Starship Asterisk*

Has anybody noticed the size of the sun on http://antwrp.gsfc.nasa.gov/apod/ap041221.html ?

This simulated view of Huygen’s probe descent seems flawed to me because the sun’s brightness and angular size is more consistent to what we’d see from the Earth or Venus.

Sun’s globe relative size, as seen from Saturn’s neighbourhood, would be around 10 times smaller than what is seen on Earth, all things being equal otherwise, isn’t it?

Unless the picture’s been taken by a 500 mm telephoto lens…

Uh? I guess it’s rather Saturn that is being depicted - you can detect faint rings (seen head-on), right?

Although, in that case, Saturn, as seen from Titan, would be about 1/5 the size of a full moon, approximately…

You can see rings. I thought it was a lense flare effect at first, but I'm pretty sure that's saturn. Click on the image for the slightly larger version.

The Size of Saturn is a function of the zoom on the imaginary camera taking this image. You could make it appear to be any size. The exposure on the virtual camera is such that Saturn's features are not visible, like the sun's features to our eyes at their comfortable exposure and apature settings. (Insert typical "Don't stare at the sun" disclaimer)

For it to be the Sun you would have to be zoomed in to a telescopic level and I imagine the rest of the picture would look very different, sort of like Mr. Nemiroff's "Gravitationally Distorted Lens" on his home page: http://antwrp.gsfc.nasa.gov/htmltest/rjn.html

M@

Technically, it WOULD have to be a telephoto lens, to render such a big Saturn in the skies (as seen from Titan). Then, given that 300-400 mm telephoto, the field of depth wouldn’t give us such extensive sharpness (from Huygens to the distant clouds)… AND because Huygens would be a moving target, a fast ISO would absolutely be required to ”freeze” the chute and probe movement.

Everything resulting in a very poor depth of field, not at all consistent with the artist’s rendition.

Notwithstanding the very poor lighting conditions in the neighbourhood of Saturn…

All in all, a nice image, out of the imagination of the imagineer (quoting Frank Zappa) !

AND DO YOU PICK APART DALI ARTWORK BECAUSE WIS CLOCKS ARE DROOPING AND WOULD THEREFORE BE NON FUNCTIONAL???

ART IS ART

BMAONE wrote:AND DO YOU PICK APART DALI ARTWORK BECAUSE WIS CLOCKS ARE DROOPING AND WOULD THEREFORE BE NON FUNCTIONAL???

ART IS ART

Mmm… BMaone… it is called Astronomy Picture of the Day, not Artistic Picture of the Day.

This is a scientific forum + BTW, I love Salvador Dali’s art, not for its scientific value but for its artistic qualities…

looks to me like a picture you could see on earth

Burokrat wrote:Has anybody noticed the size of the sun on http://antwrp.gsfc.nasa.gov/apod/ap041221.html ?

This simulated view of Huygen’s probe descent seems flawed to me because the sun’s brightness and angular size is more consistent to what we’d see from the Earth or Venus.

Sun’s globe relative size, as seen from Saturn’s neighbourhood, would be around 10 times smaller than what is seen on Earth, all things being equal otherwise, isn’t it?

Unless the picture’s been taken by a 500 mm telephoto lens…

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Mmm… BMaone… it is called Astronomy Picture of the Day, not Artistic Picture of the Day.
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You've got a point. but I like the idea of sharing ideas. normal people (ones that don't study Astronomy, that makes us abnormal) don't try to understand these things. at least they have an outlet to see neat things in APOD.

[quote="crosscountry"]looks to me like a picture you could see on earth

That’s EXACTLY what I thought when I first saw that picture. An earth inspired perspective.

Hass Traunot wrote:Technically, it WOULD have to be a telephoto lens, to render such a big Saturn in the skies (as seen from Titan). Then, given that 300-400 mm telephoto, the field of depth wouldn’t give us such extensive sharpness (from Huygens to the distant clouds)… AND because Huygens would be a moving target, a fast ISO would absolutely be required to ”freeze” the chute and probe movement.

Everything resulting in a very poor depth of field, not at all consistent with the artist’s rendition.

Notwithstanding the very poor lighting conditions in the neighbourhood of Saturn…

All in all, a nice image, out of the imagination of the imagineer (quoting Frank Zappa) !

This painting does not make sense. Saturn WOULD fill MUCH more space than what’s depicted in Titan’s skies, according to JPL’s Solar System Simulator: http://space.jpl.nasa.gov/

Saturn’s angular size, in Titan’s skies, would be approx. 5.5 deg. arc. For comparison purposes, the full Moon, as seen from the earth, is about 30 min. arc. Meaning that Saturn’s would be at least 10 times bigger than a full Moon, as seen in Titan’s sky.

If a telephoto would’ve been used for that picture, Saturn would’ve appeared much bigger, undoubtedly.

So, lets assume that a wide-angle must’ve been used: consistent with the depth of field showing in the simulation, the freezing of the probe + parachute AND the poor lighting conditions of the medium-far reaches of the solar system.

With that type of lens, Saturn WOULD appear smaller, right?

So, indeed, the picture COULD be technically realistic.

Anonymous wrote:Saturn’s angular size, in Titan’s skies, would be approx. 5.5 deg. arc. For comparison purposes, the full Moon, as seen from the earth, is about 30 min. arc. Meaning that Saturn’s would be at least 10 times bigger than a full Moon, as seen in Titan’s sky.

If a telephoto would’ve been used for that picture, Saturn would’ve appeared much bigger, undoubtedly.

So, lets assume that a wide-angle must’ve been used: consistent with the depth of field showing in the simulation, the freezing of the probe + parachute AND the poor lighting conditions of the medium-far reaches of the solar system.

With that type of lens, Saturn WOULD appear smaller, right?

So, indeed, the picture COULD be technically realistic.

So… case closed, then

Burokrat wrote:Has anybody noticed the size of the sun on http://antwrp.gsfc.nasa.gov/apod/ap041221.html ?

This simulated view of Huygen’s probe descent seems flawed to me because the sun’s brightness and angular size is more consistent to what we’d see from the Earth or Venus.

Sun’s globe relative size, as seen from Saturn’s neighbourhood, would be around 10 times smaller than what is seen on Earth, all things being equal otherwise, isn’t it?

Unless the picture’s been taken by a 500 mm telephoto lens…

Nice troll, 5 points.

Hello all. It has just come to my attention that several of you have been discussing the APOD image from 21 Dec, featuring the Huygens probe seen descending through the atmosphere of Titan.

I am the artist.

Somebody wrote that the yellow globe was the Sun, complaining it was far too small. Then somebody else noticed that it must be Saturn, as there are rings. Somebody else said that, if it's Saturn, it's still too big, since Saturn from Titan would appar smaller than the Moon from the Earth. Finally I noticed that somebody with some basic trig worked out, correctly, that this is wrong and that the true angular size of Saturn from Titan is 5.5 degrees. That's more than 11 times the size of the Moon from Earth.

I am flattered the picture got so much attention. Yes, it is Saturn, and yes it is the right size. The artist is an ex-astronomer! The only difficulty with the pic is that, at the time of the landing, Saturn, while appearing full (as I have shown it) will actually have much of one half shadowed by the rings. If I had shown the disc with the ring shadow cast on it, there might not have been any confustion.

Thanks anyway.

Another problem.

Let’s take for granted that a wide-angle was used.

If Saturn appears about as small (as the moon as seen from the earth), this means that a 17-20 mm range wide-angle lens should have been used to shoot this imaginary scene, to make Saturn appear 10 times smaller.

Such a small focal lens distort straight lines when the shooting angle is tilted down, as we see in the picture. Not as much as a fisheye, but noticeably.

Hence, the horizon line in the scene shouldn’t look straight line, but rather slightly bended (spherized).

BTW, nice art, Space Art !

I am afraid I don't understand your posting. The image shows Saturn as it would appear with no telephoto lens, 11 times larger than the Moon as seen from Earth. It is perhaps a little too small, but not a great deal. I did not imagine the scene with a wide angle lens, nor a telephoto one.

If you think that Saturn in my picture should be much bigger (some have said it looks about as big as the Moon does in Earthly skies, not eleven times bigger), then I am afraid you are mistaken. Take a photo of the Moon with no zoom, say using a focal length of about 35 mm to 50 mm and I can guarantee you that the Moon will look miniscule in it, far smaller than Saturn appears in my pic. Even when I recently photographed the full Moon at 200 mm, it still only occupied 1/20th the width of the frame.

People tend to think the Moon is a lot larger in the sky than it really is.

Cheers.

space-art wrote:Hello all. It has just come to my attention that several of you have been discussing the APOD image from 21 Dec, featuring the Huygens probe seen descending through the atmosphere of Titan.

I am the artist.

Thank you!!

space-art wrote:People tend to think the Moon is a lot larger in the sky than it really is.

Cheers.

Funny how that is true. I guess it goes back to that old debate.

space-art wrote:
People tend to think the Moon is a lot larger in the sky than it really is.

Cheers.

Visually (not scientifically), this artwork is closer to the feel of Saturn’s size (as seen from Titan) to me: http://www.space.com/php/multimedia/ima ... %20surface.

Yes, this image shows Saturn at about the right size. You could argue mine is a little small, but only a little (unless you want to say I imagined the scene with a wide-angle lens!)

But otherwise the picture is wrong. Saturn is INVISIBLE from Titan, as it's hidden by cloud -- which was why my view is above the clouds. Titan orbits in the ring plane, so the rings are ALWAYS edge-on from Titan. And the rings are far less than paper thin (in relative terms). The artist who did the image you have mentioned has put a rim on the rings!

Still, I have seen even worse.

If anybody who is reading this wants to see an alternative version of my APOD image, click on http://www.space-art.co.uk/html/whatsne ... l?quietsea

I have now added the shadow of the rings on Saturn, which I had omitted before. The shadow is substantial, and this is how Saturn will actuall appear on Jan 14 whnen Huygens arrives. I have heard, though, that Saturn will not be in the same hemisphere where Huygens will land, Never mind!

Cheers.

Dr Mark A. Garlick