APOD: Planet Earth at Twilight (2021 Apr 22)

[APOD Robot]

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]

https://www.youtube.com/watch?v=u8qTZyY84l8

Takes me back a bit! 🥰

[johnnydeep]

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?

[Chris Peterson]

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.

[neufer]

---

... 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.

[Chris Peterson]

---

... 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.

[johnnydeep]

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!

[MarkBour]

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.

[Evenstar]

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?

[WWW]

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.

Click to view full size image

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?)

.

[WWW]

(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 )



.

[WWW]

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


.

[neufer]

... 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.

Topography of the Moon

---

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.

[Chris Peterson]

No terminator will be perfectly sharp unless the body is also perfectly convex/smooth.

Only because of light scattered from surface features, though.

Topography of the Moon

---

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.

[oliverr]

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.

Planet Earth at Twilight

[Chris Peterson]

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.

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.

[oliverr]

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