APOD: Planet Earth at Twilight (2021 Apr 22)

Comments and questions about the APOD on the main view screen.
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APOD: Planet Earth at Twilight (2021 Apr 22)

Post by APOD Robot » Thu Apr 22, 2021 4:05 am

Image Planet Earth at Twilight

Explanation: No sudden, sharp boundary marks the passage of day into night in this gorgeous view of ocean and clouds over our fair planet Earth. Instead, the shadow line or terminator is diffuse and shows the gradual transition to darkness we experience as twilight. With the Sun illuminating the scene from the right, the cloud tops reflect gently reddened sunlight filtered through the dusty troposphere, the lowest layer of the planet's nurturing atmosphere. A clear high altitude layer, visible along the dayside's upper edge, scatters blue sunlight and fades into the blackness of space. This picture was taken in June of 2001 from the International Space Station orbiting at an altitude of 211 nautical miles. But you can check out the vital signs of Planet Earth Now.

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orin stepanek
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Re: APOD: Planet Earth at Twilight (2021 Apr 22)

Post by orin stepanek » Thu Apr 22, 2021 1:07 pm

https://www.youtube.com/watch?v=u8qTZyY84l8
ISS002-E-7377_1024c.JPG

Takes me back a bit! 🥰 🤩
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Orin

Smile today; tomorrow's another day!

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Re: APOD: Planet Earth at Twilight (2021 Apr 22)

Post by johnnydeep » Thu Apr 22, 2021 4:34 pm

I take it the main point to understand here is that the terminator on bodies with atmospheres will always appear diffuse due to scattering, whereas on bodies with no atmosphere the terminator will be sharp?
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Re: APOD: Planet Earth at Twilight (2021 Apr 22)

Post by Chris Peterson » Thu Apr 22, 2021 4:42 pm

johnnydeep wrote:
Thu Apr 22, 2021 4:34 pm
I take it the main point to understand here is that the terminator on bodies with atmospheres will always appear diffuse due to scattering, whereas on bodies with no atmosphere the terminator will be sharp?
Mostly... although no terminator will be perfectly sharp unless the body is illuminated by a point source. With an extended light source (like the Sun at its distance to the planets) there will always be a penumbral band.
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Re: APOD: Planet Earth at Twilight (2021 Apr 22)

Post by neufer » Thu Apr 22, 2021 5:21 pm


Chris Peterson wrote:
Thu Apr 22, 2021 4:42 pm


... no terminator will be perfectly sharp unless the body is illuminated by a point source. With an extended light source (like the Sun at its distance to the planets) there will always be a penumbral band.
No terminator will be perfectly sharp unless the body is also perfectly convex/smooth.
Art Neuendorffer

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Re: APOD: Planet Earth at Twilight (2021 Apr 22)

Post by Chris Peterson » Thu Apr 22, 2021 5:53 pm

neufer wrote:
Thu Apr 22, 2021 5:21 pm

Chris Peterson wrote:
Thu Apr 22, 2021 4:42 pm


... no terminator will be perfectly sharp unless the body is illuminated by a point source. With an extended light source (like the Sun at its distance to the planets) there will always be a penumbral band.
No terminator will be perfectly sharp unless the body is also perfectly convex/smooth.
Only because of light scattered from surface features, though.
Chris

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johnnydeep
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Re: APOD: Planet Earth at Twilight (2021 Apr 22)

Post by johnnydeep » Thu Apr 22, 2021 6:34 pm

Chris Peterson wrote:
Thu Apr 22, 2021 4:42 pm
johnnydeep wrote:
Thu Apr 22, 2021 4:34 pm
I take it the main point to understand here is that the terminator on bodies with atmospheres will always appear diffuse due to scattering, whereas on bodies with no atmosphere the terminator will be sharp?
Mostly... although no terminator will be perfectly sharp unless the body is illuminated by a point source. With an extended light source (like the Sun at its distance to the planets) there will always be a penumbral band.
I see. Because for a disc source like the nearby Sun, it won't drop below the horizon all at once, but gradually, thereby slowly decreasing in intensity, rather than "blinking off". Makes perfect sense - thanks!
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Re: APOD: Planet Earth at Twilight (2021 Apr 22)

Post by MarkBour » Thu Apr 22, 2021 6:55 pm

Very pleasant APOD today. It actually looks a lot like a shot of Jupiter.


It might be interesting to try to figure out what are the giveaways that it is Earth, not Jupiter ... or to describe some of the differences from an image of any other thickly-atmosphered planet, such as Venus, for that matter.
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Re: APOD: Planet Earth at Twilight (2021 Apr 22)

Post by Evenstar » Thu Apr 22, 2021 10:31 pm

I've heard that Earth is losing O2 in its atmosphere but science does not understand why. All the while CO2 keeps rising. Apparently there is no simple way of stripping O2 from the carbon? If one could do this where would the carbon go--probably grab the O2 right back?
<Evenstar>

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Re: APOD: Planet Earth at Twilight (2021 Apr 22)

Post by WWW » Thu Apr 22, 2021 11:38 pm

Evenstar wrote:
Thu Apr 22, 2021 10:31 pm
I've heard that Earth is losing O2 in its atmosphere but science does not understand why. All the while CO2 keeps rising. Apparently there is no simple way of stripping O2 from the carbon? If one could do this where would the carbon go--probably grab the O2 right back?
There are simple ways to replenish O2 here on Earth.
An activity related to Ardor day is one.

Earth day is another popular day to do this. Anyone plant a tree for Earth Day ?
(I've also heard/read here that "tomorrows another day" - ...for planting a tree maybe?)

.
Last edited by WWW on Fri Apr 23, 2021 1:44 am, edited 2 times in total.

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Re: APOD: Planet Earth at Twilight (2021 Apr 22)

Post by WWW » Fri Apr 23, 2021 1:35 am

WWW wrote:
Thu Apr 22, 2021 11:38 pm
Evenstar wrote:
Thu Apr 22, 2021 10:31 pm

(I've also heard/read here that "tomorrows another day" - ...for planting a tree?)

.

In addition, 1 week after tomorrow is actually National Arbor day....

(Some US states differ see: https://www.arborday.org/celebrate/dates.cfm )



.

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Re: APOD: Planet Earth at Twilight (2021 Apr 22)

Post by WWW » Fri Apr 23, 2021 1:41 am

Making O2 on other worlds:
Did everyone see the recent news of Perseverance making oxygen on Mars from CO2 ?
https://www.nasa.gov/press-release/nasa ... red-planet


.

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Re: APOD: Planet Earth at Twilight (2021 Apr 22)

Post by neufer » Fri Apr 23, 2021 2:31 am

Chris Peterson wrote:
Thu Apr 22, 2021 5:53 pm
neufer wrote:
Thu Apr 22, 2021 5:21 pm
Chris Peterson wrote:
Thu Apr 22, 2021 4:42 pm

... no terminator will be perfectly sharp unless the body is illuminated by a point source. With an extended light source (like the Sun at its distance to the planets) there will always be a penumbral band.
No terminator will be perfectly sharp unless the body is also perfectly convex/smooth.
Only because of light scattered from surface features, though.
A 2 kilometer high crater rim on the Moon will stick out as a bright feature 80 kilometers (2.6º) into the dark side of the terminator.

Even the bright side of the terminator is indistinct because:
  • 1) bright 2 kilometer high crater rims produce shadows up to 80 kilometers (2.6º) long.

    2) the terminator, itself, is but a discontinuity in the first derivative of the surface brightness.
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Re: APOD: Planet Earth at Twilight (2021 Apr 22)

Post by Chris Peterson » Fri Apr 23, 2021 4:38 am

neufer wrote:
Fri Apr 23, 2021 2:31 am
Chris Peterson wrote:
Thu Apr 22, 2021 5:53 pm
neufer wrote:
Thu Apr 22, 2021 5:21 pm

No terminator will be perfectly sharp unless the body is also perfectly convex/smooth.
Only because of light scattered from surface features, though.
A 2 kilometer high crater rim on the Moon will stick out as a bright feature 80 kilometers (2.6º) into the dark side of the terminator.

Even the bright side of the terminator is indistinct because:
  • 1) bright 2 kilometer high crater rims produce shadows up to 80 kilometers (2.6º) long.

    2) the terminator, itself, is but a discontinuity in the first derivative of the surface brightness.
Like I said, not a continuous curve. But without scatter off of surface features, the terminator would still be sharp. Every point would either be fully illuminated, or not illuminated at all. Nothing in-between.
Chris

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Re: APOD: Planet Earth at Twilight (2021 Apr 22)

Post by oliverr » Tue May 04, 2021 11:22 am

Technical question regarding this APOD:
the file extension of this APOD is upper case - is this to be expected more, or a one-time thing?

Asking because of working with the APOD images through NASA's APOD-API…
and the issue here with this particular image.
APOD Robot wrote:
Thu Apr 22, 2021 4:05 am
Image Planet Earth at Twilight

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Re: APOD: Planet Earth at Twilight (2021 Apr 22)

Post by Chris Peterson » Tue May 04, 2021 12:41 pm

oliverr wrote:
Tue May 04, 2021 11:22 am
Technical question regarding this APOD:
the file extension of this APOD is upper case - is this to be expected more, or a one-time thing?

Asking because of working with the APOD images through NASA's APOD-API…
and the issue here with this particular image.
APOD Robot wrote:
Thu Apr 22, 2021 4:05 am
Image Planet Earth at Twilight
As a rule, the image on the main page for an APOD is modified by the editors, and the click-through image is as provided by its author, including its filename. You should be able to support any valid filename, with any valid characters and any mix of case.
Chris

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Re: APOD: Planet Earth at Twilight (2021 Apr 22)

Post by oliverr » Tue May 04, 2021 12:47 pm

Chris Peterson wrote:
Tue May 04, 2021 12:41 pm
As a rule, the image on the main page for an APOD is modified by the editors, and the click-through image is as provided by its author, including its filename. You should be able to support any valid filename, with any valid characters and any mix of case.
Thanks for your prompt reply and shading some light on the procedure, Chris. Very helpful :ssmile: