and pretty hard to believe. It might be worth asking a question on biologySE. Simple absence of sunlight could not explain why many flowers apparently bloom during the same night. It could be explained with thresholds of irradiance/temperature/humidity, though. $\endgroup$
    – Eric Duminil
    Jun 15, 2021 at 6:31
  • $\begingroup$ @EricDuminil Indeed, let's do it: biology.stackexchange.com/q/101227/59521 $\endgroup$
    – Roger V.
    Jun 15, 2021 at 7:32
  • $\begingroup$ Thanks. A few points : I think you're jumping a bit too fast on conclusions: "The claim that is easily disproved", "this flower blooms at full moon"... I'm not sure that we know this. As far as I can tell, you've found two papers which might indicate that it's the case. I'm not sure that (r = +0.59 to +0.91) is a "strong correlation". Anyway, let's see what happens on biologySE. You've got many typos there, BTW. $\endgroup$
    – Eric Duminil
    Jun 15, 2021 at 7:36
  • $\begingroup$ @EricDuminil Indeed. I adjusted the wording of my question. $\endgroup$
    – Roger V.
    Jun 15, 2021 at 7:40
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  1. The physical difference between sunlight and the light scattered off of the moon aka moonlight is that it is slightly linearly polarized in nature (cite:https://doi.org/10.1007/978-3-662-09387-0_8).

  2. However, I am not sure if night blooming cereus uses this.

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    $\begingroup$ Polarization of light can be caused by specular reflection or Rayleigh scattering. The scattering of sunlight from the moon's surface is diffuse reflection and does not generally cause any significant polarization as far as I'm aware. So I think point 1. is not correct. If you google "polarized moonlight" you may find some mentions, but they are talking about the polarization of the moonlit sky due to Rayleigh scattering in the atmosphere which is exactly the same as for the sunlit sky. $\endgroup$
    – Emil
    Jun 13, 2021 at 15:29
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    $\begingroup$ @Emil It is slightly linearly polarized (cite: link.springer.com/chapter/10.1007%2F978-3-662-09387-0_8). $\endgroup$
    – vasanth
    Jun 14, 2021 at 10:20
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    $\begingroup$ Emil is right, it's not moonlight that's polarized, your link also talks about polarization of the sky, not the lunar disk. $\endgroup$
    – Ruslan
    Jun 14, 2021 at 22:38
  • $\begingroup$ @vasanth, thanks for the reference. The second paragraph indeed describes how the direct moonlight can be weakly polarized depending on the moon phase. So evidently the reflection of light from the moon surface is not fully diffuse. Unfortunately the cited original source is behind a paywall. Very interesting. I retract my objection. :) $\endgroup$
    – Emil
    Jun 15, 2021 at 8:34
  • $\begingroup$ Thank you for the retraction @emil. $\endgroup$
    – vasanth
    Jun 15, 2021 at 8:50

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