APOD: Looking Back at an Eclipsed Earth (2012 May 30)

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APOD: Looking Back at an Eclipsed Earth (2012 May 30)

Postby APOD Robot » Wed May 30, 2012 4:08 am

Image Looking Back at an Eclipsed Earth

Explanation: What's that dark spot on planet Earth? It's the shadow of the Moon. The above image of Earth was taken last week by MTSAT during an annular eclipse of the Sun. The dark spot appears quite unusual as clouds are white and the oceans are blue in this color corrected image. Earthlings residing within the dark spot would see part of the Sun blocked by the Moon and so receive less sunlight than normal. The spot moved across the Earth at nearly 2,000 kilometers per hour, giving many viewers less than two hours to see a partially eclipsed Sun. MTSAT circles the Earth in a geostationary orbit and so took the above image from about three Earth-diameters away. Sky enthusiasts might want to keep their eyes pointed upward this coming week as a partial eclipse of the Moon will occur on June 4 and a transit of Venus across the face of the Sun will occur on June 5.

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Re: APOD: Looking Back at an Eclipsed Earth (2012 May 30)

Postby bystander » Wed May 30, 2012 4:42 am

Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
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Re: APOD: Looking Back at an Eclipsed Earth (2012 May 30)

Postby emc » Wed May 30, 2012 12:12 pm

Darkness creeps
The soul creaks
What writhing, wiggling wormy things doth bask in the dim
I shudder, I sicken, I’m sightless
Oh when will this end… this despair amongst the Day
What stealthy demons lurk amidst a shadow… laughing insanely in its playground
But take heart my love, have patience! Relief is but Time away
Darkness seeps only until Light casts it back to the deep…
Until darkness comes again

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Re: APOD: Looking Back at an Eclipsed Earth (2012 May 30)

Postby APODFORIST » Wed May 30, 2012 1:01 pm

The dark spot appears quite unusual as clouds are white and the oceans are blue in this color corrected image

What is unusual with white clouds, blue water and a dark grey shadow?

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Re: APOD: Looking Back at an Eclipsed Earth (2012 May 30)

Postby neufer » Wed May 30, 2012 1:56 pm

APODFORIST wrote:

The dark spot appears quite unusual as clouds are white and the oceans are blue in this color corrected image

What is unusual with white clouds, blue water
and a dark grey shadow?

The spot appears to be too dark for purely penumbral shadow.

MACBETH: Out, out, brief candle!
    Life's but a walking shadow, a poor player
    That struts and frets his hour upon the stage
    And then is heard no more: it is a tale
    Told by an idiot, full of sound and fury,
    Signifying nothing.
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Re: APOD: Looking Back at an Eclipsed Earth (2012 May 30)

Postby ta152h0 » Wed May 30, 2012 5:27 pm

looks like a storm moving in the lower right of the image. a giant spiral thing.
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Re: APOD: Looking Back at an Eclipsed Earth (2012 May 30)

Postby flash » Wed May 30, 2012 8:38 pm

ta152h0 wrote:looks like a storm moving in the lower right of the image. a giant spiral thing.

Not so big as the other giant spiral things usually seen here. :roll:

http://apod.nasa.gov/apod/ap091230.html

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Re: APOD: Looking Back at an Eclipsed Earth (2012 May 30)

Postby Beyond » Wed May 30, 2012 10:10 pm

flash wrote:
ta152h0 wrote:looks like a storm moving in the lower right of the image. a giant spiral thing.

Not so big as the other giant spiral things usually seen here. :roll:

http://apod.nasa.gov/apod/ap091230.html

That's because 'here', is a lot smaller than the place those other giant spiral things are.
To find the Truth, you must go Beyond.

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Re: APOD: Looking Back at an Eclipsed Earth (2012 May 30)

Postby Tszabeau » Wed May 30, 2012 10:37 pm

Wow! When did they discover that planet? Let's go there this summer.

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Re: APOD: Looking Back at an Eclipsed Earth (2012 May 30)

Postby saturno2 » Wed May 30, 2012 11:04 pm

This image is beautiful.

Well, It will be interesting to see the partial eclipse of the Moon on June 4 and the transit of Venus on June 5. :!:

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Re: APOD: Looking Back at an Eclipsed Earth (2012 May 30)

Postby ta152h0 » Thu May 31, 2012 8:59 pm

I also find it striking how thin the atmosphere really is when scaled with the diameter on the earth, clearly visible in this APOD. Something coming in at 60000 mph could easily penetrate, specially at angles close to perpendicular.
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Re: APOD: Looking Back at an Eclipsed Earth (2012 May 30)

Postby Chris Peterson » Thu May 31, 2012 11:51 pm

ta152h0 wrote:I also find it striking how thin the atmosphere really is when scaled with the diameter on the earth, clearly visible in this APOD. Something coming in at 60000 mph could easily penetrate, specially at angles close to perpendicular.

Lots of stuff comes in all the time, most faster than that. It's more likely to survive if nearly horizontal, however. But very little actually does make it to the ground. To survive through even that thin layer of air and hit the ground still carrying part of its original speed requires a very massive body- typically a few tens of meters across. Luckily, bodies that large are comparatively rare.

Even allowing for the fact that the Earth continuously resculpts its surface, you only have to compare it to the Moon to get a sense of just how useful our atmosphere is in protecting us.
Chris

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Re: APOD: Looking Back at an Eclipsed Earth (2012 May 30)

Postby Beyond » Fri Jun 01, 2012 12:12 am

And all the pollution just may help a bit also. :mrgreen:
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Re: APOD: Looking Back at an Eclipsed Earth (2012 May 30)

Postby neufer » Fri Jun 01, 2012 1:20 am

Tszabeau wrote:
Wow! When did they discover that planet?

Click to play embedded YouTube video.
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Re: APOD: Looking Back at an Eclipsed Earth (2012 May 30)

Postby flash » Fri Jun 01, 2012 3:41 pm

Chris Peterson wrote:
ta152h0 wrote:I also find it striking how thin the atmosphere really is when scaled with the diameter on the earth, clearly visible in this APOD. Something coming in at 60000 mph could easily penetrate, specially at angles close to perpendicular.

Lots of stuff comes in all the time, most faster than that. It's more likely to survive if nearly horizontal, however. But very little actually does make it to the ground. To survive through even that thin layer of air and hit the ground still carrying part of its original speed requires a very massive body- typically a few tens of meters across. Luckily, bodies that large are comparatively rare.

Even allowing for the fact that the Earth continuously resculpts its surface, you only have to compare it to the Moon to get a sense of just how useful our atmosphere is in protecting us.

Assuming the atmosphere extends to 100 miles, a vertical dive at 60,000 mph gives only 6 seconds to vaporize before it hits the surface. That doesn't seem to be a lot of time. But then the energy is proportional to v2 and 60,000 squared is a big number.

Why is it that coming in at 60,000 mph horizontally (rather than vertically) make it more likely to survive? Doesn't that just give the energy more time to act?

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Re: APOD: Looking Back at an Eclipsed Earth (2012 May 30)

Postby Chris Peterson » Fri Jun 01, 2012 4:15 pm

flash wrote:Assuming the atmosphere extends to 100 miles, a vertical dive at 60,000 mph gives only 6 seconds to vaporize before it hits the surface. That doesn't seem to be a lot of time. But then the energy is proportional to v2 and 60,000 squared is a big number.

Exactly. The huge forces mean there's no way for a body traveling at (a relatively slow) 27 km/s to maintain that speed in the atmosphere. So the actual descent time becomes much longer than 6 seconds. It need not vaporize all at once; more likely is that it will break up into many small pieces, which either burn up or slow down and survive as meteorites (which land without any of the original velocity).

A 1-meter diameter meteoroid traveling 27 km/s experiences a drag force of more than 800,000 N at a height of 50 km. If it's stony, that's an acceleration of -50 G. That's enough to start breaking up most meteoroids. If a body somehow survived with that size and speed to 30 km, the acceleration increases to over 700 G. Even iron bodies will break up under those conditions.

Why is it that coming in at 60,000 mph horizontally (rather than vertically) make it more likely to survive? Doesn't that just give the energy more time to act?

Because it spends more time in the upper atmosphere, slowing down without encountering the destructive forces (as above) created by moving fast in thicker air. Remember it's that v2 term that dominates. If you can reduce v before the body descends into the deeper atmosphere, it's more likely that material will survive. The meteors that are observed to produce meteorites are usually slow to begin with (~20 km/s) and enter at a shallow angle. They need to be large enough that they can lose 90% or more of their mass to ablation and still leave something behind, but not break apart (or not break apart while still moving very fast) and lose all their mass in a nearly instantaneous burst of ablative loss.
Chris

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Re: APOD: Looking Back at an Eclipsed Earth (2012 May 30)

Postby neufer » Fri Jun 01, 2012 4:18 pm

flash wrote:
Assuming the atmosphere extends to 100 miles, a vertical dive at 60,000 mph gives only 6 seconds to vaporize before it hits the surface. That doesn't seem to be a lot of time. But then the energy is proportional to v2 and 60,000 squared is a big number.

Why is it that coming in at 60,000 mph horizontally (rather than vertically) make it more likely to survive? Doesn't that just give the energy more time to act?

Most of the kinetic energy is dissipated by shock wave heating of an atmospheric column.

That atmospheric column can be made very long and (therefore) the temperatures involved can be kept reasonable by entering horizontally.
Art Neuendorffer


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