APOD: Rocket Plume Shadow Points to the Moon (2024 Feb 11)

[APOD Robot]

Rocket Plume Shadow Points to the Moon

Explanation: Why would the shadow of a rocket's launch plume point toward the Moon? In early 2001 during a launch of the space shuttle Atlantis, the Sun, Earth, Moon, and rocket were all properly aligned for this photogenic coincidence. First, for the space shuttle's plume to cast a long shadow, the time of day must be either near sunrise or sunset. Only then will the shadow be its longest and extend all the way to the horizon. Finally, during a Full Moon, the Sun and Moon are on opposite sides of the sky. Just after sunset, for example, the Sun is slightly below the horizon, and, in the other direction, the Moon is slightly above the horizon. Therefore, as Atlantis blasted off, just after sunset, its shadow projected away from the Sun toward the opposite horizon, where the Full Moon happened to be.

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[Pete_M]

Good explanation, but implicit is that the Space Shuttle must take off in an easterly direction to take advantage of the Earth’s angular velocity. So you could have a picture like this at sunrise, but the shuttle would be coming back towards you over your head, not away from you.

[florid_snow]

I've always loved this image, but I feel compelled to make a "well, actually" comment, please forgive me, but I don't think the word "coincidence" is appropriate and instead one could conceivably substitute "inevitable" because of the caveat that we have a Full Moon, not just any moon.

During a full moon, there is no location that a rocket could take-off and have its shadow point elsewhere than towards the full moon, such an occurrence is inevitable, not a coincidence. Even at noon, when you can't see the moon, the shadow will still be pointing in the correct direction. And at sunrise and sunset, the full moon doesn't just "happen" to be on the horizon sometimes, if it is a full moon, it is always on the horizon at sunrise and sunset, by definition. It is impossible for this shadow alignment to be different in a fundamental way during a full moon, no matter what time of day, or latitude of launch, so it is not a coincidence, it is a fun inevitability of geometry and definitions. At least, seems kinda fun to me.

[Chris Peterson]

Explanation: Why would the shadow of a rocket's launch plume point toward the Moon? In early 2001 during a launch of the space shuttle Atlantis, the Sun, Earth, Moon, and rocket were all properly aligned for this photogenic coincidence. First, for the space shuttle's plume to cast a long shadow, the time of day must be either near sunrise or sunset. Only then will the shadow be its longest and extend all the way to the horizon. Finally, during a Full Moon, the Sun and Moon are on opposite sides of the sky. Just after sunset, for example, the Sun is slightly below the horizon, and, in the other direction, the Moon is slightly above the horizon. Therefore, as Atlantis blasted off, just after sunset, its shadow projected away from the Sun toward the opposite horizon, where the Full Moon happened to be.

Something important that is missed in this explanation is the geometry of the shadow with respect to the observer. The shadow isn't a column of darkness, but a plane of very slightly dimmer air. It is only visible when the observer is on that same plane, and looking through a long path of attenuated air. Very few people would have been able to see this shadow when the image was taken, and the shadow would have faded from sight if the imager had moved very far to the left or right of this location.
_

[Ann]

There is too much math for me here, even though I feel I should have understood Chris' post.

Anyway. Here's my take on the colors of the APOD:

So I take it that the brown color of the exhaust is the natural color of whatever is in there. I assume that the reddish, yellow and white parts of the top of the plume are reddish sunset light, yellow sunset light and full white sunlight. I take it that the pink part of the sky is the Belt of Venus, and the dark blue part of the sky below the pink part is the Earth's shadow.

The Belt of Venus. Note the dark blue rising shadow of the Earth near the horizon. Author unknown.

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What is most striking to me, of course, is the intensely blue color of the shadow of the shuttle plume. Could the color of the shadow have anything to do with the blue hour?

Blue hour by Marianna Bucina Roca in Gloucester, Massachusetts.

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Wikipedia wrote:

The blue hour (from French l'heure bleue; pronounced [lœʁ blø]) is the period of twilight (in the morning or evening, around the nautical stage) when the Sun is at a significant depth below the horizon. During this time, the remaining sunlight takes on a mostly blue shade. This shade differs from the colour of the sky on a clear day, which is caused by Rayleigh scattering.

The blue hour occurs when the Sun is far enough below the horizon so that the sunlight's blue wavelengths dominate due to the Chappuis absorption caused by ozone.

I hope you're taking notes.

Anyway. Shadows are often very blue on sunny days when there is snow on the ground, as in this photo from Malmö on January 18, 2024:

The shadows are very blue here, as you can see.

But are shadows in the sky really blue for the same reason as shadows on snow on a sunny day (when shadows get their color from the blue light of the sky)?

Help, Chris! Why is the shadow of the shuttle plume so blue in the APOD? Is it, perhaps, just a consequence of the processing?

Ann

[johnnydeep]

Explanation: Why would the shadow of a rocket's launch plume point toward the Moon? In early 2001 during a launch of the space shuttle Atlantis, the Sun, Earth, Moon, and rocket were all properly aligned for this photogenic coincidence. First, for the space shuttle's plume to cast a long shadow, the time of day must be either near sunrise or sunset. Only then will the shadow be its longest and extend all the way to the horizon. Finally, during a Full Moon, the Sun and Moon are on opposite sides of the sky. Just after sunset, for example, the Sun is slightly below the horizon, and, in the other direction, the Moon is slightly above the horizon. Therefore, as Atlantis blasted off, just after sunset, its shadow projected away from the Sun toward the opposite horizon, where the Full Moon happened to be.

Something important that is missed in this explanation is the geometry of the shadow with respect to the observer. The shadow isn't a column of darkness, but a plane of very slightly dimmer air. It is only visible when the observer is on that same plane, and looking through a long path of attenuated air. Very few people would have been able to see this shadow when the image was taken, and the shadow would have faded from sight if the imager had moved very far to the left or right of this location.
_
contrailshadow.jpg

I'm having trouble understanding this. Isn't there a a 3D volume of dimmer air with a cross section that matches the Sun-illuminated surface of the column of exhaust? True, our eyes can only "perceive" a single plane, but that's true of all things we see since we don't see all points in a volume simultaneously!

[Chris Peterson]

Explanation: Why would the shadow of a rocket's launch plume point toward the Moon? In early 2001 during a launch of the space shuttle Atlantis, the Sun, Earth, Moon, and rocket were all properly aligned for this photogenic coincidence. First, for the space shuttle's plume to cast a long shadow, the time of day must be either near sunrise or sunset. Only then will the shadow be its longest and extend all the way to the horizon. Finally, during a Full Moon, the Sun and Moon are on opposite sides of the sky. Just after sunset, for example, the Sun is slightly below the horizon, and, in the other direction, the Moon is slightly above the horizon. Therefore, as Atlantis blasted off, just after sunset, its shadow projected away from the Sun toward the opposite horizon, where the Full Moon happened to be.

Something important that is missed in this explanation is the geometry of the shadow with respect to the observer. The shadow isn't a column of darkness, but a plane of very slightly dimmer air. It is only visible when the observer is on that same plane, and looking through a long path of attenuated air. Very few people would have been able to see this shadow when the image was taken, and the shadow would have faded from sight if the imager had moved very far to the left or right of this location.
_
contrailshadow.jpg

I'm having trouble understanding this. Isn't there a a 3D volume of dimmer air with a cross section that matches the Sun-illuminated surface of the column of exhaust? True, our eyes can only "perceive" a single plane, but that's true of all things we see since we don't see all points in a volume simultaneously!

Maybe it's easier to visualize as a curtain, which "hangs" from the exhaust trail in a direction away from the Sun. If you were facing that curtain, you wouldn't see anything, because you're only looking through a few meters of unilluminated air. But if you're looking at the curtain from the end, along its length, that's now 1000s of meters of unilluminated air... so it looks noticeably darker. And it also looks like a column rather than a sheet.

[Ann]

Something important that is missed in this explanation is the geometry of the shadow with respect to the observer. The shadow isn't a column of darkness, but a plane of very slightly dimmer air. It is only visible when the observer is on that same plane, and looking through a long path of attenuated air. Very few people would have been able to see this shadow when the image was taken, and the shadow would have faded from sight if the imager had moved very far to the left or right of this location.
_
contrailshadow.jpg

I'm having trouble understanding this. Isn't there a a 3D volume of dimmer air with a cross section that matches the Sun-illuminated surface of the column of exhaust? True, our eyes can only "perceive" a single plane, but that's true of all things we see since we don't see all points in a volume simultaneously!

Maybe it's easier to visualize as a curtain, which "hangs" from the exhaust trail in a direction away from the Sun. If you were facing that curtain, you wouldn't see anything, because you're only looking through a few meters of unilluminated air. But if you're looking at the curtain from the end, along its length, that's now 1000s of meters of unilluminated air... so it looks noticeably darker. And it also looks like a column rather than a sheet.

You didn't answer my question about the blue color of the plume shadow, but since I am the Color Commentator, I should probably answer that question myself.

And wait! I think perhaps I've got it. The shadow is similar to a crepuscular ray, or rather an anticrepuscular ray, since it is seen from the direction opposite the Sun:

Blue crepuscular ray over Houston in 2016. Image credit: Probably ABC13 Houston.

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ABC13 wrote:

HOUSTON (KTRK) -- What caused this mysterious blue streak across Houston's morning sky? Meteorologist Travis Herzog says this is a thunderstorm cloud shadow illuminated by crepuscular rays.

Not sure I like the expression "illuminated by crepuscular rays". The blue streak is a crepuscular ray, and it is a shadow, and a shadow is characterized by light having been taken away, not light having been added.

I guess the dark (blue) crepuscular ray is indeed similar to the blue shadow of a tree on snowy ground on a sunny day: The color of the shadow is similar to the color of the sky, plus the color of the background, minus any direct sunlight, and that is why a shadow on a sunny day is not just darker but also bluer than the surrounding ground or sky.

As for the geometry of the plume shadow in the APOD, let the math idiot compare the plume shadow with an edge-on galaxy. These tend to have a reasonably high surface brightness along their disks. But face-on galaxies can be very faint indeed, unless they have a bright center. Their disks tend to be hard to see.

Edge-on spiral galaxy NGC 4565 is comparatively easy to see. Credit: Ken Crawford.

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Sketch of faint face-on galaxy M74. The artist, Frank LILL (Francis) wrote: 'M 74 is a spiral face-on galaxy, of 9.4 visual magnitude, but of very low surface brightness so that it barely stands out from sky background. It deserves its name, the Phantom Galaxy, NGC628. In the constellation of Pisces, it is about 30 million light years away. It is a blurry spot at the eyepiece with a weak central core, but the spiral arms appear when using EAA. It reminds a little of M101. Only about 1/3 of the surface of the galaxy is reported in the sketch, the extensions are missed. However the angular shape of the northern arm is clearly there.'

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[Chris Peterson]

Not sure I like the expression "illuminated by crepuscular rays". The blue streak is a crepuscular ray, and it is a shadow, and a shadow is characterized by light having been taken away, not light having been added.

Well, there's a terminology issue here. Which are the "rays"... the light parts or the shadows? I think it is more common to refer to the illuminated segments as "crepuscular rays" and the dark as "crepuscular shadows". That view is also consistent with this Wikipedia comment on the subject:

Crepuscular rays usually appear orange because the path through the atmosphere at sunrise and sunset passes through up to 40 times as much air as rays from a high Sun at midday. Particles in the air scatter short-wavelength light (blue and green) through Rayleigh scattering much more strongly than longer-wavelength yellow and red light.

[Ann]

Not sure I like the expression "illuminated by crepuscular rays". The blue streak is a crepuscular ray, and it is a shadow, and a shadow is characterized by light having been taken away, not light having been added.

Well, there's a terminology issue here. Which are the "rays"... the light parts or the shadows? I think it is more common to refer to the illuminated segments as "crepuscular rays" and the dark as "crepuscular shadows". That view is also consistent with this Wikipedia comment on the subject:

Crepuscular rays usually appear orange because the path through the atmosphere at sunrise and sunset passes through up to 40 times as much air as rays from a high Sun at midday. Particles in the air scatter short-wavelength light (blue and green) through Rayleigh scattering much more strongly than longer-wavelength yellow and red light.

Okay, Chris. Got it.

(Well, actually, I didn't find the Wikipedia entry helpful at all - since it didn't even mention dark crepuscular shadows - but I agree with you that "crepuscular shadow" is a much, much better term the "crepuscular ray" for the straight dark shadows across the sky.)

Ann

[johnnydeep]

Something important that is missed in this explanation is the geometry of the shadow with respect to the observer. The shadow isn't a column of darkness, but a plane of very slightly dimmer air. It is only visible when the observer is on that same plane, and looking through a long path of attenuated air. Very few people would have been able to see this shadow when the image was taken, and the shadow would have faded from sight if the imager had moved very far to the left or right of this location.
_
contrailshadow.jpg

I'm having trouble understanding this. Isn't there a a 3D volume of dimmer air with a cross section that matches the Sun-illuminated surface of the column of exhaust? True, our eyes can only "perceive" a single plane, but that's true of all things we see since we don't see all points in a volume simultaneously!

Maybe it's easier to visualize as a curtain, which "hangs" from the exhaust trail in a direction away from the Sun. If you were facing that curtain, you wouldn't see anything, because you're only looking through a few meters of unilluminated air. But if you're looking at the curtain from the end, along its length, that's now 1000s of meters of unilluminated air... so it looks noticeably darker. And it also looks like a column rather than a sheet.

Ok, thanks. When put this way, I think I understand!