APOD: Moon Shadow versus Sun Reflection (2017 Jul 17)

Comments and questions about the APOD on the main view screen.
User avatar
APOD Robot
Otto Posterman
Posts: 2880
Joined: Fri Dec 04, 2009 3:27 am

APOD: Moon Shadow versus Sun Reflection (2017 Jul 17)

Postby APOD Robot » Mon Jul 17, 2017 4:05 am

Image Moon Shadow versus Sun Reflection

Explanation: What are those lights and shadows crossing the Earth? As the featured five-second time-lapse video progresses, a full day on planet Earth is depicted as seen from Japan's Himawari-8 satellite in geostationary orbit high above the Pacific Ocean. The Sun rises to the right and sets to the left, illuminating the half of Earth that is most directly below. A reflected image of the Sun -- a Sun glint -- is visible as a bright spot that moves from right to left. More unusual, though, is the dark spot that moves from the lower left to upper right That is the shadow of the Moon, and it can only appear when the Moon goes directly between the Earth and the Sun. Last year, on the day these images were taken, the most deeply shadowed region experienced a total eclipse of the Sun. Next month a similarly dark shadow will sweep right across the USA.

<< Previous APOD This Day in APOD Next APOD >>

User avatar
Ann
4725 Å
Posts: 8255
Joined: Sat May 29, 2010 5:33 am

Re: APOD: Moon Shadow versus Sun Reflection (2017 Jul 17)

Postby Ann » Mon Jul 17, 2017 4:41 am

That video is short but sweet! :D

Ann
Color Commentator

BDanielMayfield
Don't bring me down
Posts: 1347
Joined: Thu Aug 02, 2012 11:24 am
AKA: Bruce
Location: South Texas

Re: APOD: Moon Shadow versus Sun Reflection (2017 Jul 17)

Postby BDanielMayfield » Mon Jul 17, 2017 8:05 am

Yes, a sweet view of a very nice place.

Imagine however that a planet just like this with oceans, land, clouds and a large moon is not just 36,000 km away, but say, 36 light years away. Let's also say that it orbits a sunlike star in an Earth-like orbit. How large and what types of telescopes would be required to detect it?

Bruce

P. S. It doesn't transit its star from our point of view. The great majority of exoplanets don't, after all.

P. S. S. Both types of eclipses would occur from our point of view though.
"Happy are the peaceable ... "

User avatar
Coil_Smoke
Ensign
Posts: 81
Joined: Sun Nov 17, 2013 7:57 am

Re: APOD: Moon Shadow versus Sun Reflection (2017 Jul 17)

Postby Coil_Smoke » Mon Jul 17, 2017 9:49 am

Ann wrote:That video is short but sweet! :D

Ann

And it reveals so much. Plus, doing it all in one take.( It might be a while for take 2 ) I started to think about the relative motion(s) required. The movement of our Moon and observing craft. Why the apparent speed/motion of our Star's reflection is similar to that of the Moon's shadow and why the Sun's reflection appears to move at all? And... Got a headache 8-)

Click to play embedded YouTube video.

User avatar
neufer
Vacationer at Tralfamadore
Posts: 14011
Joined: Mon Jan 21, 2008 1:57 pm
Location: Alexandria, Virginia

Re: APOD: Moon Shadow versus Sun Reflection (2017 Jul 17)

Postby neufer » Mon Jul 17, 2017 11:25 am

If the sun-glint and the moon shadow had collided would there have been an explosion :?:
Art Neuendorffer

User avatar
Chris Peterson
Abominable Snowman
Posts: 13000
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

Re: APOD: Moon Shadow versus Sun Reflection (2017 Jul 17)

Postby Chris Peterson » Mon Jul 17, 2017 1:46 pm

neufer wrote:If the sun-glint and the moon shadow had collided would there have been an explosion :?:

An implosion. Of physics.
Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

User avatar
Chris Peterson
Abominable Snowman
Posts: 13000
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

Re: APOD: Moon Shadow versus Sun Reflection (2017 Jul 17)

Postby Chris Peterson » Mon Jul 17, 2017 1:51 pm

BDanielMayfield wrote:Imagine however that a planet just like this with oceans, land, clouds and a large moon is not just 36,000 km away, but say, 36 light years away. Let's also say that it orbits a sunlike star in an Earth-like orbit. How large and what types of telescopes would be required to detect it?

To detect a planet at such a distance doesn't require all that large of a telescope. What it requires is a system to block the light of the star, and no scattering atmosphere. In other words, a space-based telescope with an occulting disk located a few kilometers in front of it (maybe a few hundred kilometers). In any case, a perfectly feasible design with our existing technology.

To resolve anything on that planet, however, would require a telescope aperture many kilometers in diameter- well beyond what we're currently able to achieve.
Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

User avatar
MarkBour
Science Officer
Posts: 438
Joined: Mon Aug 26, 2013 2:44 pm
Location: Illinois, USA

Re: APOD: Moon Shadow versus Sun Reflection (2017 Jul 17)

Postby MarkBour » Mon Jul 17, 2017 5:19 pm

Chris Peterson wrote:
neufer wrote:If the sun-glint and the moon shadow had collided would there have been an explosion :?:

An implosion. Of physics.

The sun glint is very polite, so it will never do that.

I was wondering why the sun-glint gets very bright in a few spots on the earth that I assumed would be ocean surface. The video is so short it is hard to give a pointer to the places. The nicest one is when the glint is quite a bit east of Australia. What causes those?
Capture.GIF
You do not have the required permissions to view the files attached to this post.
Mark Goldfain

User avatar
Chris Peterson
Abominable Snowman
Posts: 13000
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

Re: APOD: Moon Shadow versus Sun Reflection (2017 Jul 17)

Postby Chris Peterson » Mon Jul 17, 2017 5:28 pm

MarkBour wrote:I was wondering why the sun-glint gets very bright in a few spots on the earth that I assumed would be ocean surface. The video is so short it is hard to give a pointer to the places. The nicest one is when the glint is quite a bit east of Australia. What causes those?

It looks like we're getting specular reflection off of clouds. The ocean reflection doesn't look much different before or after the encounter with those particular clouds. I'd presume it occurs because of some preferential orientation of ice crystals at the cloud tops.
Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

User avatar
neufer
Vacationer at Tralfamadore
Posts: 14011
Joined: Mon Jan 21, 2008 1:57 pm
Location: Alexandria, Virginia

Re: APOD: Moon Shadow versus Sun Reflection (2017 Jul 17)

Postby neufer » Mon Jul 17, 2017 6:44 pm

Chris Peterson wrote:
MarkBour wrote:
I was wondering why the sun-glint gets very bright in a few spots on the earth that I assumed would be ocean surface. The video is so short it is hard to give a pointer to the places. The nicest one is when the glint is quite a bit east of Australia. What causes those?

It looks like we're getting specular reflection off of clouds. The ocean reflection doesn't look much different before or after the encounter with those particular clouds. I'd presume it occurs because of some preferential orientation of ice crystals at the cloud tops.

At work we watched loops from geostationary satellites every day at lunchtime and
no one ever suggested that any of the sun-glint was specular reflection off of clouds.

    Down dropt the breeze, the sails dropt down,
    'Twas sad as sad could be;
    And we did speak only to break
    The silence of the sea!

    All in a hot and copper sky,
    The bloody Sun, at noon,
    Right up above the mast did stand,
    No bigger than the Moon.

    Day after day, day after day,
    We stuck, nor breath nor motion;
    As idle as a painted ship
    Upon a painted ocean.
Art Neuendorffer

User avatar
Chris Peterson
Abominable Snowman
Posts: 13000
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

Re: APOD: Moon Shadow versus Sun Reflection (2017 Jul 17)

Postby Chris Peterson » Mon Jul 17, 2017 7:01 pm

neufer wrote:
Chris Peterson wrote:
MarkBour wrote:
I was wondering why the sun-glint gets very bright in a few spots on the earth that I assumed would be ocean surface. The video is so short it is hard to give a pointer to the places. The nicest one is when the glint is quite a bit east of Australia. What causes those?

It looks like we're getting specular reflection off of clouds. The ocean reflection doesn't look much different before or after the encounter with those particular clouds. I'd presume it occurs because of some preferential orientation of ice crystals at the cloud tops.

At work we watched loops from geostationary satellites every day at lunchtime and
no one ever suggested that any of the sun-glint was specular reflection off of clouds.

So did anybody suggest what it was? Internal refraction inside water droplets, like with glories? Some sort of asymmetric backscatter phenomenon? Any of these seem plausible.
Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

De58te
Ensign
Posts: 60
Joined: Mon Sep 30, 2013 6:35 pm

Re: APOD: Moon Shadow versus Sun Reflection (2017 Jul 17)

Postby De58te » Mon Jul 17, 2017 7:22 pm

So that is the Moon shadow that had been following Cat Stevens. I had thought he was referring to the dark side of a New Moon or a First Quarter. But that would mean Cat Stevens would have had to been on the Moon for that shadow to follow him. This makes more sense.

User avatar
neufer
Vacationer at Tralfamadore
Posts: 14011
Joined: Mon Jan 21, 2008 1:57 pm
Location: Alexandria, Virginia

Re: APOD: Moon Shadow versus Sun Reflection (2017 Jul 17)

Postby neufer » Mon Jul 17, 2017 9:02 pm

Chris Peterson wrote:
neufer wrote:
Chris Peterson wrote:
It looks like we're getting specular reflection off of clouds. The ocean reflection doesn't look much different before or after the encounter with those particular clouds. I'd presume it occurs because of some preferential orientation of ice crystals at the cloud tops.

At work we watched loops from geostationary satellites every day at lunchtime and
no one ever suggested that any of the sun-glint was specular reflection off of clouds.

So did anybody suggest what it was? Internal refraction inside water droplets, like with glories? Some sort of asymmetric backscatter phenomenon? Any of these seem plausible.
Art Neuendorffer

User avatar
geckzilla
Ocular Digitator
Posts: 8514
Joined: Wed Sep 12, 2007 12:42 pm

Re: APOD: Moon Shadow versus Sun Reflection (2017 Jul 17)

Postby geckzilla » Mon Jul 17, 2017 9:08 pm

I think the shiny spots are places where the water is relatively calm and the less shiny spots are where the water is rougher.
Just call me "geck" because "zilla" is like a last name.

MikeA

Re: APOD: Moon Shadow versus Sun Reflection (2017 Jul 17)

Postby MikeA » Mon Jul 17, 2017 9:42 pm

I think Geckzilla is on the right track - but it's not because the water is smooth but that the prevailing wavelets on the water happen to be at an angle that maximizes the sun's reflection. You can observe this effect on any lake or ocean.

User avatar
neufer
Vacationer at Tralfamadore
Posts: 14011
Joined: Mon Jan 21, 2008 1:57 pm
Location: Alexandria, Virginia

Re: APOD: Moon Shadow versus Sun Reflection (2017 Jul 17)

Postby neufer » Mon Jul 17, 2017 11:02 pm

MikeA wrote:
I think Geckzilla is on the right track - but it's not because the water is smooth but that the prevailing wavelets on the water happen to be at an angle that maximizes the sun's reflection. You can observe this effect on any lake or ocean.

But it happens at the same time and at the central point that the sun-glint track hits it.

Ergo, the angle is zero.
Art Neuendorffer

User avatar
Chris Peterson
Abominable Snowman
Posts: 13000
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

Re: APOD: Moon Shadow versus Sun Reflection (2017 Jul 17)

Postby Chris Peterson » Tue Jul 18, 2017 2:07 am

neufer wrote:It is a still water area devoid of clouds.

That's certainly plausible, although I remain unconvinced that the area is actually cloud free.
Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com


Return to “The Bridge: Discuss an Astronomy Picture of the Day”

Who is online

Users browsing this forum: CommonCrawl [Bot] and 1 guest