APOD: A Blue Moon Halo over Antarctica (2015 Aug 11)

[APOD Robot]

A Blue Moon Halo over Antarctica

Explanation: Have you ever seen a halo around the Moon? Such 22 degree rings around the Moon -- caused by ice crystals falling in the Earth's atmosphere -- are somewhat rare. OK, but have you ever seen a blue moon? Given the modern definition of blue moon -- the second full moon occurring in a calendar month -- these are also rare. What is featured above might therefore be considered doubly rare -- a halo surrounding a blue moon. The featured image was taken late last month near Zhongshan Station in Antarctica. Visible in the foreground are a power generating house and a snowmobile. What might seem to be stars in the background are actually illuminated snowflakes near the camera.

<< Previous APOD

This Day in APOD

Next APOD >>

[/b]

[Guest]

Great shot from a very cold place. What is illuminating the clouds on the lower left? Since the moon is full, the sun is behind the photographer.

[alter-ego]

Great shot from a very cold place. What is illuminating the clouds on the lower left? Since the moon is full, the sun is behind the photographer.

Not clouds. Artificial lighting from behind the building illuminating what appears to be a snow-covered hill in the background.

[Boomer12k]

Awesome looking....Snow Cats, GO!!!

:---[===] *

[Boomer12k]

Took me a little time to realize those are not stars but snow....
Constellations of Snow....

:---[===] *

[Altair2015]

The moon is not at the center of the halo (closer to lower left). Why?

[Michael Spencer]

Why is the halo apparently asymmetrical on the moon?
Cheers,
Mike

[isoparix]

Why is the halo apparently asymmetrical on the moon?
Cheers,
Mike

This is because the camera was not pointed directly at the moon.

It's not distortion, it's just geometry. Imagine contructing a huge flat piece of graph paper, pinning it to a wall, and photographing that. The bits directly in front of the camera wil maintain, more or less, the equal areas and right angles of the grid. However, the further away from the centre you get, the less the image of this distant view will resemble the 'correct' central view.

Not a fault of the lens, or the camera, just the geometry of projections.

[henrystar]

Took me a little time to realize those are not stars but snow....
Constellations of Snow....

:---[===] *

Oval snowflakes? Hmmm…. could be UFOs.

[stevenmaines]

What accounts for the size/diameter of the ring? I have seen smaller rings here in the south.

[Chris Peterson]

What accounts for the size/diameter of the ring? I have seen smaller rings here in the south.

A halo like this is always about the same size, around the Sun or Moon- 44° across the inner diameter (and it extends outward much farther, but most of that is difficult or impossible to see). The size is determined by the geometry of the ice crystals that create it.

There is another phenomenon called a corona that creates a kind of halo or glow around the Moon, and it's much smaller than the 44° halo phenomenon seen in today's image.

That said, I'd be cautious about relating the size of something seen in an image to something you've seen with your eyes. In many images (including this one) there is little to provide any scale. This could be a very small halo imaged with a long focal length lens, or a very large one imaged with a short focal length lens.

[Joe Stieber]

That said, I'd be cautious about relating the size of something seen in an image to something you've seen with your eyes. In many images (including this one) there is little to provide any scale. This could be a very small halo imaged with a long focal length lens, or a very large one imaged with a short focal length lens.

However, there are a couple of hints that it's a wide-angle view. They are... (1) the converging verticals of the buildings in the foreground and (2) the far greater diameter of the halo with respect to the half-degree diameter of the moon (and the moon image is actually disproportionately large because of overexposure).

Anyway, knowing that it's a 22 degree radius halo, a rough measurement shows the frame is about 110 degrees wide -- indeed, a wide-angle shot. With a DSLR camera, that would be approximately the field of an 8 mm focal length lens with an APS-C sensor or 12 mm focal length with a "full-frame" sensor (like 35 mm film). Since the moon is not centered in the frame, geometric distortion from the very wide angle would account for the obvious non-centered position of the moon inside the halo.

BTW, this picture was also featured as a recent Optics Picture of the Day, along with a non-flash version having no snowflakes in the foreground.

[John Bragg]

I appreciated the picture of the blue moon over Antarctica. I have to say, though, that moon halos are quite common in the Pacific Northwest of the US. Rare or common-- they are always quite lovely! Thanks.

[MarkBour]

Why is the halo apparently asymmetrical on the moon?
Cheers,
Mike

This is because the camera was not pointed directly at the moon.

It's not distortion, it's just geometry. Imagine contructing a huge flat piece of graph paper, pinning it to a wall, and photographing that. The bits directly in front of the camera wil maintain, more or less, the equal areas and right angles of the grid. However, the further away from the centre you get, the less the image of this distant view will resemble the 'correct' central view.

Not a fault of the lens, or the camera, just the geometry of projections.

Interesting explanation. If I understand it, you're really talking about the geometry of the camera's light sensor, which is probably a flat planar surface, so turning this surface relative to the distant light source (the Moon) can cause the halo to land on the surface off-center and distorted. If I'm picturing this right, it seems it must also stretch the halo (image) into an ellipse. I don't know if I should go further, but our human eyes, with their spherical retina, may be pretty immune to this effect.

Meanwhile, it's too bad about the explosion of the neighboring facility ...

[Chris Peterson]

Interesting explanation. If I understand it, you're really talking about the geometry of the camera's light sensor, which is probably a flat planar surface, so turning this surface relative to the distant light source (the Moon) can cause the halo to land on the surface off-center and distorted. If I'm picturing this right, it seems it must also stretch the halo (image) into an ellipse.

It's just an inherent problem with mapping a spherical scene onto a plane. Different lenses offer different projections (and hence, different sorts of aberration), but in essence, a degree of FOV on the optical axis and a degree of FOV far off to the side map to different physical distances at the image plane. So a circle ends up looking distorted.

I don't know if I should go further, but our human eyes, with their spherical retina, may be pretty immune to this effect.

Our eyes have all sorts of optical aberrations, as well. Our brain does a good job fixing them for us so we don't notice.