APOD: The Umbra of Earth (2011 Dec 15)

[APOD Robot]

The Umbra of Earth

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 most easily seen during a lunar eclipse. For example, last Saturday the Full Moon slid across the southern half of Earth's umbral shadow, entertaining moonwatchers around much of the planet. In the total phase of the eclipse, the Moon was completely within the umbra for 51 minutes. Recorded from Beijing, China, this composite eclipse image uses successive pictures from totality (center) and partial phases to trace out a large part of the umbra's curved edge. Background stars are visible in the darker eclipse phases. The result shows the relative size of the shadow's cross section at the distance of the Moon, as well as the Moon's path through Earth's umbra.

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[Wah!]

This is mine, post at HKT2011-12-12 00:07
http://www.hkastroforum.net/viewtopic.p ... 983#233638

Click to view full size image

[Guest]

i love it

[bactame]

So that's how Eratosthenes got the size of the Earth, huh. Neat.

[SJoe]

It's weird. I never get tired of pictures of the moon.

[mikeysaling]

great image - and the one submitted in this discussion.

[biddie67]

Beautiful photo and fascinating how info about sizes and distance can be figured from this ... kudos to Mr. Wang!!

(( And also to Wah! above ))

[bob p.]

There is something I don't undestand....why are the stars in the background of the moon not blocked by the earth's shadow?

[supamario]

There is something I don't undestand....why are the stars in the background of the moon not blocked by the earth's shadow?

A better question would be, why isnt the moon blocked out by the earth's shadow. Stars are point sources of light and do not get blocked out.
The moon? Still visible because of reflected light from the sun and also the earth. The reddish colour comes from simultaneous sunrise and sunsets from the edges of the earth and
can also be dust in the atmosphere.

[DStarr]

My question is about umbra vs. penumbra. Isn't the reddish portion of the picture the umbra - where the only earthlight is diffused thru our atmosphere? And isn't the lighter gray and larger shadow the penumbra, where some direct sunlight peaking around the earth's limb is hitting the moon, hence the "partial shadow?"

[APODFORIST]

This is a cool picture. You can imagine the size of earth as seen from moon.

[orin stepanek]

This is mine, post at HKT2011-12-12 00:07
http://www.hkastroforum.net/viewtopic.p ... 983#233638

Click to view full size image

Nice picture Wah! Very well done!

[flash]

Nice images!

Has anyone ever seen a photo of an eclipse of the Sun by the Earth as seen from the Moon? What would such an eclipse properly be called? (It is an eclipse of the Sun, so it should be a Solar eclipse, but that seems confusing. The same event as seen from the Earth is called a Lunar eclipse! There ought to be nomenclature that is independent of the location of the observer...) Given all the cameras we've sent there I assume that there must be photos of the Earth taken during a "Lunar" eclipse such as this most recent one where the Earth comes between the Sun and the Moon. And given the relatively larger size of the Earth I assume that a higher percentage of Solar eclipses seen from the Moon would be total (as opposed to Solar eclipses as seen from the Earth). There is no possibility of it being annular) But then, as seen from the surface of the Moon, unless the camera were completely in the umbra, the solar eclipse would not be total.

Similar subject: Has a photo ever been taken of a transit of the Sun by the Earth from Mars? How often does that occur?

[TheKadieNebula]

This is a very nice picture!!! I love it, you can sort of see the comparison of sizes between the Earth and the Moon.

[Chris Peterson]

Has anyone ever seen a photo of an eclipse of the Sun by the Earth as seen from the Moon?

Yes- I've seen a few images. For example. It has also been imagined by artists- indeed, on my short list of really bad APODs is such a rendering, which for the sake of sensibility I won't link.

What would such an eclipse properly be called? (It is an eclipse of the Sun, so it should be a Solar eclipse, but that seems confusing. The same event as seen from the Earth is called a Lunar eclipse!

It should be called a solar eclipse seen from the Moon. Or a lunar eclipse seen from the Earth.

There ought to be nomenclature that is independent of the location of the observer...)

Well, I guess you could call it a Sun-Earth-Moon syzygy. But otherwise, terms like "eclipse", "transit", "conjunction" and the like are very much dependent on viewpoint, so if there is any chance of confusion (which there usually is not), it is important to be clear about that viewpoint. Note that the above referenced article is quite careful in this regard.

Similar subject: Has a photo ever been taken of a transit of the Sun by the Earth from Mars? How often does that occur?

No. Such transits are rare- the last one was on 1984 May 11 (there were no operating probes there at the time), and the next one will be on 2084 November 10.

[Ann]

Chris wrote:
syzygy

Surely one of my favorite English words! I haven't googled it yet, so I don't yet know what it means, although I have definitely heard it before.

Some other of my favorite English words are selenelion, and, off the astronomical chart, palimpsest. Going back to astronomy, I like the word "ephemeris", too. I'm not too sure what it means, although I think it has to do with the position of the Moon and asteroids and things like that. I know, I could google it, but I'm too lazy. But I like the word!

Forrest Gump said that life is like a box of chocolates, and you never know what you will get. I think that a collection of fantastic words are like a box of verbal pralines.

Now let's see - I wonder which of these little delicacies is the syzygy?

Ann

P.S. The Moon images are nice. I like how today's APOD, as well as the image by Wah!, brings out the contrast in size between the Earth (and its shadow) and the Moon.

[YPmule]

We live in the mountains of central Idaho, so only could catch the first half of the eclipse. My smarter half stood outside in 10 degrees (F) for a couple of hours to get this shot of the eclipsed moon setting behind the mountain.

Click to view full size image

Larger size:
http://local-color.smugmug.com/AIR/The- ... KSV5-XL-LB

[neptunium]

What would such an eclipse properly be called? (It is an eclipse of the Sun, so it should be a Solar eclipse, but that seems confusing. The same event as seen from the Earth is called a Lunar eclipse!

It should be called a solar eclipse seen from the Moon. Or a lunar eclipse seen from the Earth.

How about planetary solar and lunar eclipse?

[Chris Peterson]

How about planetary solar and lunar eclipse?

That seems even more confusing!

"Eclipse" just means one astronomical body blocks another. It must always be qualified with the position of the observer, although in most cases that is obvious from the context.

[rr_carroll]

Excuse me for nitpicking, but I believe the APOD picture does not show the outline of the Earth's shadow. The shadow moves across the stars at a little less than 1 degree/day, due to the Earth's annual motion around the Sun. So during the several hours needed to get all the Moon images, the shadow moves noticeably. This seems most apparent on the right, where the two shadow edges don't quite match up. Edit: I'm assuming the camera is tracking at a sidereal rate, since we see stars as points.

APODFORIST, I think Earth's shadow (at the Moon's distance) is about 6000 miles across. During a solar eclipse, the Moon' umbra on the Earth shrinks down to just a few miles across - that is, it loses about 1000 miles on all sides. The angles for the Earth during a lunar eclipse are about the same, so it should also be reduced about 1000 miles all the way around.

[APODFORIST]

APODFORIST, I think Earth's shadow (at the Moon's distance) is about 6000 miles across. During a solar eclipse, the Moon' umbra on the Earth shrinks down to just a few miles across - that is, it loses about 1000 miles on all sides. The angles for the Earth during a lunar eclipse are about the same, so it should also be reduced about 1000 miles all the way around.

Thanks for your advice - you are right. Diameters earth/moon are 12724km/3476km which means earth's diameter is more than 3.6 times wider than moon's diameter.

[DStarr]

And for each lunar eclipse on the earth there is a concurrent solar eclipse on the moon.

[eaglekepr]

Edit: I'm assuming the camera is tracking at a sidereal rate, since we see stars as points.

Since the image is a composite of 5 images of the moon, and since there are no stars in the "end" photos of the moon (due to the shorter exposure required to capture the mostly lit moon) I'd expect the stars to be taken from the center/full eclipse image and just included as part of the composite, not necessarily due to the effect of "tracking". Each of the 5 images used for the composite have different exposure times due to the large luminosity swing from full moon to eclipsed moon.