APOD: Two Worlds, One Sun (2015 May 12)

Comments and questions about the APOD on the main view screen.
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APOD: Two Worlds, One Sun (2015 May 12)

Postby APOD Robot » Tue May 12, 2015 4:08 am

Image Two Worlds, One Sun

Explanation: How different does sunset appear from Mars than from Earth? For comparison, two images of our common star were taken at sunset, one from Earth and one from Mars. These images were scaled to have same angular width and featured here side-by-side. A quick inspection will reveal that the Sun appears slightly smaller from Mars than from Earth. This makes sense since Mars is 50% further from the Sun than Earth. More striking, perhaps, is that the Martian sunset is noticeably bluer near the Sun than the typically orange colors near the setting Sun from Earth. The reason for the blue hues from Mars is not fully understood, but thought to be related to forward scattering properties of Martian dust. The terrestrial sunset was taken in 2012 March from Marseille, France, while the Martian sunset was captured last month by NASA's robotic Curiosity rover from Gale crater on Mars.

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Re: APOD: Two Worlds, One Sun (2015 May 12)

Postby Boomer12k » Tue May 12, 2015 4:21 am

Amazing!!!
One Familiar...One Alien....BOTH BEAUTIFUL...

Ann must be intrigued...

It still has our...."curiosity"...

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Re: APOD: Two Worlds, One Sun (2015 May 12)

Postby Ann » Tue May 12, 2015 4:33 am

Boomer12k wrote:Amazing!!!
One Familiar...One Alien....BOTH BEAUTIFUL...

Ann must be intrigued...

It still has our...."curiosity"...

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Well, the really fascianting thing is that Earth's skies are normally blue, but our sunsets are typically orange. The Martian skies are typically orange or peach, but their sunsets are (at least sometimes) blue.

Weird.

Ann
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the old blind man

Re: APOD: Two Worlds, One Sun (2015 May 12)

Postby the old blind man » Tue May 12, 2015 5:12 am

We're mostly done with the one on the left, so hopefully the one on the right stays (mostly) pristine.

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Re: APOD: Two Worlds, One Sun (2015 May 12)

Postby Cousin Ricky » Tue May 12, 2015 10:12 am

Another obvious difference is that the Sun appears flattened from Earth, due to differential atmospheric refraction, but not from Mars. Mars' thinner atmosphere must have a considerably smaller index of refraction.

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Re: APOD: Two Worlds, One Sun (2015 May 12)

Postby Redbone » Tue May 12, 2015 10:49 am

I did not know that Mars is 50% farther from the Sun than Earth. ~150 * .5 = 75. 75 + 150 = 225, almost exactly the ~228 million kilometers that mars averages. That is not a coincidence. Thanks.

Animal of Stone

Re: APOD: Two Worlds, One Sun (2015 May 12)

Postby Animal of Stone » Tue May 12, 2015 11:07 am

Wonderful idea to transport my imagination to another world. Cant wait til postcards from the Red Planet are a reality.

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Re: APOD: Two Worlds, One Sun (2015 May 12)

Postby Craine » Tue May 12, 2015 12:01 pm

It makes me wonder if Mars has anything like the Green Flash at sunset.
But then what color would it have? Would it be green? or perhaps more bluish, maybe even purple?
Though more likely the atmosphere is too thin to do anything like that.

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Re: APOD: Two Worlds, One Sun (2015 May 12)

Postby CURRAHEE CHRIS » Tue May 12, 2015 12:06 pm

Animal of Stone wrote:Wonderful idea to transport my imagination to another world. Cant wait til postcards from the Red Planet are a reality.


Agreed!!! It really had a sci-fi quality to this mornings APOD. What a neat compare and contrast!!

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Re: APOD: Two Worlds, One Sun (2015 May 12)

Postby neufer » Tue May 12, 2015 12:48 pm

Craine wrote:
It makes me wonder if Mars has anything like the Green Flash at sunset.
But then what color would it have? Would it be green? or perhaps more bluish, maybe even purple?
Though more likely the atmosphere is too thin to do anything like that.

The thin Martian atmosphere will still act as a prism but a very weak one that will be overwhelmed by dust pollution.

Even on Earth the Green Flash is a very low pollution phenomenon.
http://en.wikipedia.org/wiki/Green_flash#Green_rim wrote:
<<As an astronomical object sets or rises in relation to the horizon, the light it emits travels through Earth's atmosphere, which works as a prism separating the light into different colors. The color of the upper rim of an astronomical object could go from green to blue to violet depending on the decrease in concentration of pollutants, as they spread throughout an increasing volume of atmosphere. The lower rim of an astronomical object is always red.

A green rim is very thin and is difficult or impossible to see with the naked eye. In usual conditions, a green rim of an astronomical object gets fainter when an astronomical object is very low above the horizon because of atmospheric reddening, but sometimes the conditions are right to see a green rim just above the horizon.>>

Also, the Earth's Green Flash benefits greatly from orange/yellow Chappuis absorption bands of ozone forming a clean dark band break between the Sun's red image and the Sun's green image. There is no ozone on Mars.
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Re: APOD: Two Worlds, One Sun (2015 May 12)

Postby gvann » Tue May 12, 2015 12:54 pm

Question: on Earth, you can stare at the setting sun without fear of eye damage because the atmosphere attenuates sunlight by a large factor. The photo of the Martian sunset gives the impression that you could do the same on Mars. Is it true? Cousin Ricky has already noted the absence of the familiar flattening of the Sun's disk because of the thin atmosphere. Does it also mean that you would not be able to really witness such a sunset, except through a thick protective filter? If so, would you see the outlines of the surrounding hills as they appear in the picture?

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Re: APOD: Two Worlds, One Sun (2015 May 12)

Postby neufer » Tue May 12, 2015 1:04 pm

Click to play embedded YouTube video.
CURRAHEE CHRIS wrote:
Animal of Stone wrote:
Wonderful idea to transport my imagination to another world. Cant wait til postcards from the Red Planet are a reality.
Agreed!!! It really had a sci-fi quality to this mornings APOD. What a neat compare and contrast!!
http://en.wikipedia.org/wiki/Coneheads wrote:
<<The Coneheads are an alien family, natives of the planet Remulak, who find themselves stranded on Earth. When questioned by Earth neighbors as to their strange behavior, they invariably reply that they are from France.>>
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Re: APOD: Two Worlds, One Sun (2015 May 12)

Postby ChuckN » Tue May 12, 2015 3:06 pm

City=MARSeille - Planet=MARS
^^^^

othermoons

Re: APOD: Two Worlds, One Sun (2015 May 12)

Postby othermoons » Tue May 12, 2015 3:20 pm

In the Mars image, what causes the color change in the outer disks of sun light? Just that the changes have definite boundaries where the color hue changes from blue to ~green, then mauve.

Wayne Warren

Re: APOD: Two Worlds, One Sun (2015 May 12)

Postby Wayne Warren » Tue May 12, 2015 3:23 pm

Red sunsets on Earth are a result of blue light being scattered away by the atmosphere. Mars' atmosphere is much less dense, so more blue light gets through and the Sun looks bluer. Scattering is proportional to the inverse fourth power of the wavelength. The properties of the Martian atmosphere must have an effect too.

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Re: APOD: Two Worlds, One Sun (2015 May 12)

Postby Chris Peterson » Tue May 12, 2015 3:46 pm

Wayne Warren wrote:Red sunsets on Earth are a result of blue light being scattered away by the atmosphere. Mars' atmosphere is much less dense, so more blue light gets through and the Sun looks bluer. Scattering is proportional to the inverse fourth power of the wavelength. The properties of the Martian atmosphere must have an effect too.

That's just Rayleigh scattering, which is usually dominant in our thick atmosphere. In the Martian atmosphere, thin on gas and heavy on dust, Mie and other scattering mechanisms play a large role, I'm sure. These are very difficult to analyze and assess.
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Re: APOD: Two Worlds, One Sun (2015 May 12)

Postby neufer » Tue May 12, 2015 3:50 pm

Wayne Warren wrote:
Red sunsets on Earth are a result of blue light being scattered away by the atmosphere. Mars' atmosphere is much less dense, so more blue light gets through and the Sun looks bluer. Scattering is proportional to the inverse fourth power of the wavelength. The properties of the Martian atmosphere must have an effect too.

Red skies and blue sunsets on Mars are the result of
red light being scattered by moderately large red dust particles.

Dust scattering is proportional to the inverse fourth power of the wavelength
only for infrared wavelengths much longer than the dust particle size of ~3 µm in diameter.

http://en.wikipedia.org/wiki/Martian_so ... heric_dust wrote:
<<Mars is covered with vast expanses of sand and dust and its surface is littered with rocks and boulders. The dust is occasionally picked up in vast planet-wide dust storms. Mars dust is very fine and enough remains suspended in the atmosphere to give the sky a reddish hue. The reddish hue is due to rusting iron minerals presumably formed a few billion years ago when Mars was warm and wet, but now that Mars is cold and dry, modern rusting may be due to a superoxide that forms on minerals exposed to ultraviolet rays in sunlight.

The Martian atmospheric dust particles are generally 3 µm in diameter. It is important to note that while the atmosphere of Mars is thinner, Mars also has a lower gravitational acceleration, so the size of particles that will remain in suspension cannot be estimated with atmospheric thickness alone. Electrostatic and van der Waals forces acting among fine particles introduce additional complexities to calculations. Rigorous modeling of all relevant variables suggests that 3 µm diameter particles can remain in suspension indefinitely at most wind speeds, while particles as large as 20 µm diameter can enter suspension from rest at surface wind turbulence as low as 2 ms−1 or remain in suspension at 0.8 ms−1.>>
Art Neuendorffer

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Re: APOD: Two Worlds, One Sun (2015 May 12)

Postby Mulsar » Tue May 12, 2015 5:45 pm

It would be interesting to see the sun at noon on both planets to see a more accurate depiction of the relative size of the sun from both worlds.

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Re: APOD: Two Worlds, One Sun (2015 May 12)

Postby Ron-Astro Pharmacist » Tue May 12, 2015 5:47 pm

Eclipses have been caught too.

http://en.wikipedia.org/wiki/Solar_eclipses_on_Mars

Can't seem to locate how often Earth or Venus might supply Curiosity or Opportunity an eclipse view?

Earth-from-Mars-with-Phobos-Deimos-Aug31-am.jpg


Picture is a Stellarium-generated image from http://astrobob.areavoices.com/2012/08/ ... from-mars/

I'm sure an eclipse by Earth or Venus would be "annularly". :wink:
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Re: APOD: Two Worlds, One Sun (2015 May 12)

Postby neufer » Tue May 12, 2015 6:42 pm

Ron-Astro Pharmacist wrote:
Can't seem to locate how often Earth or Venus might supply Curiosity or Opportunity an eclipse view?

I'm sure an eclipse by Earth or Venus would be "annularly". :wink:

With Mars at 1.5 AU,
    Earth can never be more than ("a rainbow") 42º [= sin-1(2/3)] away from the Sun.
Earth is just another morning or evening star for Mars unless it is too close to the Sun to be seen.
Art Neuendorffer

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Re: APOD: Two Worlds, One Sun (2015 May 12)

Postby FloridaMike » Tue May 12, 2015 6:44 pm

Ron-Astro Pharmacist wrote:
I'm sure an eclipse by Earth or Venus would be "annularly". :wink:


Would not it be described as a transit?
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Re: APOD: Two Worlds, One Sun (2015 May 12)

Postby neufer » Tue May 12, 2015 7:40 pm

Click to play embedded YouTube video.
FloridaMike wrote:
Ron-Astro Pharmacist wrote:
I'm sure an eclipse by Earth or Venus would be "annularly". :wink:

Would not it be described as a transit?

Transit of Earth (and Moon) from Mars
Strakul

Stellarium simulation of the transit of Earth and its Moon as seen from Mars. The ground and atmosphere have been disabled to show the full transit. Date: November 10, 2084.
Art Neuendorffer

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Re: APOD: Two Worlds, One Sun (2015 May 12)

Postby Ron-Astro Pharmacist » Tue May 12, 2015 7:55 pm

That's a good point Mike and very cool Art.

When is an eclipse not an eclipse?

When it transits annularly.


That makes our solar events, all called eclipses, frequently a misnomer. Wonder where the defined transition occurs?
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Re: APOD: Two Worlds, One Sun (2015 May 12)

Postby sarabiga » Wed May 13, 2015 11:05 am

So the Blue Planet's sunsets are red, while the Red Planet's sunsets are blue... how poetic! 8-)

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Re: APOD: Two Worlds, One Sun (2015 May 12)

Postby nikki » Wed May 13, 2015 7:24 pm

Here is "Martian Sunset" on Earth. This picture I took on 22nd of May 2014 with OLYMPUS FE-280 from LJUBLJANA, when southwestern wind brought Saharian dust to Europe.
http://www.astronom.si/forum/attachment ... 1400784909


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