APOD: Shuttle Plume Shadow Points to the Moon (2011 Nov 27)

[Chris Peterson]

IMO, serendipitous image/s, indeed.

Yes. Because, as previously noted, not only do the Sun, Moon, and plume need to be collinear (or coplanar, if you prefer), but the imager needs to be close to that plane, as well. Because otherwise, the shadow will be all but invisible.

I observe contrail shadows all the time, and image them when I can. The only time you can see them is when you, the Sun, and the shadow are approximately on the same plane. Otherwise, you simply aren't looking through a thick enough layer of air for the shadow contrast to be significant.

Certainly, an imager can plan for a shot like this (within reason- at a launch facility, there are simply places where it will be impossible to place a camera). But catching a shadow like this without planning is the very definition of coincidental, or serendipitous.

[zloq]

Hmm - I think you both are missing my main point. The shadow itself was visible over a wide range of locations perpendicular to the "plane" - and those locations all had a great lookin' shadow - but it didn't go right through the center of the moon. The center of a given plane would have the best looking shadow - but as you move either way, the location of the shadow relative to the moon changes - and the shadow still looks pretty good. You are free to move left and right to where the shadow makes a bullseye right through the center of the moon - as shown in the apod image. It's the fact that the shadow makes a perfect bullseye that is so compelling about the image - and that's because the photographer dialed it in by changing location and choosing the exact time - since the plume, sun, and moon are all moving, and both refraction and parallax are involved. It's a dynamic scene, and what you see depends on your location and the changing solar refraction - not just the astronomical geometry.

I don't know how to make this any clearer - the same photographer has another image with a great shadow - but it's offset from the center of the moon because it was earlier and in a slightly different location. A different photographer much farther away also has a great shadow and it is way off from the center of the moon. The apod image would have been much less compelling if the shadow even slightly missed the center of the moon, but it's perfect - and that is what bothered me. It's perfect because he moved to where he needed to be, and chose the exact moment as a dynamic scene unfolded.

The contrail image above is amusing because it actually flares out at the base - as opposed to getting smaller due to perspective. A rocket contrail is different because it is roughly vertical, and it isn't straight - which means it forms many "planes" where a good shadow can appear from a wide range of locations. Additionally, the shadow in the apod image is different from a contrail because the sun has set and the earth below is in shadow. That means that the dark volume of atmosphere associated with the shadow doesn't start at the eye (as in does in the contrail example), but instead starts way up in the sky where the sun is shining. That increases the role of location and parallax in the location of the shadow relative to the moon.

In fact - I think people could plan the apod shot if they have a rocket launch around a full moon at dawn or sunset. Just be in an area where you can drive perpendicular to the plane to find where the shadow exactly hits the moon as the scene unfolds.

zloq

[Chris Peterson]

Hmm - I think you both are missing my main point. The shadow itself was visible over a wide range of locations perpendicular to the "plane" - and those locations all had a great lookin' shadow - but it didn't go right through the center of the moon.

Are you sure about that? My experience is that unless a contrail shadow is actually cast onto clouds, you just don't see that shadow unless you are fairly near the plane defined by the contrail and the Sun. The degree to which even humid atmosphere scatters light is just generally too small, and as you get off the plane, the shadow itself becomes planar, not linear. Certainly, this is a denser trail than typical contrails, and is more vertical. I don't think the direction matters, although the density ought to.

Could you point me again to an image that shows the shadow from well off the plane? I'm not finding it in the examples given.

[zloq]

This isn't a thin, straight, horizontal contrail high up in daylight with the sun overhead shining down. It is a nearly vertical, wide, jagged and multi-segmented rocket contrail after sunset with the sun shining up from below. This makes the shadow a cross between a crepuscular ray and a volumetric shadow, since the shadow volume does not actually include the viewer - though it does involve an accumulation of contrast through a slab-like volume for each roughly straight part of the plume. I pointed to two relevant images in my notes above where the shadow is off from the moon compared to the perfection in the apod example - for the same event - indicating the role of location and time on the alignment. Finally, I indicated that the photographer apparently changed his location in realtime during this important photo-op, and improved on the alignment of the shadow with the moon as shown by the change in the two pictures.

I have seen many jet contrail volumetric shadows in daylight similar to your picture. I have never seen a rocket launch just after sunset - but I can visualize the key differences relevant to this apod, and the role parallax would play since the bottom of the shadow is "up there" and its location in the sky is dependent on view location. I also know the direction of the shadow will depend on the time-dependent refraction of the sun as it sets - which has no analog for a jet contrail - and allows time to dial in the best alignment for a given location.

zloq

[Indigo_Sunrise]

zloq, I misunderstood your previous post, and have corrected mine.