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Scorpio Supermoon (UV)

Doug Kerr

Well-known member
Hi Asher,

That does not look edible any more! Give me back my Swiss cheese moon!

Asher
Indeed.

While the subject of the "supermoon" is up, let me put on my photometric hat and point out a serious flaw is the commonly-reported description of this phenomenon.

It is generally said that the moon in this state "has an apparent diameter of about 14% greater than 'normal' and a brightness about 30% greater than 'normal' ".

Indeed, the moon in this state has an apparent diameter about 14% greater than 'normal', but of course its "brightness" (luminance) is essentially unchanged from 'normal'. There is no geometric nor other phenomenon that would change the brightness of the moon. In its "super" appearance is is illuminated by the customary light source (the sun) at only a minuscule difference in distance from it.

Note that luminance ("brightness") can be considered to be the amount of luminous intensity "emitted" per unit area of the surface as we see it. If the illuminated object is moved closer to us, the overall geometry is such that we see the "brightness" as unchanged. (The illuminance upon the retina of the eye is unchanged.)

What is, photometrically, about 30% greater than normal is the total luminous intensity of the entire lunar disk as we see it. In the "super" state, that disk subtends to us about a 30% greater solid angle than "normal" (it is apparently a disk of the same luminance but of 30% greater size). Thus the total luminous intensity (toward us) of the disk is about 30% greater than "normal".

Some attempts to describe the situation more accurately then "the moon will be about 30% brighter than 'normal' " say, "The total light from the moon will be about 30% greater than 'normal' ". Sadly, that does not do it either. The total luminous flux ("light") emerging from the illuminated moon in its "super" state is essentially exactly the same as "normal".

But in fact the total luminous flux reflected from the moon that strikes the Earth is (as a first order approximation) about 30% greater than "normal". (Just as is the total amount of luminous flux from the moon that passes through the iris into our eye.) But that's not because we have a "brighter" object - just one of the same brightness that is, to us, and to "the Earth", "larger".

The situation is no different than if we were to first hold a 7" diameter white disk in the sun and then later hold an 8" diameter disk of the same material in the sun. The second disk would not be "brighter".

Best regards,

Doug
 

Asher Kelman

OPF Owner/Editor-in-Chief
Doug,

If the distance to the sun is reduced at any time, then, (if somehow we could distinguish this change), the moon will appear brighter, no matter where it is. The question is not whether or not we could measure that difference, (because I would wager that we could), but whether our eyes could distinguish the minute change.

Do we know?

Asher
 

Doug Kerr

Well-known member
Hi, Asher,

Doug,

If the distance to the sun is reduced at any time, then, (if somehow we could distinguish this change), the moon will appear brighter, no matter where it is. The question is not whether or not we could measure that difference, (because I would wager that we could), but whether our eyes could distinguish the minute change.
Well, the distance from the sun to the moon does vary, cyclically. Considering when the moon is in the "full moon" phase (when we see essentially its full disk), its distance from the sun varies from about 146.4 M km to 152.4 M km.

In that case, we can expect that the difference in the moon's luminance over this cycle is about 1:1.070; that is, at its greatest it is about 7% greater than at its least. In photographic terms, that is about 0.1 stop.

I'm sure that could be quite readily measured (and I'm sure has been).

Note that the luminance as we see it is also affected by atmospheric absorption, which among other things is affected by the altitude of the moon.​

Even if we were able to have both moons present side by side, could we distinguish that difference? Not likely. We certainly would have no awareness of that change during the actual "cycle".

Best regards,

Doug
 

Asher Kelman

OPF Owner/Editor-in-Chief
Hi, Asher,



Well, the distance from the sun to the moon does vary, cyclically. Considering when the moon is in the "full moon" phase (when we see essentially its full disk), its distance from the sun varies from about 146.4 M km to 152.4 M km.

In that case, we can expect that the difference in the moon's luminance over this cycle is about 1:1.070; that is, at its greatest it is about 7% greater than at its least. In photographic terms, that is about 0.1 stop.

I'm sure that could be quite readily measured (and I'm sure has been).

Note that the luminance as we see it is also affected by atmospheric absorption, which among other things is affected by the altitude of the moon.​

Even if we were able to have both moons present side by side, could we distinguish that difference? Not likely. We certainly would have no awareness of that change during the actual "cycle".

@Doug,

So it's about 0.1 of a stop; not much at all! But if one views onlythe brightest section of the gray scale, one should be spreading out our light perception over a much narrow range of brightness. Do we know the limits of our ability to reset the range of the numbers of shades of light qe can distinguish?

@Klaus,

I hope you can forgive me going so much off topic, but your creative UV image got me thinking about how we observe things!

Asher
 

Doug Kerr

Well-known member
Hi, Asher,

@Doug,

So it's about 0.1 of a stop; not much at all! But if one views only the brightest section of the gray scale, one should be spreading out our light perception over a much narrow range of brightness.
Dunno about that.

Do we know the limits of our ability to reset the range of the numbers of shades of light qe can distinguish?
No. I don't know much about that. I will perhaps poke around in that area.

Best regards,

Doug
 
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