APOD: Moon, Venus, and Planet Earth (2013 Sep 19)

[APOD Robot]

Moon, Venus, and Planet Earth

Explanation: In this engaging scene from planet Earth, the Moon shines through cloudy skies following sunset on the evening of September 8. Despite the fading light, the camera's long exposure still recorded a colorful, detailed view of a shoreline and western horizon looking toward the island San Gabriel from Colonia del Sacramento, Uruguay. Lights from Buenos Aires, Argentina are along the horizon on the left, across the broad Rio de la Plata estuary. The long exposure strongly overexposed the Moon and sky around it, though. So the photographer quickly snapped a shorter one to merge with the first image in the area around the bright lunar disk. As the the second image was made with a telephoto setting, the digital merger captures both Earth and sky, exaggerating the young Moon's slender crescent shape in relation to the two nearby bright stars. The more distant is bluish Spica, alpha star of the constellation Virgo. Closest to the Moon is Earth's evening star, planet Venus, emerging from a lunar occultation.

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[Ann]

Nice picture, but the heading is a bit confusing. We can clearly see the surface of a planet, and we are looking up at the sky, and in the sky we can see three lights. Three lights? One is the Moon, and the two others are - Venus, and planet Earth? Suddenly the perspective shifts, at least for me, and I'm wondering what planet we are supposed to be standing on and what we are supposed to be looking at in the sky. Are we on the Earth at all, or is this an alien sky, where we can observe the Earth from a distance?

But oh - the third light in the sky is blue star Spica.

I get it. We are on the Earth. The heading just forgot Spica.

Well, it's a nice picture, to be sure.

Ann

[Beyond]

Hey, three out of four isn't bad. But there's also another star further left and a little lower.

[Elder]

From Earth, the Moon is seen against a black sky. Therefore it can't appear darker than its surroundings. This is true whether the night is clear or party cloudy.

In this picture, the Moon's dark parts should actually be observably lighter than its surroundings, because at thin crescent the Moon receives a considerable amount of light reflected from Earth.

This darkness is what causes the outlandish feeling with this picture. But such astronomical ignorance is not suitable for an APOD picture. As it is, this picture is merely another photoshop wallpaper, and belongs here as much as ponies in morning mist.

[Markus Schwarz]

But such astronomical ignorance is not suitable for an APOD picture. As it is, this picture is merely another photoshop wallpaper, and belongs here as much as ponies in morning mist.

Yes, today's APOD is more art than science. However, keep in mind that most APODs, in particular those from Hubble (see yesterday's discussion), result from a manipulation of the raw data to yield an aesthetic image. Also, the night sky holds a fascination besides just scientific value. Therefore, I don't mind when APOD captures this aspect as well once in a while. If the ponies in the morning mist had an interesting sky in the background, I am sure it would pass for an APOD.

[Indigo_Sunrise]

'Surreal' comes to mind....

[fausto.lubatti]

... little words can express the beauty of this picture!

[Joe Stieber]

From Earth, the Moon is seen against a black sky. Therefore it can't appear darker than its surroundings. This is true whether the night is clear or party cloudy.

In this picture, the Moon's dark parts should actually be observably lighter than its surroundings, because at thin crescent the Moon receives a considerable amount of light reflected from Earth.

This darkness is what causes the outlandish feeling with this picture. But such astronomical ignorance is not suitable for an APOD picture. As it is, this picture is merely another photoshop wallpaper, and belongs here as much as ponies in morning mist.

The text does say that a reduced-exposure telephoto image of the moon was inserted, which would account for the relative darkness you note, so the APOD doesn't really suffer the "astronomical ignorance" you suggest. The light reflected from earth onto the crescent moon is known as "earthshine." Aesthetic evaluation is, of course, a personal judgment.

Anyway, regarding the moon's apparent size in the picture, the spacing between Venus and Spica at the time was about 3.5 degrees, which is about seven moon diameters. In the picture, the moon's diameter is slightly greater than the spacing between Venus and Spica; however, it's difficult to be precise with this spacing since I don't know if the moon or Venus (or neither) was centered on its original position when the telephoto segment was inserted.

Initially, I was also puzzled by Venus appearing about the same brightness as Spica in the picture, but then I realized that since Venus was so close to the moon, it must have been within the reduced-exposure insert. Venus was around magnitude -4.1 vs. +1.0 for Spica. That 5.1 magnitude difference means that Venus should appear 110x brighter than Spica.

The star to the left of the moon is Gamma Hydrae. Below Gamma, and below-left of the moon, five stars of Corvus (including the bent trapezoidal stick figure) can be seen in the large version of the picture (although they seem a bit out of place for we northern-hemisphere observers ).

[geckzilla]

This darkness is what causes the outlandish feeling with this picture. But such astronomical ignorance is not suitable for an APOD picture. As it is, this picture is merely another photoshop wallpaper, and belongs here as much as ponies in morning mist.

The only thing ignorant at APOD these days are the knee-jerk reactions people hastily type up after seeing something they don't like. Thankfully, individuals like Markus and Joe are here to help clarify. Anyway, it's perfectly fine not to like something, but please be friendly with your comments. I'd love to see a photo of ponies in the morning mist...

[stephen63]

As far as Landscape images go, this is one of the more appealing ones to me. The haze around the moon adds to it's beauty. You can be rest assured that the photographer went to a considerable amount of effort to make this capture. What a nice surprise it must have been to learn it would be the APOD for today! He may or may not care, but reading negative comments about the validity of your composition must surely sting a little.

[neufer]

But oh - the third light in the sky is blue star Spica.

I get it. We are on the Earth. The heading just forgot Spica.

I was also puzzled by Venus appearing about the same brightness as Spica in the picture, but then I realized that since Venus was so close to the moon, it must have been within the reduced-exposure insert. Venus was around magnitude -4.1 vs. +1.0 for Spica. That 5.1 magnitude difference means that Venus should appear 110x brighter than Spica.

• DeSpicable

[Chris Peterson]

Yes, today's APOD is more art than science. However, keep in mind that most APODs, in particular those from Hubble (see yesterday's discussion), result from a manipulation of the raw data to yield an aesthetic image. Also, the night sky holds a fascination besides just scientific value. Therefore, I don't mind when APOD captures this aspect as well once in a while. If the ponies in the morning mist had an interesting sky in the background, I am sure it would pass for an APOD.

I generally have no problem with artistic or stylized images on APOD. This one does bother me a little, though. There is more than the usual manipulation of intensity or color going on here. We have the Moon being presented at a different angular scale than the background sky. This is a geometrically impossible image- it actually misrepresents the scene (as opposed to showing it in a way our eyes couldn't normally see). For me, at least, that pushes it a little beyond what I think of as a reasonable APOD.

[Chris Peterson]

Initially, I was also puzzled by Venus appearing about the same brightness as Spica in the picture, but then I realized that since Venus was so close to the moon, it must have been within the reduced-exposure insert. Venus was around magnitude -4.1 vs. +1.0 for Spica. That 5.1 magnitude difference means that Venus should appear 110x brighter than Spica.

The fact that these were caught with separate exposures is also revealed by the fact that Venus shows no drift, while Spica does.

Images on a monitor have at most 256 levels of intensity (and on most monitors, it's closer to 64 levels). That means that you can barely make out a pixel reduced 110x from saturation. A good monitor can display 6 astronomical magnitudes between the noise floor and saturation. In other words, don't expect anything on a monitor to be remotely close to actual brightness levels. In this image, both Venus and Spica are saturated, so we have no way to compare their brightnesses at all. In general, with astronomical images, stellar and planetary points are near or above saturation, and we can only distinguish their actual intensities by the size of disc they produce. In this case, of course, Venus and Spica were exposed differently, so we can't even do that.

[Beyond]

• DeSpicable

Daffy Duck used to say that a lot about Bugs Bunny.

[geckzilla]

I generally have no problem with artistic or stylized images on APOD. This one does bother me a little, though. There is more than the usual manipulation of intensity or color going on here. We have the Moon being presented at a different angular scale than the background sky. This is a geometrically impossible image- it actually misrepresents the scene (as opposed to showing it in a way our eyes couldn't normally see). For me, at least, that pushes it a little beyond what I think of as a reasonable APOD.

Oh, the moon has been scaled up? I didn't realize it.

[FloridaMike]

Oh, the moon has been scaled up? I didn't realize it.

• DeSpicable

HA!

[Chris Peterson]

Oh, the moon has been scaled up? I didn't realize it. :?

My point exactly. Even though you can sort of figure that out from the caption, it isn't obvious, and I think most people are going to miss it. You did, and I know you are one of the more knowledgeable, critical viewers here. In this image, the Moon appears 230 arcminutes in diameter, more than seven times its actual size.

[MargaritaMc]

Joe Steiber wrote:
The star to the left of the moon is Gamma Hydrae. Below Gamma, and below-left of the moon, five stars of Corvus (including the bent trapezoidal stick figure) can be seen in the large version of the picture (although they seem a bit out of place for we northern-hemisphere observers).

Thank you, Joe! I've been puzzling at that all day and, I shamefacedly admit, I didn't think that I needed to look as from the southern hemisphere... Once I turned my star map upside down, Corvus became clear. But I wouldn't have 'got' the Gamma of Hydra.

Margarita

[bkinmonth]

A lovely picture, and I applaud the photographer for the extra artistic efforts. But does the caption really refer to Venus as a "star"?

[neufer]

A lovely picture, and I applaud the photographer for the extra artistic efforts.
But does the caption really refer to Venus as a "star"?

Why not? It refers to Earth as a planet.

Planet: from Ancient Greek ἀστὴρ πλανήτης (astēr planētēs), meaning "wandering star"

[Ann]

• DeSpicable

Daffy Duck used to say that a lot about Bugs Bunny.

Well, Bugs Bunny is an orange kind of bunny, or at least he only seems to like orange things, so it stands to reason that he would find blue star Spica despicable.

And Daffy Duck probably only likes the dark side of the Moon, or maybe black holes, and certainly not Spica!

Ann

[Ann]

Joe Steiber wrote:
The star to the left of the moon is Gamma Hydrae. Below Gamma, and below-left of the moon, five stars of Corvus (including the bent trapezoidal stick figure) can be seen in the large version of the picture (although they seem a bit out of place for we northern-hemisphere observers).

Thank you, Joe! I've been puzzling at that all day and, I shamefacedly admit, I didn't think that I needed to look as from the southern hemisphere... Once I turned my star map upside down, Corvus became clear. But I wouldn't have 'got' the Gamma of Hydra.

Margarita

Margarita is speaking for me, too. Thank you, Joe.

Ann

[stephen63]

In this image, the Moon appears 230 arcminutes in diameter, more than seven times its actual size.

How did you figure that out? Just curious.

[Anthony Barreiro]

... . As it is, this picture is merely another photoshop wallpaper, and belongs here as much as ponies in morning mist.

[Chris Peterson]

In this image, the Moon appears 230 arcminutes in diameter, more than seven times its actual size.

How did you figure that out? Just curious.

When the image was made, Spica and Venus were 216 arcminutes apart. Since they are 66 pixels apart in the image, scale is 3.27 arcmin/pixel. The apparent diameter of the Moon is 70 pixels, which at this scale is 229 arcminutes. At the time, the angular diameter of the Moon was 31.4 arcminutes. So the lunar image is scaled up by 7.3 times compared with the astrometric background.