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APOD: Shipwreck at Moonset (2018 Nov 24)
Posted: Sat Nov 24, 2018 5:09 am
by APOD Robot
Shipwreck at Moonset
Explanation: A crescent Moon is about to sink under the western horizon in this sea and night skyscape. The atmospheric photo was taken on September 11 from the desert shore along the
Skeleton Coast of Namibia. So close to moonset, the moonlight is reddened and dimmed by the low, long line-of-sight across the Atlantic. But near the center of the frame
Venus still shines brightly, its light reflected in calm ocean waters. The celestial beacon above the brilliant evening star is bright planet Jupiter.
Namibia's Skeleton Coast was so named for the many seal and whale bones that were once strewn along the shoreline. In more recent times it's better known
for shipwrecks.
Re: APOD: Shipwreck at Moonset (2018 Nov 24)
Posted: Sat Nov 24, 2018 10:35 am
by De58te
Weird. Where I live, Canada, even with the Moon Illusion where the Moon appears bigger near the horizon, I have never seen Venus twice the size of the moon. It usually appears like Jupiter is in the picture.
Re: APOD: Shipwreck at Moonset (2018 Nov 24)
Posted: Sat Nov 24, 2018 2:20 pm
by Ann
De58te wrote: ↑Sat Nov 24, 2018 10:35 am
Weird. Where I live, Canada, even with the Moon Illusion where the Moon appears bigger near the horizon, I have never seen Venus twice the size of the moon. It usually appears like Jupiter is in the picture.
I'm not the right person to answer you here.
As you can see, however, the Moon is new(?) and very low in the sky, near the horizon. Presumably, the Moon would look downright faint here to visual observers.
Venus and Jupiter, by contrast, would be unreddened by dust and visually bright, clearly brighter than the faint and reddened Moon. Venus would be so bright, in fact, that its brightness would cause "pixel bleeding" and make the planet look larger in the picture than it would to the eye. Another possibility is that the night when this photo was taken was misty, and the mist made the planets look larger than they usually do.
Ann
Re: APOD: Shipwreck at Moonset (2018 Nov 24)
Posted: Sat Nov 24, 2018 2:46 pm
by Chris Peterson
De58te wrote: ↑Sat Nov 24, 2018 10:35 am
Weird. Where I live, Canada, even with the Moon Illusion where the Moon appears bigger near the horizon, I have never seen Venus twice the size of the moon. It usually appears like Jupiter is in the picture.
No, it never
looks bigger than the Moon. But the longer the exposure, the bigger it gets in an image. Diffraction, scatter, and internal reflections. At the scale of this image, Venus is no bigger than a pixel. We're not seeing an
image of Venus at all, just a big blob of light that came off that body.
Re: APOD: Shipwreck at Moonset (2018 Nov 24)
Posted: Sat Nov 24, 2018 3:24 pm
by BDanielMayfield
Chris Peterson wrote: ↑Sat Nov 24, 2018 2:46 pm
De58te wrote: ↑Sat Nov 24, 2018 10:35 am
Weird. Where I live, Canada, even with the Moon Illusion where the Moon appears bigger near the horizon, I have never seen Venus twice the size of the moon. It usually appears like Jupiter is in the picture.
No, it never
looks bigger than the Moon. But the longer the exposure, the bigger it gets in an image. Diffraction, scatter, and internal reflections. At the scale of this image, Venus is no bigger than a pixel. We're not seeing an
image of Venus at all, just a big blob of light that came off that body.
So if Venus was no bigger than a a pixel in this shot it would make no difference what phase Venus was in? I was wondering because it's "blob of light" appears perfectly round. (And btw, what phase was Venus in back on Sep 11?
Re: APOD: Shipwreck at Moonset (2018 Nov 24)
Posted: Sat Nov 24, 2018 3:43 pm
by Chris Peterson
BDanielMayfield wrote: ↑Sat Nov 24, 2018 3:24 pm
Chris Peterson wrote: ↑Sat Nov 24, 2018 2:46 pm
De58te wrote: ↑Sat Nov 24, 2018 10:35 am
Weird. Where I live, Canada, even with the Moon Illusion where the Moon appears bigger near the horizon, I have never seen Venus twice the size of the moon. It usually appears like Jupiter is in the picture.
No, it never
looks bigger than the Moon. But the longer the exposure, the bigger it gets in an image. Diffraction, scatter, and internal reflections. At the scale of this image, Venus is no bigger than a pixel. We're not seeing an
image of Venus at all, just a big blob of light that came off that body.
So if Venus was no bigger than a a pixel in this shot it would make no difference what phase Venus was in? I was wondering because it's "blob of light" appears perfectly round. (And btw, what phase was Venus in back on Sep 11?
The only impact Venus's phase has on a wide field image is the amount of light it reflects. An overexposed Venus will always appear round (unless distorted by the optics), regardless of phase. BTW, this is true of the Moon, as well. Even a thin crescent moon looks like a big round blob in an image that is sufficiently overexposed.
Re: APOD: Shipwreck at Moonset (2018 Nov 24)
Posted: Sat Nov 24, 2018 4:26 pm
by neufer
BDanielMayfield wrote: ↑Sat Nov 24, 2018 3:24 pm
what phase was Venus in back on Sep 11?
Venus was ~34 arc-seconds wide on Sep 11
(very close to its maximum brightness of magnitude -4.8
when it is ~37 arc-seconds wide).
Jupiter is about 34 degrees above the surface
of the ocean or ~3600 Venus widths.
The Moon's width is about the geometric mean.
Re: APOD: Shipwreck at Moonset (2018 Nov 24)
Posted: Sat Nov 24, 2018 7:19 pm
by Fred the Cat
Originally from Astoria, Oregon, we often spent time peering at the wreck of the
Peter Iredale. You get that
sinking feeling for those people of the
Namibian coast wrecks.
Do good photographers sense those emotions prior to the “click” as the moon sinks into the horizon? My horizon is east, so the moon is on the rise. Maybe they’ll build a
launch pad on the mountain top.
Re: APOD: Shipwreck at Moonset (2018 Nov 24)
Posted: Sat Nov 24, 2018 8:13 pm
by neufer
https://en.wikipedia.org/wiki/Skeleton_Coast wrote:
<<The name Skeleton Coast was coined by John Henry
Marsh as the title for the 1944 book he wrote chronicling the shipwreck of the Dunedin Star. The Skeleton Coast is the northern part of the Atlantic coast of Namibia and south of Angola from the Kunene River south to the Swakop River, although the name is sometimes used to describe the entire Namib Desert coast. The Bushmen of the Namibian interior called the region "The Land God Made in Anger", while Portuguese sailors once referred to it as "The Gates of Hell".
On the coast, the upwelling of the cold Benguela current gives rise to dense ocean fogs (called cassimbo by the Angolans) for much of the year. The winds blow from land to sea,
rainfall rarely exceeds 10 millimetres annually and the climate is highly inhospitable. There is a constant, heavy surf on the beaches. In the days before engine-powered ships and boats, it was possible to get ashore through the surf but impossible to launch from the shore. The only way out was by going through a
marsh hundreds of miles long and only accessible via a hot and arid desert.>>
https://en.wikipedia.org/wiki/Astoria,_Oregon#Climate wrote:
<<
Astoria lies within the Mediterranean climate zone (Köppen Csb). There are an average of 191 days with measurable precipitation. The wettest "water year", defined as October 1 through September 30 of the next year, was from 1915–16 with 2,744 mm and the driest from 2000–2001 with 1,130 mm. Astoria is tied with Lake Charles, Louisiana, and Port Arthur, Texas, as the city with the highest average relative humidity in the contiguous United States. The average relative humidity in Astoria is 89% in the morning and 73% in the afternoon.>>
