APOD: Venus and the Sisters (2012 Apr 06)

APOD Robot · Post by **APOD Robot** » Fri Apr 06, 2012 4:08 am

Venus and the Sisters

Explanation: After wandering about as far from the Sun on the sky as Venus can get, the brilliant evening star crossed paths with the Pleiades star cluster earlier this week. The beautiful conjunction was enjoyed by skygazers around the world. Taken on April 2, this celestial group photo captures the view from Portal, Arizona, USA. Also known as the Seven Sisters, even the brighter naked-eye Pleiades stars are seen to be much fainter than Venus. And while Venus and the sisters do look star-crossed, their spiky appearance is the diffraction pattern caused by multiple leaves in the aperture of the telephoto lens. The last similar conjunction of Venus and Pleiades occurred nearly 8 years ago. As it did then, Venus will again move on to cross paths with the disk of the Sun in June.

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Ann · Post by **Ann** » Fri Apr 06, 2012 10:20 am

No comments yet. Well, I think it's a very fine picture. The individual Pleiads stand out crisp and clear, in spite of the overwhelming brightness of Venus. And Venus itself is really impressive, of course!

APOD Robot wrote:

Also known as the Seven Sisters, even the brighter naked-eye Pleiades stars are seen to be much fainter than Venus.

Well, what a difference distance makes. Alcyone, brightest of the Pleiads, shines more than 900 times brighter than the Sun in visual light from a distance of about 400 light-years, giving it an absolute magnitude of around -2.6. I'm not too sure about the current distance to Venus, but it's got to be less than an AU, less than 8 light-minutes. It makes me wonder: If we could see Venus fully illuminated by the Sun from a distance of some 30 light-years (a parsec - the distance that is used to determine absolute magnitudes) - what would the absolute magnitude of Venus be, and how much fainter than Alcyone would it be?

Ann

orin stepanek · Post by **orin stepanek** » Fri Apr 06, 2012 12:44 pm

Ann: I don't think you could see Venus at 30 light years; not very clearly anyway!

But; that's probably what you're getting at!

Maybe the Kepler probe could pick it out if it were in a stellar system!

Guest · Post by **Guest** » Fri Apr 06, 2012 12:44 pm

Ann wrote:No comments yet. Well, I think it's a very fine picture. The individual Pleiads stand out crisp and clear, in spite of the overwhelming brightness of Venus. And Venus itself is really impressive, of course!

APOD Robot wrote:

Also known as the Seven Sisters, even the brighter naked-eye Pleiades stars are seen to be much fainter than Venus.
Well, what a difference distance makes. Alcyone, brightest of the Pleiads, shines more than 900 times brighter than the Sun in visual light from a distance of about 400 light-years, giving it an absolute magnitude of around -2.6. I'm not too sure about the current distance to Venus, but it's got to be less than an AU, less than 8 light-minutes. It makes me wonder: If we could see Venus fully illuminated by the Sun from a distance of some 30 light-years (a parsec - the distance that is used to determine absolute magnitudes) - what would the absolute magnitude of Venus be, and how much fainter than Alcyone would it be?

Ann

Is that one of those trick questions that your going to post on the internet to ridicule me for not knowing the answer? Well, it all depends on what kind of car you drive and what music you are listening to, at the time.

TNT · Post by **TNT** » Fri Apr 06, 2012 1:39 pm

Wow! A nice image of the Seven Sisters plus our own sister planet!

starstruck · Post by **starstruck** » Fri Apr 06, 2012 1:56 pm

I really love today's picture . . brilliant Venus and the jewels of the Sisters; a wonderful capture. This photo must have been taken shortly before the closest conjunction, which I missed from my location due to poor weather conditions at the time, but I saw it with my own eyes last night and, though the separation was greater, it was magical to see.

Ann · Post by **Ann** » Fri Apr 06, 2012 4:29 pm

starstruck wrote:I really love today's picture . . brilliant Venus and the jewels of the Sisters; a wonderful capture. This photo must have been taken shortly before the closest conjunction, which I missed from my location due to poor weather conditions at the time, but I saw it with my own eyes last night and, though the separation was greater, it was magical to see.

Beautifully put, starstruck. Like you, I was prevented from seeing the closest conjunction on April 3 due to the weather, but I saw it the next day. It was splendid on April 4, too.

Ann

Case · Post by **Case** » Fri Apr 06, 2012 5:12 pm

Ann wrote:If we could see Venus fully illuminated by the Sun from a distance of some 30 light-years (a parsec - the distance that is used to determine absolute magnitudes) - what would the absolute magnitude of Venus be, and how much fainter than Alcyone would it be?

Venus-Earth distance today: 0.63 AU
Venus magnitude today: -4.31
Venus absolute magnitude today: +28.27
Venus fully lit absolute magnitude: +26.81
Alcyone magnitude: +2.85
Alcyone absolute magnitude: -2.41
Magnitude difference: 29.22
Brightness difference factor: (2.512)^29.22 ≈ 488,000,000,000

Ann · Post by **Ann** » Fri Apr 06, 2012 5:24 pm

So Venus would be almost two trillion times fainter than Alcyone, then?

Thanks, Case!

Ann

Case · Post by **Case** » Fri Apr 06, 2012 5:42 pm

Ann wrote:So Venus would be almost two trillion times fainter than Alcyone, then?

At first, I didn't account for “fully illuminated”, so I edited the numbers. With the new numbers, it is a factor ~4 (3.8) less from previous numbers.

Ann · Post by **Ann** » Fri Apr 06, 2012 6:00 pm

Forgive my limited understanding of mathematese. So the new figure would make Venus five billion times fainter than Alcyone, is that right?

Ann

rstevenson · Post by **rstevenson** » Fri Apr 06, 2012 6:34 pm

Ann wrote:Forgive my limited understanding of mathematese. So the new figure would make Venus five billion times fainter than Alcyone, is that right?

Ann

Almost 500 billion. But "billion" is a tricky word to define, having as it does different common usages in different parts of the world. (See Long and short scales.) In the unambiguous scientific notation (using Case's final figure) Alcyone is 4.88 x 10¹¹ times brighter than Venus.

Rob

ta152h0 · Post by **ta152h0** » Fri Apr 06, 2012 7:03 pm

not a painting, for sure

StarRiderDad · Post by **StarRiderDad** » Fri Apr 06, 2012 8:41 pm

One of the interesting optical artifacts in this picture, beside the diffraction pattern, is the faint ghost image of Venus in the upper left. Ghosts are common in refactive optical systems (e.g. telephoto lenses) and in this particular case the ghost clearly shows the phase of Venus meaning that the ghost is not too far out of focus.

Diana · Post by **Diana** » Fri Apr 06, 2012 10:31 pm

Since we are being picky with numbers, let's be picky with the term "brightness" and the two bodies' relitive brightness.

Can we say that Venus' brightness is a reflective measurement and the Pleiades star's is a emission-type brightness?

Oh, unless those brightness numbers for the star include the brightness of the surrounding reflecting nebulus/dust clouds.
In that case there is added brightness from the back side of the star reflecting off that surrounding material. hummmm

Diana

ta152h0 · Post by **ta152h0** » Fri Apr 06, 2012 11:27 pm

I just googled the Observatory and discovered it is at elevation 4700 feet in southeast Arizona. Amazing clarity on this image for still being imaged with a lot of " air over the camera ".

Chris Peterson · Post by **Chris Peterson** » Sat Apr 07, 2012 1:29 am

Diana wrote:Since we are being picky with numbers, let's be picky with the term "brightness" and the two bodies' relitive brightness.

Can we say that Venus' brightness is a reflective measurement and the Pleiades star's is a emission-type brightness?

Yes, but it doesn't matter in this context. What is being referred to is intensity- the number of photons reaching the observer per unit time. The photons don't know or care if they were emitted directly or reflected from cloud tops.

Ann · Post by **Ann** » Sat Apr 07, 2012 4:44 am

rstevenson wrote:
Ann wrote:Forgive my limited understanding of mathematese. So the new figure would make Venus five billion times fainter than Alcyone, is that right?

Ann
Almost 500 billion. But "billion" is a tricky word to define, having as it does different common usages in different parts of the world. (See Long and short scales.) In the unambiguous scientific notation (using Case's final figure) Alcyone is 4.88 x 10¹¹ times brighter than Venus.

Rob

I know "billion" is tricky, but I was speaking English. In Swedish, a "biljon" is a trillion - that is, a "biljon" is 1,000,000,000,000. So what is a billion called in Swedish, then? It's a "miljard". A "miljon" is really a million.

Ann

Chris Peterson · Post by **Chris Peterson** » Sat Apr 07, 2012 4:59 am

Ann wrote:I know "billion" is tricky, but I was speaking English. In Swedish, a "biljon" is a trillion - that is, a "biljon" is 1,000,000,000,000. So what is a billion called in Swedish, then? It's a "miljard". A "miljon" is really a million.

A "billion" is pretty unambiguously 10^9 in any scientific discussion. The fact that some countries are still using a different, confusing, obsolete, and illogical set of terminology should not impact a discussion in a technical forum. You're not going to find milliards in scientific papers.

geckzilla · Post by **geckzilla** » Sat Apr 07, 2012 5:33 am

Try large numbers with a Chinese person. Larger sets of numbers are grouped by 10,000 instead of 1,000. So 1,000,000,000 instead ends up as 10,0000,0000 so there's not a simple word for billon. Instead, it's 10 [word for 1,0000,0000]. Something like that. Ten one-hundred millions I guess is the closest translation. (If someone here is Chinese and spotted an error in my post I would be happy to have it fixed.)

Ann · Post by **Ann** » Sat Apr 07, 2012 5:48 am

Chris Peterson wrote:
Ann wrote:I know "billion" is tricky, but I was speaking English. In Swedish, a "biljon" is a trillion - that is, a "biljon" is 1,000,000,000,000. So what is a billion called in Swedish, then? It's a "miljard". A "miljon" is really a million.
A "billion" is pretty unambiguously 10^9 in any scientific discussion. The fact that some countries are still using a different, confusing, obsolete, and illogical set of terminology should not impact a discussion in a technical forum. You're not going to find milliards in scientific papers.

Right. I know, Chris. And an important reason why milliards are missing in scientific papers is that English doesn't use milliards, and English is the lingua franca of the community of science of today.

Carl von Linné, Swedish biologist who classified very many species of the flora and fauna in the 18th century, gave us all Latin designations, because Latin was the language of science back then. And that's why we are called Homo Sapiens and not Intelligent Human. (Or even Klok Människa.)

Ann

DavidLeodis · Post by **DavidLeodis** » Sat Apr 07, 2012 11:01 am

What does the "multiple leaves in the aperture of the telephoto lens" mean

I know I should try to find out but I'm surely not the only one not to know, so I thought I would ask.

Chris Peterson · Post by **Chris Peterson** » Sat Apr 07, 2012 2:11 pm

DavidLeodis wrote:What does the "multiple leaves in the aperture of the telephoto lens" mean :?: I know I should try to find out but I'm surely not the only one not to know, so I thought I would ask. :?

: iris.jpg (4.78 KiB) Viewed 2941 times

A standard camera iris, for adjusting the size of the aperture, is made up of multiple leaves. This is a five-leaf iris, which would produce ten diffraction spikes around a bright point. Today's image was made using a six-leaf aperture, which produces just six spikes.

Chris Peterson · Post by **Chris Peterson** » Sat Apr 07, 2012 2:14 pm

geckzilla wrote:Try large numbers with a Chinese person. Larger sets of numbers are grouped by 10,000 instead of 1,000. So 1,000,000,000 instead ends up as 10,0000,0000 so there's not a simple word for billon. Instead, it's 10 [word for 1,0000,0000]. Something like that. Ten one-hundred millions I guess is the closest translation. (If someone here is Chinese and spotted an error in my post I would be happy to have it fixed.)

There would probably be no confusion at all for a Chinese scientist reading a scientific paper written in English. That was my point. Of course, different numbering systems can cause confusion in other contexts.

geckzilla · Post by **geckzilla** » Sat Apr 07, 2012 3:17 pm

Of course not. Not a scientist, anyway. For everyday communication between me and my fiance's dad, however... It gets confusing.

Starship Asterisk*

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