APOD: Lunar Eclipses (2013 Apr 25)

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APOD: Lunar Eclipses (2013 Apr 25)

Post by APOD Robot » Thu Apr 25, 2013 4:06 am

Image Lunar Eclipses

Explanation: The dark, inner shadow of planet Earth is called the umbra. Shaped like a cone extending into space, it has a circular cross section and is most easily seen during a lunar eclipse. But the complete cross section is larger than the Moon's angular size in the stages of an eclipse. Still, this thoughtful composite illustrates the full extent of the circular shadow by utilizing images from both partial and total eclipses passing through different parts of the umbra. The images span the years 1997 to 2011, diligently captured with the same optics, from Voronezh, Russia. Along the bottom and top are stages of the partial lunar eclipses from September 2006 and August 2008 respectively. In the rightside bottom image, the Moon is entering the umbra for the total eclipse of September 1997. At left bottom, the Moon leaves the umbra after totality in May 2004. Middle right, center, and left, are stages of the total eclipse of June 2011, including the central, deep red total phase. During today's brief partial lunar eclipse seen only from the eastern hemisphere, the Moon will just slightly graze the umbra's lower edge.

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Re: APOD: Lunar Eclipses (2013 Apr 25)

Post by bystander » Thu Apr 25, 2013 4:42 am

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Re: APOD: Lunar Eclipses (2013 Apr 25)

Post by mister T » Thu Apr 25, 2013 9:49 am

Important science:

An examination of this composite reveals a very irregular edge to the Earths shadow upon Luna.

This is pretty conclusive evidence that Earth must have an atmosphere. :idea:

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Re: APOD: Lunar Eclipses (2013 Apr 25)

Post by Chris Peterson » Thu Apr 25, 2013 12:55 pm

mister T wrote:Important science:

An examination of this composite reveals a very irregular edge to the Earths shadow upon Luna.

This is pretty conclusive evidence that Earth must have an atmosphere. :idea:
The soft edge is penumbral, indicating that the Sun isn't a point source. I don't think there is anything in this image suggestive of Earth's atmosphere (except for the red cast during totality). That is, I think the image would look the same if we didn't have an atmosphere.
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Re: APOD: Lunar Eclipses (2013 Apr 25)

Post by neufer » Thu Apr 25, 2013 1:37 pm

Chris Peterson wrote:
mister T wrote:
An examination of this composite reveals a very irregular edge to the Earths shadow upon Luna.

This is pretty conclusive evidence that Earth must have an atmosphere. :idea:
The soft edge is penumbral, indicating that the Sun isn't a point source. I don't think there is anything in this image suggestive of Earth's atmosphere (except for the red cast during totality). That is, I think the image would look the same if we didn't have an atmosphere.
Wouldn't the soft umbral edge as well as the visibility during totality
also constitute evidence that Earth must have an atmosphere :?:
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Re: APOD: Lunar Eclipses (2013 Apr 25)

Post by Chris Peterson » Thu Apr 25, 2013 2:06 pm

neufer wrote:Wouldn't the soft umbral edge as well as the visibility during totality
also constitute evidence that Earth must have an atmosphere :?:
Certainly, the color during totality indicates an atmosphere (as I said). But I don't think there's anything about the shadow edges that do. The umbral/penumbral transitions are the result of the half degree size of the Sun, which certainly must dominate the small effect of atmospheric absorption and refraction (at this scale, I don't think the atmosphere is even one pixel wide).
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Re: APOD: Lunar Eclipses (2013 Apr 25)

Post by neufer » Thu Apr 25, 2013 3:34 pm

Chris Peterson wrote:
Click to play embedded YouTube video.
neufer wrote:
Wouldn't the soft umbral edge as well as the visibility during totality
also constitute evidence that Earth must have an atmosphere :?:
Certainly, the color during totality indicates an atmosphere (as I said). But I don't think there's anything about the shadow edges that do. The umbral/penumbral transitions are the result of the half degree size of the Sun, which certainly must dominate the small effect of atmospheric absorption and refraction (at this scale, I don't think the atmosphere is even one pixel wide).
Even a black & white photo would indicate that the moon is far from dark at totality and lacks a sharp sunrise terminator.
http://www.nasa.gov/mission_pages/sdo/news/eclipse-spring2013.html wrote: NASA’s SDO Observes Earth, Lunar Transits in Same Day
Karen C. Fox, NASA’s Goddard Space Flight Center, 03.11.13

<<On March 2, 2013, NASA’s Solar Dynamics Observatory (SDO) entered its semiannual eclipse season, a period of three weeks when Earth blocks its view of the sun for a period of time each day. On March 11, however, SDO was treated to two transits. Earth blocked SDO’s view of the sun from about 2:15 to 3:45 a.m. EDT. Later in the same day, from around 7:30 to 8:45 a.m. EDT, the moon moved in front of the sun for a partial eclipse.

When Earth blocks the sun, the boundaries of Earth’s shadow appear fuzzy, since SDO can see some light from the sun coming through Earth’s atmosphere. The line of Earth appears almost straight, since Earth -- from SDO’s point of view -- is so large compared to the sun.

The eclipse caused by the moon looks far different. Since the moon has no atmosphere, its curved shape can be seen clearly, and the line of its shadow is crisp and clean. Any spacecraft observing the sun from an orbit around Earth has to contend with such eclipses, but SDO's orbit is designed to minimize them as much as possible, with only two three-week eclipse seasons each year.>>
http://en.wikipedia.org/wiki/Transit_of_Venus wrote:
<<The existence of an atmosphere on Venus was concluded by Mikhail Lomonosov on the basis of his observation of the transit of Venus of 1761 from the Saint Petersburg. He used a two-lens achromat refractor and a weak solar filter (smoked glass) and reported seeing a bump or bulge of light ("Lomonosov's arc") off the solar disc as Venus began to exit the Sun. Lomonosov attributed that effect to refraction of solar rays through an atmosphere; he also reported the appearance of a sliver around the part of Venus that had just entered the Sun's disk during the initial phase of transit.

In 2012, Pasachoff and Sheehan reported, based on knowing what Venus's atmosphere would look like because of Pasachoff and Schneider's observations of the 2004 transit of Venus, that what Lomonosov reported was not Venus's atmosphere. To make a decisive test, a group of researchers carried out experimental reconstruction of Lomonosov's discovery of Venusian atmosphere with antique refractors during the transit of Venus on 5–6 June 2012. They observed the "Lomonosov's arc" and other aureole effects due to Venus's atmosphere and concluded that Lomonosov's telescope was fully adequate to the task of detecting the arc of light around Venus off the Sun's disc during ingress or egress if proper experimental techniques as described by Lomonosov in his 1761 paper are employed.>>
http://www.universetoday.com/13873/plutos-atmosphere/ wrote:
Pluto Atmosphere
by Fraser Cain, Universe Today, April 25, 2008

<<Yes, that’s right, Pluto does have an atmosphere. Well, the Pluto atmosphere is not the ocean of air we have here on Earth, but Pluto’s thin envelope of gases do surround the dwarf planet for part of its orbit around the Sun.

It’s important to understand that the orbit of Pluto is very elliptical, bringing it closer and then more distant at various points of its orbit. At the closest point, the surface of solid nitrogen heats up enough that it sublimates – changes directly from a solid to a gas. These clouds of nitrogen surround Pluto, but it doesn’t have enough gravity to keep them together, so they can escape out into space. And then, as Pluto gets further from the Sun again, it cools down, and the atmosphere freezes and solidifies back down on the surface of Pluto.

In 1988, astronomers discovered that Pluto has an atmosphere by watching how it passed in front of a more distant star – called a planetary transit. Instead of dimming the moment it went behind Pluto, the star was first obscured by the atmosphere, so that astronomers could measure its thickness and composition. It currently has 0.30 Pa on the surface and its height extends 60 km above the surface.

More precise observations were done in 2002, when astronomers were surprised to find that Pluto’s atmosphere had actually thickened since it had first been discovered. Astronomers think this is a seasonal phenomenon. The nitrogen on Pluto’s surface was exposed to sunlight following a 120-year winter. The nitrogen became a gas, but it took time to get going as an atmosphere.

As Pluto is now traveling away from the Sun, the Pluto atmosphere won’t last long. Astronomers think it will begin to disappear by 2015. This is one of the big reasons NASA sent its New Horizons spacecraft – to study Pluto’s atmosphere before it’s gone for good.>>
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Re: APOD: Lunar Eclipses (2013 Apr 25)

Post by Chris Peterson » Thu Apr 25, 2013 3:50 pm

neufer wrote:Even a black & white photo would indicate that the moon is far from dark at totality and lacks a sharp sunrise terminator.
Yes, although I don't really see your point, other than continuing to reiterate what I said.

I don't get the point of the other references, either. A lunar eclipse is nothing like a transit. I've seen nothing to make me think there is anything about the edges of the Earth's shadow seen on the Moon during a lunar eclipse that indicate we have an atmosphere.
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Re: APOD: Lunar Eclipses (2013 Apr 25)

Post by neufer » Thu Apr 25, 2013 4:09 pm

Chris Peterson wrote:
neufer wrote:
Even a black & white photo would indicate that the moon is far from dark at totality and lacks a sharp sunrise terminator.
Yes, although I don't really see your point, other than continuing to reiterate what I said.
My point is:

1) I agree with you that the most obvious indication of Earth's atmosphere (in today's APOD) is the red color.

2) I agree with Mister T that there are other obvious indications of Earth's atmosphere (in today's APOD) besides the red color.
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Re: APOD: Lunar Eclipses (2013 Apr 25)

Post by Chris Peterson » Thu Apr 25, 2013 4:40 pm

neufer wrote:2) I agree with Mister T that there are other obvious indications of Earth's atmosphere (in today's APOD) besides the red color.
Perhaps you could tell me what they are in simple words, as I can't find them, in the images, in Mr T's comment, or in your references.
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Re: APOD: Lunar Eclipses (2013 Apr 25)

Post by Anthony Barreiro » Thu Apr 25, 2013 5:06 pm

It is really cool to be able to see Earth's entire umbra as revealed by lunar eclipses at different ecliptic latitudes! Bravo to Mr. Vin'yaminov for this well-conceived and well-executed composite image!

I too was impressed by the fuzzy irregularity of Earth's umbra. It makes sense to me that the angular size of the Sun and refraction through Earth's atmosphere would both contribute to some fuzziness of the umbra. And given that this is a composite image, I wonder if the composite umbra might appear more irregular than would the single, real-time umbra during a single eclipse.

Neufer, for what it's worth, the contrasting images of the Earth and the Moon eclipsing the Sun as seen from SDO seem to me to nicely illustrate the effect of Earth's atmosphere on the edge of the umbra. And the image of Nancy Reagan sitting on Mr. T's lap will doubtless haunt my dreams tonight.
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Re: APOD: Lunar Eclipses (2013 Apr 25)

Post by Boomer12k » Thu Apr 25, 2013 5:18 pm

"Blood on the Moon"....

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Re: APOD: Lunar Eclipses (2013 Apr 25)

Post by neufer » Thu Apr 25, 2013 5:54 pm

Chris Peterson wrote:
neufer wrote:
2) I agree with Mister T that there are other obvious indications of Earth's atmosphere (in today's APOD) besides the red color.
Perhaps you could tell me what they are in simple words, as I can't find them, in the images, in Mr T's comment, or in your references.
There are no sharp demarcations of brightness within the region colored red.
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Re: APOD: Lunar Eclipses (2013 Apr 25)

Post by neufer » Thu Apr 25, 2013 5:55 pm

.
.
Image
Anthony Barreiro wrote:
Neufer, for what it's worth, the contrasting images of the Earth and the Moon eclipsing the Sun as seen from SDO seem to me to nicely illustrate the effect of Earth's atmosphere on the edge of the umbra. And the image of Nancy Reagan sitting on Mr. T's lap will doubtless haunt my dreams tonight.
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Re: APOD: Lunar Eclipses (2013 Apr 25)

Post by bystander » Thu Apr 25, 2013 6:17 pm

Your Discovery News Eclipse Guide for 2013
Discovery News | Mark Thompson | 2013 Apr 25

This year sees a relatively quiet, yet interesting time for eclipse hunters with three lunar eclipses and two solar eclipses.
...
Today's partial lunar eclipse will be visible for parts of South America, Europe, Africa, Asia and Australia starting at 18:03 UT and ending at 22:11 UT.

Following hot on its heels is a solar eclipse on May 10, 2013, but this solar eclipse won't be total instead, it is an annular eclipse.
...
Another lunar eclipse takes place on May 25, 2013, but again the moon is slightly out of alignment with the sun and Earth so this isn't a total eclipse. It is not even classed as a partial eclipse, instead the moon skirts through what is known as the penumbral shadow (or partial shadow) of the Earth where some sunlight still reaches it.
...
The next eclipse is another penumbral lunar eclipse taking place on Oct. 18, 2013, and will be visible from America, Europe, Africa and Asia.
...
On Nov. 3, 2013, parts of the Northern Hemisphere are treated to what is known as a hybrid solar eclipse where a combination of annular and total will be seen.
...
More details of when and where to look to see these eclipses can be found on NASA's Eclipse website.
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Re: APOD: Lunar Eclipses (2013 Apr 25)

Post by Chris Peterson » Thu Apr 25, 2013 8:45 pm

I'm watching today's eclipse now... it's pretty subtle, but a nice moonrise all the same. Tried imaging it, but only have my tablet, and that's just not capturing it properly.
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Re: APOD: Lunar Eclipses (2013 Apr 25)

Post by Chris Peterson » Thu Apr 25, 2013 8:50 pm

neufer wrote:
Chris Peterson wrote:
neufer wrote:
2) I agree with Mister T that there are other obvious indications of Earth's atmosphere (in today's APOD) besides the red color.
Perhaps you could tell me what they are in simple words, as I can't find them, in the images, in Mr T's comment, or in your references.
There are no sharp demarcations of brightness within the region colored red.
I'm afraid I don't get it. I'm sticking with my original assessment that there's nothing in the nature of the shadows to lead one to conclude one way or the other that the Earth has an atmosphere.
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Re: APOD: Lunar Eclipses (2013 Apr 25)

Post by Anthony Barreiro » Thu Apr 25, 2013 8:53 pm

bystander wrote:Your Discovery News Eclipse Guide for 2013
Discovery News | Mark Thompson | 2013 Apr 25

This year sees a relatively quiet, yet interesting time for eclipse hunters with three lunar eclipses and two solar eclipses.
...
Another lunar eclipse takes place on May 25, 2013, but again the moon is slightly out of alignment with the sun and Earth so this isn't a total eclipse. It is not even classed as a partial eclipse, instead the moon skirts through what is known as the penumbral shadow (or partial shadow) of the Earth where some sunlight still reaches it.
...
More details of when and where to look to see these eclipses can be found on NASA's Eclipse website.
The May 25 lunar eclipse will be the first eclipse in Saros Series 150. Thus this eclipse will be the slightest possible penumbral eclipse, with barely one percent of the Moon's southern limb passing briefly through the Earth's penumbra. This eclipse will be completely unnoticeable, except perhaps photometrically. Saros Series 150 will continue until June 30, 3275, when the last eclipse of the series will be the barest possible penumbral eclipse of the Moon's northern limb.

It's amazing to me that ancient Babylonian astrologers observed and calculated that two eclipses occurring 18 years and 11 days apart have the same geometry, and thus they could predict the date, time, and appearance of solar and lunar eclipses far into the future.
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Re: APOD: Lunar Eclipses (2013 Apr 25)

Post by Anthony Barreiro » Thu Apr 25, 2013 8:53 pm

Chris Peterson wrote:I'm watching today's eclipse now... it's pretty subtle, but a nice moonrise all the same. Tried imaging it, but only have my tablet, and that's just not capturing it properly.
Where are you?
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Re: APOD: Lunar Eclipses (2013 Apr 25)

Post by Chris Peterson » Thu Apr 25, 2013 9:33 pm

Anthony Barreiro wrote:
Chris Peterson wrote:I'm watching today's eclipse now... it's pretty subtle, but a nice moonrise all the same. Tried imaging it, but only have my tablet, and that's just not capturing it properly.
Where are you?
Sorry, should have mentioned that. I was watching from Cambridge in the UK.
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Re: APOD: Lunar Eclipses (2013 Apr 25)

Post by FLPhotoCatcher » Fri Apr 26, 2013 3:37 am

The reason the earth's shadow is not a smooth circle is because of the darker areas on the moon, not clouds in the earth's atmosphere. Apparently the photographer increased the contrast so that the dark-light boundary is sharper, and this made the darker areas just outside the umbra even darker.

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Re: APOD: Lunar Eclipses (2013 Apr 25)

Post by neufer » Fri Apr 26, 2013 1:01 pm

FLPhotoCatcher wrote:
The reason the earth's shadow is not a smooth circle is because of the darker areas on the moon, not clouds in the earth's atmosphere. Apparently the photographer increased the contrast so that the dark-light boundary is sharper, and this made the darker areas just outside the umbra even darker.
That's the reason the isophotes are not smooth circles.

But it is not the reason that the isophote gradient for the penumbral boundary is much more sharply defined than the isophote gradient for the umbral boundary (when it should, in fact, be just the reverse).
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Re: APOD: Lunar Eclipses (2013 Apr 25)

Post by alter-ego » Mon May 27, 2013 5:39 am

Chris Peterson wrote:
mister T wrote:Important science:

An examination of this composite reveals a very irregular edge to the Earths shadow upon Luna.

This is pretty conclusive evidence that Earth must have an atmosphere. :idea:
The soft edge is penumbral, indicating that the Sun isn't a point source. I don't think there is anything in this image suggestive of Earth's atmosphere (except for the red cast during totality). That is, I think the image would look the same if we didn't have an atmosphere.
neufer wrote:
Chris Peterson wrote:
neufer wrote:
Wouldn't the soft umbral edge as well as the visibility during totality
also constitute evidence that Earth must have an atmosphere :?:
Certainly, the color during totality indicates an atmosphere (as I said). But I don't think there's anything about the shadow edges that do. The umbral/penumbral transitions are the result of the half degree size of the Sun, which certainly must dominate the small effect of atmospheric absorption and refraction (at this scale, I don't think the atmosphere is even one pixel wide).
Even a black & white photo would indicate that the moon is far from dark at totality and lacks a sharp sunrise terminator.
There are really two separate questions here:
1. Does the Earth's atmosphere impart a visible fuzziness or change in the umbral edge?
2. Do the APOD Eclipse pictures reveal such atmospheric effects?

First, the atmosphere does visibly blur the ideal geometrically sharp umbral edge.
Historically, lunar eclipse crater timings have indicated the apparent umbral radius is bigger that predicted to the tune of 1% or 2% (at visible wavelengths). It is generally accepted that Earth's atmosphere is the main reason for this scaling factor, but there are unpredictable eclipse-to-eclipse shadow size variations. You can read about Earth shadow enlargement here at NASA Eclipse website. The observable umbral edge is defined as an inflection point separating the umbra / penumbral intensity regions. The penumbra is also enlarged, but that edge is not observable. The radius of the umbral-edge inflection point is ≈ 1 arcminute larger than the ideal, sharp, geometrical shadow edge, and occurs at intensity levels ≈ 2% of the un-eclipsed intensity.

Second, the component partial eclipse photos do not reveal any effects suggesting Earth has an atmosphere.
For these images, I verified the umbral edge transition region occurs well within the dark edges, and the small contrast variations cannot be discerned. As Chris said, the obvious visible soft shadow edges are the simple result of blocking an extended source. (Just look an object's shadow on a sunny day, it is fuzzy for that reason). The composite image of several separate partial eclipse images do accurately display the size of Earth's dark circular shadow (umbra), but not necessarily the correct penumbra intensity gradient. Spatially, 1 arcminute is readily resolvable in these images, but the subtle inflection point brightness variations are not. There are several complicating factors that prevent visibility of the umbral edge (inflection point) in the images. They include a wide intensity range which makes it difficult to detect the few percent brightness variation at the inflection point, brightness non-uniformity from lunar surface reflectance variations, and possible image post-processing steps. However, the photos consistently do show noticeably steeper gradients than expected (higher isophote density, which might be what Art noticed).

Just what is the nature of the penumbral shadow and the atmosphere's impact on the umbral edge?
Wanting to know more detail, I found a very nice paper presenting an accurate measurement of the entire penumbra of a total eclipse in 1945 (The Radiant-Energy Gradient of the Earth's Penumbral Shadow, Morrison 1946. During a single eclipse, 10 (film) pictures were acquired using a telescope and camera, and the negatives analyzed with a microphotometer :!: along equatorial scans. The penumbral (relative) intensity profile was extracted by normalizing the "intensity" of each eclipsed location with the un-eclipsed "intensity" from the exact location obtained from an earlier or later image from his eclipse sequence. I digitized Morrison's graph data so I could add information and see behavior more readily. Morrison did not plot a predicted gradient for comparison, maybe because of the computational difficulty 66 years ago. Unable to find such a prediction, I went ahead and computed the pure geometrical penumbra gradient. I thought that solar limb darkening would be important, especially near totality, so I also digitized a modern drift scan of the sun, and used it in the penumbra gradient prediction. Surprisingly :shock: , the predicted penumbra gradient fit Morrison's data VERY WELL over refraction-free region, and solar limb darkening was needed for the good fit at lower penumbral intensities. The 1945 lunar eclipse circumstances were carefully accounted for in the penumbral gradient plot. Also in the plot, both the geometrical and enlarged radii for the umbra and penumbra edges are indicated. Morrison's data clearly begins to deviate at a radius ≈3' larger than the geometrical edge. Morrison claimed this deviation is due to refraction of the Earth's atmosphere, and said his data at longer wavelengths (680nm to 870nm) would enhance this effect due to less absorption in the atmosphere. At visible wavelengths (higher absorption), less light would get to the lunar surface, and the inflection point would occur at a lower gradient intensity, and therefore a smaller radius closer to the ideal geometric umbra edge. The bottom line is:
The refraction-induced, enlarged umbral radius is visible, but correctly recording the intensity gradient perturbation so that the inflection point can be identified appears challenging to do in a single, typical partial eclipse picture. In fact, unless the inflection point is directly observed (e.g. by a crater timing or clear "kink" in the digitized intensity profile), any refractive effect will most likely not show up because of conspiring effects which introduce artifact gradients and/or bury the gradient umbral inflection point feature. I.e. just because you see a fuzzy, dark edge in an eclipse picture doesn't mean you've identified the umbral edge.
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