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Hekate's Lair
2007-Aug-15, 11:38 AM
I am writing astronomical presentations for the planets in my website and while I was searching some info about Venus I run across this:
http://www.science-frontiers.com/sf137/sf137p02.htm

Could someone please explain some more about this issue? The ashen light of venus is a fact or not?

Romanus
2007-Aug-15, 01:35 PM
The Ashen Light has been reported for a long, long time; unfortunately, you'll find no easy answers here, for IIRC, the jury is still out on whether it's real or a contrast effect; to my knowledge it has never been observed by a space telescope or space probe. Even for those who assume it's real, no one can agree on what causes it.

A book I recommend that may talk a bit more about it (though I haven't read it in a while) is Patrick Moore's Venus, which recently went through a new edition.

Kyle Edwards
2007-Aug-15, 01:55 PM
I know there have been pictures taken of the night side of Venus in IR, because of the heat it radiates, but I do not know about in visible light.

Venus is going to be at inferior conjunction Friday, and I have been looking at and imaging Venus in the last few days, and have not seen the Ashen Light at all. It does not show up in my images where the crescent is overexposed.

grant hutchison
2007-Aug-15, 05:09 PM
There was some discussion of the evidence and theory underlying the ashen light on this thread (http://www.bautforum.com/questions-answers/60052-why-can-i-see-dark-side-venus.html).

Grant Hutchison

Hekate's Lair
2007-Aug-15, 06:21 PM
Thanks a lot, now I got a better idea of the whole thing.

It seems that ashen light of Venus is visible sometimes but not all times.

As for what could cause it, I would be clueless but when I read this in the other ashen light thread


There was some recent research on this, as I recall reported in New Scientist, that the Venusian atmosphere actually glows. A useful rule of thumb is that if something is glowing a dull red, it's at about 1000 degrees, for practical purposes regardless of material.

I remembered something I'd read in an old astronomy book, that the atmosphere of Venus glows.

We do not have to depend upon the spectroscope for evidence that Venus has a dense atmosphere, for we can, in a manner, see her atmosphere, in consequence of its refractive action upon the sunlight that strikes into it near the edge of the planet's globe. This illumination of Venus's atmosphere is witnessed both when she is nearly between the sun and the earth, and when, being exactly between them, she appears in silhouette against the solar disk. During a transit of this kind, in 1882, many observers, and the present writer was one, saw a bright atmospheric bow edging a part of the circumference of Venus when the planet[Pg 55] was moving upon the face of the sun—a most beautiful and impressive spectacle.

Even more curious is an observation made in 1866 by Prof. C.S. Lyman, of Yale College, who, when Venus was very near the sun, saw her atmosphere in the form of a luminous ring. A little fuller explanation of this appearance may be of interest.

When approaching inferior conjunction—i.e., passing between the earth and sun—Venus appears, with a telescope, in the shape of a very thin crescent. Professor Lyman watched this crescent, becoming narrower day after day as it approached the sun, and noticed that its extremities gradually extended themselves beyond the limits of a semicircle, bending to meet one another on the opposite side of the invisible disk of the planet, until, at length, they did meet, and he beheld a complete ring of silvery light, all that remained visible of the planet Venus! The ring was, of course, the illuminated atmosphere of the planet refracting the sunlight on all sides around the opaque globe.

http://www.gutenberg.org/files/18431/18431-h/18431-h.htm

Hekate's Lair
2007-Aug-16, 09:58 PM
Are there any contemporary photos that show ashen light of Venus?

RJ Emery
2007-Aug-17, 01:15 AM
I am writing astronomical presentations for the planets in my website and while I was searching some info about Venus I run across this:
http://www.science-frontiers.com/sf137/sf137p02.htm

Could someone please explain some more about this issue? The ashen light of venus is a fact or not?

Venutian ashen light has not been shown to exist and probably does not exist. While human observers claim to have seen it, we do not know what instruments they were using at the time.

I read recently that Percival Lowell, when he thought he was seeing canals on Mars, was actually seeing the blood vessels of his own retina, the result of the particular optics he was using. That may also be the explanation for the structure shown in the 1897 sketch accompanying the reference you cited.

Ashen light may be nothing more than an artifact of the observer's own eye coupled with the specific instrument being used to view the planet.

RJ Emery
2007-Aug-17, 11:43 AM
I am writing astronomical presentations for the planets in my website and while I was searching some info about Venus I run across this:
http://www.science-frontiers.com/sf137/sf137p02.htm

Could someone please explain some more about this issue? The ashen light of venus is a fact or not?
In the source that you cite, there is a reference to the article "Case for 'Ashen Light' Weakens," Sky & Telescope, May, 2001, p. 27. Did you read that article?

George
2007-Aug-17, 04:39 PM
This topic still intrigues me.

I have recalculated the amount of light an observer on Earth should see coming from Venus based on the same formula from the thread Grant referenced. Assuming no gain from the proposed cloud-dish effect, and using more accurate values for radius and distance, as well as a guess value of 1150 w/m2 (mid-range between arriving solar wattage and surface wattage), the returning wattage from Earthshine is 1.45x10-13 watts/m2, or 4x10-18 watts per entrance pupil to an eye opened to 6mm diameter.

In terms of photons due to Earthshine, using 550nm average wavelength which yields a coincidental value of 4x10-18 Joules/photon, the eye will receive about 1 photon per second per sq. meter on Venus due to Earthshine from Venus.

The surface area of Venus is about 460 million sq. km. A crescent Venus might have say 180 million sq. km of shadowed region. This is 1.8x1014 sq. meters. Thus, an equal number of photons should be entering our eye for the entire Earthshined dark region of Venus.

I am bound to be doing something wrong, but I am stuck with other issues at present. I will review this later and likely edit it, unless another can fix it. Yet, I wanted to kick this Earthshine idea around some more.

Shooting from the hip, I wonder if cloud structures on Earth, at times, act to focus the light. Clouds form vertically and the planets are spherical. If a large central region were clear, say the Pacific ocean and it facing the Sun, with an outer ring of vertical cloud structures, wouldn't the effect of Mie Scattering and a dish-shaped reflective clouds increase the gain of light reflecting toward the solar direction, hence increasing the Earthshine upon Venus?

Hekate's Lair
2007-Aug-17, 06:33 PM
In the source that you cite, there is a reference to the article "Case for 'Ashen Light' Weakens," Sky & Telescope, May, 2001, p. 27. Did you read that article?

No, I only saw the basic points as mentioned in the source. Is there some link to it perhaps?

George,

excuse my ignorance (physics isn't my subject) but what exactly do you mean?

I am thinking...Venus' thick cloud layers are highly reflective. Maybe this, in compination with some optics issue from the Earth (atmosphere, clouds, light reflection...) causes the phenomenon that several have observed but telescope images cannot reproduce.

George
2007-Aug-17, 06:51 PM
George, excuse my ignorance (physics isn't my subject) but what exactly do you mean?

I am thinking...Venus' thick cloud layers are highly reflective. Maybe this, in compination with some optics issue from the Earth (atmosphere, clouds, light reflection...) causes the phenomenon that several have observed but telescope images cannot reproduce.
The Ashen light is either reflected light, light created in the atmosphere (eg lightning), or a psychological effect the observer sees.

We can observe the Earthshine acting on the dark side of the Moon that faces us, but I am exploring the possibility that the Earth, along with the other reflected light sources (asteroids, Mars, Jupiter, stars, etc.) could also shine brightly enough to weakly illuminate Venus dark side (ie Ashen light).

The prior thread I concluded that the light is too faint from Earth to offer any hope for an explanation, but I had a spare moment so I played with it here and now I am not so sure. Tonite, at home, I will review what I've learned on photon threshold limits. Grant, hopefully, will step in as he is quite good with this.

[Added: Oh, and the cloud focusing effect could only be a slight contributor since clouds, even in an ideal configuration, are not that tall so offer only a slight possible increase in light flux.]

RJ Emery
2007-Aug-17, 08:00 PM
Hekate,


I read recently that Percival Lowell, when he thought he was seeing canals on Mars, was actually seeing the blood vessels of his own retina, the result of the particular optics he was using. That may also be the explanation for the structure shown in the 1897 sketch accompanying the reference you cited.

Concerning the above, I now recall where and what I read. It was in Neil DeGrasse Tyson's book, Death by Black Hole, 2007, Chapter "Planet Parade," p. 79. Tyson wrote:

An optometrist from Saint Paul, Minnesota, named Sherman Schultz wrote a letter in response to an article in the July 2002 issue of Sky and Telescope magazine. Schultz pointed out that the optical setup Lowell preferred for viewing the Venutian surface was similar to the gizmo used to examine the interior of patients’ eyes. After seeking a couple of second opinions, the author established that what Lowell saw on Venus was actually the network of shadows cast on Lowell’s own retina by his ocular blood vessels. When you compare Lowell’s diagram of the spokes with a diagram of the eye, the two match up, canal for blood vessel. And when you combine the unfortunate fact that Lowell suffered from hypertension—which shows up clearly in the vessels of the eyeballs—with his will to believe, it’s no surprise that he pegged Venus as well as Mars with teeming with intelligent, technologically capable inhabitants.
You seem to be focusing on the theoretical and ignoring the systematic errors and distortions introduced by any observing apparatus.

--

Send me a private message with your e-mail address, and I will respond with the S&T piece referenced in the source article you cited. It is one page long.

Hekate's Lair
2007-Aug-17, 08:52 PM
That's right, I concentrate on what could be the possible cause of the phenomenon- and that's theoretical thinking. If there were only a couple of reports, I would definitely think that possible causes could be the instruments or something in the eyes of the observers. But there seem to be too many reports about the ashen light (existing or hypothetical), and in this case it is irrational to think that all the observers had something wrong with their eyes... or their instruments were not adequate... or even to consider some psychological explanation.

So I thought that it has to do with the light. Light from earth, from venus, from a combination of both? Do weather conditions play also a role? I wouldn't know, without more information. I read a lot of ancient hellenic astronomy (original texts only) and there was a whole branch of astronomy called "optics" which dealt with observational issues - how the eye sees, how the light reaches the eye, what distortions are made by distance, weather, atmosphere conditions etc. and what possible illusions could happen during observation... lot's of interesting stuff.

Thanks for the info on P. Lowell - it is quite a reasonable explanation in his case, regarding the diagram he drew for the lighted side of venus.

RJ Emery
2007-Aug-17, 10:47 PM
That's right, I concentrate on what could be the possible cause of the phenomenon- and that's theoretical thinking. If there were only a couple of reports, I would definitely think that possible causes could be the instruments or something in the eyes of the observers. But there seem to be too many reports about the ashen light (existing or hypothetical), and in this case it is irrational to think that all the observers had something wrong with their eyes... or their instruments were not adequate... or even to consider some psychological explanation.
Frankly, I think the number of ashen light claims to be few and far between -- an extremely small percent compared to all the observers over the centuries who couldn't see the glow, often at the same time and near proximity of those who claimed they could. Moreover, wherever and whenever a claim was made, there never was any independent verification. It would be great if there were at least one claim substantiated at the same time in a far different place, but there weren't any.

Visual claims of the glow are overwhelmingly unsubstantiated and rest solely in the eye and mind of the beholder, just like Lowell was the only one who could see canali features on Mars and Venus.

George
2007-Aug-18, 12:38 AM
Perhaps physics does offer a reasonable answer, afterall. The Earth's light, augmented by other light sources, might just be enough to illuminate the dark side of Venus.

See if this is correct....

The Solar Constant is well established at 1366 w/m2.

This is the amount of light energy received at Earth at 1 a.u. This is also the entire spectrum, so it is a little bit high of a number for our use since we will only want the visible portion of the spectrum, though it does comprise the majority of the Solar spectrum.

The amount of light reflected from the Earth can be determined by Earth's albedo. It is 0.367, or 36.7% of the light hitting the Earth is reflected back into space.

Multiplying the two, yields a reflected light value of 501 w/m2.

The amount of this light that reaches Venus is easily determined:
The inverse square law is used by squaring the Earth's radius divided by the distance to our crescent-phased Venus (say 42 x106 km).

Thus, arriving light from Earth, Earthshine, = (501)(6380/42x106)2
= 1.16x10-5 w/m2.

[If this concept is awkward, think of light coming from the center of the Earth and providing the 501 w/m2 just as it leaves our atmosphere. Then consider the sphere around the Earth at a radius that would touch Venus at the 42x106km. The 501 value gets spread out over this entire distant surface which varies as the square of the distance to this spherical shell. This is because the surface area of a sphere varies as the square of the radius. Thus, the ratio of the spherical areas is all we need, or, more direct, the ratio of the squares of the radii.]

Now we need to calculate how much light is reflected from Venus, which becomes the light we can see due to Earthshine. This is simply the albedo times the received amount of light. Venus has a very high albedo of 0.65 (65% of incoming light is reflected).

Reflected light from Venus due to Earthshine = (1.16x10-5)(0.65)
= 7.5x10-6w/m2.

This is now the value for the surface brightness of the dark side of Venus.

I have to convert to candela power to get a handle on how this might appear to the eye.

1 candela = 1/683 watts (@ 550nm, but were close enough)

Thus, Venus has a surface brightness of 4.8 x 10-3 cd/m2. [(683)(7.5x10-6w/m2).

However, this is as we would see it if we were in Earth's orbit. We must cut it by, say, 25% if we are viewing terrestrially. We should also cut this value down some because not all this light energy is in the visible spectrum.

Let's say a better value is only 20% of the original result. We are still around one thousandth of a cd/m2.

This value is within the scotopic vision of our eye (the region where the very sensitive rods work and color is not obtainable). If the physics above is correct, the question becomes why Ashen light isn't always seen. So, before anyone swallows what I offer, just keep chewin' until the big guns take a look at it, else you may be spittin' back up like I often do. :)

Perhaps the intense surface brightness of Venus on the Solar illuminated side washes-out the improved dark vision ability of the rods, thus diminishing the ability to see the Ashen light.

RJ Emery
2007-Aug-18, 01:04 AM
George,

Your calculations notwithstanding, if earth's albedo were the source of Venus' supposed ashen light, then it would be a readily reproducible phenomenon able to be captured on photographic film, plates or with CCD cameras. If one person saw it, then hundreds if not thousands of others would have also seen it at roughly the same time in different parts of the globe.

None of the few and isolated claims that exist were ever verified, substantiated or confirmed. I still maintain the best explanation for what these observers may have seen rests with their optical devices.

George
2007-Aug-18, 01:54 AM
You certainly could be correct, and it is the very question I pose.

I wish I could say just how much havoc the extremely bright crescent light creates in imaging. Overexposure I would bet is a real problem. Have any attempts been made to image only the dark side without the glare of the illuminated crescent?

Playing with some values... the surface brightness of the crescent of Venus could be around 500,000 cd/m2. The intensity of the Sun, according to some quick work, at the orbital distance of Venus is around 2.3 x 106 cd/m2 (using a 5777K eff. surface temp. for the Sun). Cutting that by more than half for the visible portion only and multiplying this by the 65% albedo, will exceed my estimate of 500K for Venus. However, I do not know enough about how albedo measurements are determined to say what any reflectance value would be off a limb region. Nevertheless, the contrast is probably close to a billion times and may play a significant roll in diminishing the dark side of Venus. I wish I could say. Hopefully, someone around here does know these imaging issues.

George
2007-Aug-18, 02:13 AM
I did a little quick search and found this site (http://www.pacifier.com/~tpope/Venus_Page.htm) which imaged Venus at exposure times under 1/100 of a second. It is not surprising the Ashen light would not be captured in normal Venus images.

I also found this (http://www-ssc.igpp.ucla.edu/personnel/russell/papers/ashen/) which states...

Known as the Ashen Light these emissions were first reported by Giovanni Riccioli on January 9, 1643 /1/ and have been observed on many hundreds of occasions since. For example, Levine /2/ reports 129 sightings made between 1954 and 1962. Ashen Light has been observed simultaneously and independently by 2 professional astronomers at least once /3/ and by up to 4 independent amateur observers on many occasions /4/. ] this

The numbers are the footnotes, of course, that allow verification of their claim.

Are you sure the Ashen light claims are so unsubstantiated?

RJ Emery
2007-Aug-18, 02:32 AM
You certainly could be correct, and it is the very question I pose. ... Have any attempts been made to image only the dark side without the glare of the illuminated crescent?
I would have no idea. There are great unsolved problems in astronomy, astrophysics and cosmology, and then there are far lesser ones like the ashen light controversy.

I looked at this ashen light issue some time ago, thinking it was perhaps something within the realm of solving by dedicated amateurs. However, after reviewing the works of professionals on this topic, I came to the conclusion it is a nonexistent problem. Until someone, somewhere can produce incontrovertible evidence of its existence, attention is better spent elsewhere.

RJ Emery
2007-Aug-18, 02:57 AM
... Are you sure the Ashen light claims are so unsubstantiated?
Yes. Follow the footnotes yourself and see how rock solid the claims are. Of particular interest would be a description of the optics used when said observations were made.

George
2007-Aug-18, 02:01 PM
Yes. Follow the footnotes yourself and see how rock solid the claims are. Of particular interest would be a description of the optics used when said observations were made.
Verifiable evidence seems somewhat lacking, but it is likely the Ashen light is real.

D.P. Cruikshank (NASA) seems convinced it is real and has a nice paper on it which includes a drawing from R. M. Baum in this paper here (http://articles.adsabs.harvard.edu//full/1992ASPC...33...43C/0000043.000.html).

Apparently, Napier in 1971 offered the Earthshine idea, too. [No surprise.] Yet, it did not gain much favor.

Hornblower
2007-Aug-20, 01:54 AM
I do not think Earthshine on Venus would make it, even near inferior conjunction.

Here is my rough and dirty way of estimating the brightness:

Full Earth as seen from Venus at inferior conjunction is about magnitude -7, which is about 21 magnitudes fainter than the Sun as seen from Venus. To estimate that brightness, I started with the full Moon at about -12.7. From Venus it would be 100 times the distance, bringing it down a factor of 10,000 or magnitude -2.7. The Earth, with its higher albedo and size, should be roughly 50 times the brightness, or a little over 4 magnitudes.

Venus at superior conjunction full phase is near -4. Bring it in to inferior conjunction distance with the same luminance, it would be about -8.

Earthshine at that distance would be down about 21, for a magnitude of roughly +13, spread out over about a square arcminute. It would be fainter at a large enough elongation to get the planet into a dark sky.

Deep sky fuzzies that faint are a challenge to see even in a dark sky, with no bright objects in the field of view. I am reasonably certain, based on my own observing experience, that the glare from the sunlit portion of Venus would obliterate it.

If the ashen light is real, it thus is my opinion that it must be vastly brighter than Earthshine on Venus.

RJ Emery
2007-Aug-20, 02:33 AM
Three main theories for ashen light are:

1) Glowing rocks from the hot Venetian surface made visible by a rare thinning of clouds in the otherwise thick atmosphere,
2) An aurora type display, even less plausible as it is generally recognized that a magnetic field is necessary to concentrate solar particles to make an aurora visible (Venus has no magnetic field), and
3) A myth, perpetuated by those with vivid imaginations and poor eyesight or optics or both.

I favor the last theory.

With respect to the aurora display, Venus' atmosphere also works against this theory, as it lacks the chemical composition that enables auroras on earth to generate their effect.

George
2007-Aug-20, 05:48 PM
Full Earth as seen from Venus at inferior conjunction is about magnitude -7, which is about 21 magnitudes fainter than the Sun as seen from Venus. To estimate that brightness, I started with the full Moon at about -12.7. From Venus it would be 100 times the distance, bringing it down a factor of 10,000 or magnitude -2.7. The Earth, with its higher albedo and size, should be roughly 50 times the brightness, or a little over 4 magnitudes.
I crunched your Moon reference and got a -7.00. Nice rough estimate you have!

Combining a full Moon to augment the light illuminating Venus only changes it to -7.02. If we happen to have alignment with Mars and Jupiter, too -- do I look I am stretching this :) -- yields only a combined mag. of -7.037. Not much help. Maybe a Pacific cyclone in the summer, when the Antarctic ice is more exposed toward Venus, will improve Earth's albedo. :)


Venus at superior conjunction full phase is near -4. Bring it in to inferior conjunction distance with the same luminance, it would be about -8.

Earthshine at that distance would be down about 21, for a magnitude of roughly +13, spread out over about a square arcminute. That matches fairly close to the surface brightness numbers I have, if the 500,000 cd/m^2 is compared to about 1 thousandth of a cd/m^2 as mentioned in the earlier post.


Deep sky fuzzies that faint are a challenge to see even in a dark sky, with no bright objects in the field of view. I am reasonably certain, based on my own observing experience, that the glare from the sunlit portion of Venus would obliterate it.
That makes sense. However, what about at 200x when Venus is over 3 arc degrees in diameter. If most or all of the bright region were kept outside the field of view, and the observer had his or her full adative vision, could the Ashen light be possible? Yet, this method may not have been used.

Assuming adaptive vision is needed, is adaptive vision lost instantly by all observers when seeing something with a surface brightness in the 500,000 cd/m^2 range?


If the ashen light is real, it thus is my opinion that it must be vastly brighter than Earthshine on Venus. You are likely right. The atmospheric reductions in brightness for Venus near conjunction will further reduce Earthshine's surface brightness close to the eye's minimal scotopic threshold. Since it is still within this range, perhaps Earthshine augments whatever might be happening in the Venutian atmosphere.

The mystery lingers.

tony873004
2007-Aug-20, 07:57 PM
Let me give a shot at the logic and numbers:


Earthshine is easily visible on a crescent Moon with the naked eye.
Venus has a higher albedo than the Moon, thus increasing the visibility of Earthshine
Venus is further than the Moon, thus decreasing the visibility of Earthshine.
You need to look through a telescope to see Earthshine on Venus. The naked eye works fine for the Moon.
A telescope's objective lens or mirror is a much larger collector of light than the pupil of the eye, making the Earthshine on Venus more obvious.
A telescope also magnifies the image, which spreads its light out over a greater area, making the Earthshine on Venus less obvious.



making a formula with all these factors:

(Venus Albedo / Moon Albedo) * (Moon Distance / Venus Distance)2 * (Diameter of Primary Mirror / Diameter of Eye Pupil)2 * (1 / Magnification)2 = brightness of Earthshine on Venus through a telescope / brightness of Earthshine on Moon viewed with the naked eye.


Plugging in some numbers:

Venus Albedo: 0.65
Moon Albedo: 0.12
Moon Distance: 384,000
Venus Distance: 38,400,000 (~ inferior conjunction distance, chosen to make a nice 1/100 ratio)
Diameter of Primary Mirror: 200mm (~8 inch telescope)
Diameter of Eye Pupil: 2mm (non-dark adapted pupil. I can see Earthshine on the Moon without dark adapting my eyes.
Magnification: 25x (enough to easily resolve the disk and crescent shape of Venus)

(.65*3840002*2002)/(.12*38400000^2*22*252) = 0.00867, or roughly 1% as bright as Earthshine on the Moon.

Numbers to play with:

Distance to Venus. I used inferior conjunction, but realistically larger numbers should be chosen, causing the Earthshine to dim
Mirror diameter: a 1 meter mirror would brighten the Earthshine on Venus to about 22% as bright as Earthshine on the Moon.


But can the human eye see something as faint as 1% that of Earthshine on the Moon? That’s a difference of 5 magnitudes. I would guess it can. Consider that people have reported seeing shadows cast in Venus light on bright surfaces here on Earth. That means our eyes are sensitive enough to pick up Venus shine against a contrasting black shadow. But when Venus is at its brightest, it is a crescent in our sky. But Earth is a gibbous in Venus’ sky. A gibbous is much brighter than a crescent. Despite having only about half of Venus’ albedo, the gibbous phase more than makes up this. Additionally, Earth is larger than Venus; its disk has about 10% more area. And Earth has the added bonus of having a Moon send light towards Venus as well. The Moon’s disk has about 1/13 the area of Earth’s disk and about 1/3 Earth’s albedo, so it only contributes a little. Earthshine on Venus should be much brighter than Venus shine on Earth, and our eyes can detect Venus shine on Earth.

George
2007-Aug-20, 08:44 PM
Your idea is a nice approach to it.



Venus Albedo: 0.65

Moon Albedo: 0.12
Moon Distance: 384,000
Venus Distance: 38,400,000 (~ inferior conjunction distance, chosen to make a nice 1/100 ratio)
Diameter of Primary Mirror: 200mm (~8 inch telescope)
Diameter of Eye Pupil: 2mm (non-dark adapted pupil. I can see Earthshine on the Moon without dark adapting my eyes.
Magnification: 25x (enough to easily resolve the disk and crescent shape of Venus)Couple issues I may not understand....

I thought the inferior conjunction distance should be about 42 million km. From JPL's Horizon ephemeris generator.... it will be 43.2 million if we want a 8 deg. elongation with 1% illumination.


(.65*3840002*2002)/(.12*38400000^2*22*252) = 0.00867, or roughly 1% as bright as Earthshine on the Moon.
Shouldn't the equation use the inverse fourth power for distance change? [When light is reflected light it is the inverse square for both directions; glowing objects are just inverse square only.]


Consider that people have reported seeing shadows cast in Venus light on bright surfaces here on Earth. That means our eyes are sensitive enough to pick up Venus shine against a contrasting black shadow. Nice point. Yet, Venus is reflecting sunlight that is about 3x the flux as here at Earth. Regardless, we have already seen that the Earth would be about -7 in mag. seen from Venus, far brighter than the shadow-casting Venus at -4.3.

Hornblower
2007-Aug-20, 11:37 PM
Your idea is a nice approach to it.

[/LIST]Couple issues I may not understand....

I thought the inferior conjunction distance should be about 42 million km. From JPL's Horizon ephemeris generator.... it will be 43.2 million if we want a 8 deg. elongation with 1% illumination.


Shouldn't the equation use the inverse fourth power for distance change? [When light is reflected light it is the inverse square for both directions; glowing objects are just inverse square only.]
Very good question. As the object is moved away and the observer remains alongside the light source, the total brightness drops off as inverse fourth power, but it is concentrated in an apparent area that is reduced by inverse square. Thus the observed surface brightness, defined for the sake of argument as light units per square arcminute, is counterintuitively independent of the observer's distance and varies as the inverse square of the light source's distance.


Nice point. Yet, Venus is reflecting sunlight that is about 3x the flux as here at Earth. Regardless, we have already seen that the Earth would be about -7 in mag. seen from Venus, far brighter than the shadow-casting Venus at -4.3.
Let us not forget that at or near inferior conjunction we would be dealing with bright twilight.

I would make an attempt with Venus at a favorable elongation of about 30o, to get it in a dark sky and high enough to beat the worst of atmospheric extinction. My rough calculation based on the increased distance from Earth and Earth's gibbous phase gives a reduction of the surface brightness by about a factor of two. A nebula of the same apparent size and total magnitude would not be too difficult in a dark sky with a sufficiently large telescope, but again the challenge would be glare from the bright crescent. I would put an occulting bar in the field stop of the eyepiece to hide the crescent, and look for a good refractor with the greatest possible freedom from scattering in the lenses. Since we are now looking at a total magnitude of about +14, I would want at least a 10-inch scope. The wild card here is atmospheric forward scattering of light adjoining the crescent, something the occulting bar would not stop.

Remember, seeing a large light and shadow pattern that fills your retina is much easier than seeing a small spot of the same light.

tony873004
2007-Aug-20, 11:46 PM
I thought the inferior conjunction distance should be about 42 million km

You're right. But 38 million isn't that different. I choose it simply because I saw the ratio of 1:100 used here and was too lazy to look up the actual value. But realistically, we want to choose a number much larger than either of these numbers since Venus is usually much further than inferior conjunction, up to 4 times farther, when the Ashen light is reported. This brings the brightness down by a factor of 16.


Shouldn't the equation use the inverse fourth power for distance change? [When light is reflected light it is the inverse square for both directions; glowing objects are just inverse square only.]

No, not in this case. Comparing Earthshine on the Moon to Earthshine on Venus, Earth is considered the light source. Its distance to the Sun remains constant. Inverse 4th is for the brightness of distant objects, such as Pluto or Sedna. Sunlight must travel 40 AU to get to Pluto, and another 39 AU to get back to Earth. For Sedna, it must travel 90 AU to get from the Sun to Sedna, and 89 AU to get back to Earth. So Sedna is approximately (40/90)^4 dimmer than Pluto. In the Earthshine case, the light travels 1 AU to Earth and 384000 km to get to the Moon, and 1 AU to Earth and 43,000,000 km to get to Venus. So it is only squared since the 1 AU is constant.


Nice point. Yet, Venus is reflecting sunlight that is about 3x the flux as here at Earth. Regardless, we have already seen that the Earth would be about -7 in mag. seen from Venus, far brighter than the shadow-casting Venus at -4.3.
I forgot about that 3x. It has no effect on the formula I derived, only on the Venus shine example. But as you point out, Earth is still much brighter regardless. That's the power of a gibbous vs. a crescent.

George
2007-Aug-21, 03:46 AM
As the object is moved away and the observer remains alongside the light source, the total brightness drops off as inverse fourth power, but it is concentrated in an apparent area that is reduced by inverse square. Thus the observed surface brightness, defined for the sake of argument as light units per square arcminute, is counterintuitively independent of the observer's distance and varies as the inverse square of the light source's distance. Nuts, I've always avoided surface brightness work and have been on magnitude or brightness terms where inverse fourth works. Of course, I have now switched to surface brightness for this thread and now have bombed my 4th power lick. :sad:


Let us not forget that at or near inferior conjunction we would be dealing with bright twilight. Yep, that is a big one, of course.


I would make an attempt with Venus at a favorable elongation of about 30o, to get it in a dark sky and high enough to beat the worst of atmospheric extinction. My rough calculation based on the increased distance from Earth and Earth's gibbous phase gives a reduction of the surface brightness by about a factor of two. That elongation might be pushing it as illumination is now 14%, distance is over 52 million km, and your 50% reduction from Earth is probably close.

What would be nice is some empircal data from observers: elongation, magnification, etc. Does any exist? Since this goes back 300 years, I suppose low mag. is suppose to work.


... But realistically, we want to choose a number much larger than either of these numbers since Venus is usually much further than inferior conjunction, up to 4 times farther, when the Ashen light is reported. Ah, that might be a clue. Yet at 4x further, illumination is over 70%. Do we have much observational information for those who've seen it?


No, not in this case. Comparing Earthshine on the Moon to Earthshine on Venus, Earth is considered the light source. Its distance to the Sun remains constant. Inverse 4th is for the brightness of distant objects, such as Pluto or Sedna. Sunlight must travel 40 AU to get to Pluto, and another 39 AU to get back to Earth. For Sedna, it must travel 90 AU to get from the Sun to Sedna, and 89 AU to get back to Earth. So Sedna is approximately (40/90)^4 dimmer than Pluto. Yep. I recall calculating that at 10,000 au -- most capitalize AU but should we? -- Jupiter would be invisible to the Hubble for this fourth power reason, and as we look for exoplanets. :)


In the Earthshine case, the light travels 1 AU to Earth and 384000 km to get to the Moon, and 1 AU to Earth and 43,000,000 km to get to Venus. So it is only squared since the 1 AU is constant. Oddly, I should have remembered my logic and math earlier regarding surface brightness (cd/m2). I was stuck on total brighness thinking. My glitch.

tony873004
2007-Aug-21, 06:05 AM
...Let us not forget that at or near inferior conjunction we would be dealing with bright twilight...
This is a very good point. At inferior conjunction, Venus is basically in the daytime sky. I can see the crescent moon in the daytime sky, but I can't seen any Earthine on it until the sky grows dark.

It is important to note that I used a bunch of best-case-scenerio numbers in my example. If we can't see Earthshine under the best-case scenerios, then we can't see it at all.

George
2007-Aug-21, 12:57 PM
If the surface brightness is within the scotopic range of the eye, there may still be plenty of hope. There is a fair chance my calculations (http://www.bautforum.com/astronomy/63462-venus-ashen-light.html#post1051570) are correct.

If the result is indeed around 10-3 cd/m2, then it is worth noting that this level is actually on the bright side of our rods's scotopic vision. Scotopic vision only, not mesopic, doesn't begin till around 10-2 cd/m2.

Hekate's Lair
2007-Aug-23, 12:38 PM
Correct me if I'm wrong, but if your calculations are right, then ashen light should always be visible? Or not? I mean could there be some other factor that contributes to the partial appearance of this phenomenon?

Hornblower
2007-Aug-23, 01:56 PM
Correct me if I'm wrong, but if your calculations are right, then ashen light should always be visible? Or not? I mean could there be some other factor that contributes to the partial appearance of this phenomenon?

Let's get the viewing conditions back into the discussion.

I think we have concluded that Earthshine on Venus would be visible in a large telescope if we could eliminate the glare from the bright crescent. My experience as an observer is that looking at Venus through a telescope discombobulates my night vision enough to obliterate it. As I said before, if the ashen light is real, it must be vastly brighter than Earthshine alone.

George
2007-Aug-23, 06:47 PM
Correct me if I'm wrong, but if your calculations are right, then ashen light should always be visible? Or not? I mean could there be some other factor that contributes to the partial appearance of this phenomenon?
I agree with what Hornblower has said but will add that the issue of contrast needs attention. Ignoring the flood of light from the illuminated portion of Venus, the Ashen [it is capitalized, right?] light must be bright enough to allow the eye to see enough contrast with the background. Grant has done a few posts on this in the past on sky brightness and contrast issues, but darn if I can find them at them moment. [When I have a little more time, I will find it.]

Of course, there is so much in trade-offs as to when the best time is to see the Ashen light, which is why I would enjoy seeing empirical accounts. If Venus is near inferior conjunction, then glare is minimized; yet a twilight sky will be brighter than the Ashen light and what you've gained in glare [reduction] will be easily lost due to lack of contrast of the dark side of Venus with the background sky. [Somewhat surprisingly, when you look at an object the skylight is added to the object you are observing, so there is a slight gain in magnitude for the object that would not exist otherwise. Yet, contrast is still king in this case, I believe.]

There are so many interesting variables to all this that it is hard not to play with ideas. Do some eyes hold dark adaptation better than others? These would be less susceptible to the glare problem. Are the observations of the AL at quarter Moon? What are all the possibilities of glow from the atmosphere?

Hekate's Lair
2007-Aug-23, 08:53 PM
A study of empirical accounts (especially dates and time) would help with many issues. Another question I came up with is whether the same side of Venus faces the Earth, at the times Ashen light was/is seen.

Ok, I'm a theoretical thinker and I couldn't help wondering if there is some other factor that has to do with Venus (perhaps some parts of its atmosphere, or something underneath), that enhances the visibility of Ashen light, at certain intervals...

George
2007-Aug-24, 01:05 AM
I have found that a dark night sky brightness is around 0.0002 cd/m2.

Thus, if the rough value of 0.001 cd/m2 is correct for Venus, then the contrast amount is 80%. That is quite good.

However, a twilight sky is not dark. So, I am only adding a little fodder to chew on.

Also, there is this on airglow (http://www.ing.iac.es/Astronomy/observing/conditions/skybr/skybr.html#airg) due to ionization of our atmosphere, so what of Venus?

Well, there is this (http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=39669) that shows much oxygen ionzation was found. Much of it is in the bow shock of the Solar wind. Add this to our mix! Could much of it be OIII and produce the 5007A green emission? If so, our eyes in scotopic vision is a little more sensitive here, I think. [Keep in mind this is too dim for any color determination.]

This bowshock light, or augmentation, might be more apparent as Venus approaches inferior conjuction. This is when Grant has mentioned more observations of the Ashen light have taken place.