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B So I don't understand moon eclipses

  1. Dec 6, 2016 #1
    here the classic moon diagram.

    but the problem i see with this explanation is that in the penumbral shadow the center edge is 1% illuminated while the outer edge 99% illuminated

    so if in the visible from earth edge of the eclipse it goes form 0% illumination to 1% illumination why is it so noticeable, shouldnt it be more like a degradation?

    we disccussed it in class to the question of a guy but we couldn get to an agreement so the teacher gave us an assignment on it

    could someone help me here im really lost on this one
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  3. Dec 6, 2016 #2

    Simon Bridge

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    Since you should be researching the topic - what have you done to find the answer on your own?

    Some things to bear in mind: the picture is a very simple idealization - how often would the situation depicted happen?
    is the picture to scale (does it matter)?
    where do the numbers (1%, 99% etc) come from? Are they accurate?

    Is the transition from the penumbra to the shadow a sharp one? How did you determine this?
  4. Dec 6, 2016 #3
    well the first thing i did was checking if all moon eclipse diagram told the same story an they did:


    so then i did a diagram rendering the issue discussed in class drawing a degradation:


    so my view on the asignement will be that all those diagram are a simplification of the eclipse and actually earth atmosphere plays a main role on it

    the diagram i did is as it should be if there was no earths atmosphere

    but earth atmosphere acts as a lens and makes a sharp edge on the shadow of the eclipse as visible from earth as i have witness myself

    my teacher told us recently that the educative system was based in memorization so far but this makes no sense no more with the net so its being time to focus on creativity

    so ill say that all those moon eclipse diagrams are a simplification and the problem they have is that is that simplification what explains the issue for without that simplification there wouldnt be an explanation from the jump step from grey to black

    so ill take it as an example of the flaw of memorization in detriment of creativity, once the blindman steps on the stone all behind will follow

    what im working now on is to see if i can make an eclipse diagram which renders both the degradation of the penumbra an the sharpening lens effet of earths

    the diagram of course is not to scale but we agree on class that being the angular size both of the sun and moon identical the size of the penumbra should be the same size than the moon

    the situation depicted would happen every time theres a moon eclipse

    the numbers 1% and 99% is a way to express the shadow should be an analogic degradation from black to white not a digital jump from black to grey to white

    the shadow of the eclipse has been determined as sharp by several witnesses
  5. Dec 6, 2016 #4


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    It doesn't look that sharp in the following picture (and hundreds of others just like it I found on google images). It looks pretty "fuzzy" to me.

  6. Dec 6, 2016 #5

    Simon Bridge

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    Your own diagram shows a Sun and earth the same size: is this true in real life?
    What difference does the real life scale make to the diagram - ie. Sun bigger but further away? (ie. what happens to the angular width of the penumbra?)
    Your own diagram shows a sharp drop in sunlight at the edge between the penumbra and the umbra - could this not produce the sharp outline you saw?

    Your sources are "several witnesses" and your own observation - so you would be relying on the human eye as an instrument to measure how the brightness changes across the face of the moon? How accurate is the eye at determining how sharp a brightness transition is?
    How sharp is the transition between the part of the moon in the penumbra and the part in the umbra?
    Perhaps finding photographs of total lunar eclipse online would be helpful rather than relying on your memory and stories from others? (See post #4 ... a lot depend on what you mean by "sharp".)
  7. Dec 6, 2016 #6
    in second and 6th moon it appears pretty sharp to me

    anyway however distant by geometry the size of the penumbra of the eclipse is the same than the moon size, 0,5º of angular size
  8. Dec 6, 2016 #7


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    Really? I definitely see a smooth dropoff in light intensity. Try looking for images in google. See if you can find one like the 2nd or 6th picture but much bigger so it's easier to see the shadow.
  9. Dec 6, 2016 #8

    Simon Bridge

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    What do you mean by "sharp"?

    Your diagram - now I look more closely, appears to have a gradient from 0% to 99% across the penumbra
    Can you verify that this information is correct? Could it be that the transition from say 50% to 99% happens in a relatively short space?
    Can you find a more continuous brightness curve for the penumbral shadow?
  10. Dec 6, 2016 #9
    i think we are discussing the wrong thing

    maybe we should discuss weather why not a single moon eclipse diagram paints a gradient or degradation from black to white but instead 3 drastic jumps from white to grey to white

    i suppose you realize depending where youre on the moon in the penumbra you can see more or less of the sun
  11. Dec 6, 2016 #10
    yes my diagram tried to draw a graident but dint do it very well

    here another try:
  12. Dec 6, 2016 #11


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    I think I see your issue. It might be helpful to calculate the actual angles and linear diameter of the penumbra and the umbra: I think you will find that the penumbra is way, way oversized in that picture: smaller than the umbra.
  13. Dec 6, 2016 #12
    according geometry the size of the penumbra as seen from earth has an angular size of 0.5 the same than the sun and moon

    any sun shadow as seen from origin of the objet that causes the shadow has a penumbra of 0.5º angular size
  14. Dec 6, 2016 #13


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    The earth and sun are the two objects interacting to create the shadow, not the sun and moon....
  15. Dec 6, 2016 #14
    thats what i meant, accidentally sun and moon have the same angular size, 0.5º
  16. Dec 6, 2016 #15


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    We're talking about the shape of the shadows. The shadows are created by the Earth blocking the sun's light. So the umbra is the angular diameter of the Earth as seen from the moon and the penumbra is the additional angular diameter of the Sun as seen from the moon.

    ...but anyway, since we are most concerned about the penumbra and it is indeed half a degree, let's think about what that means:

    To someone on the moon, they see a circular sun and a circular Earth that overlap during an eclipse. A person on one "edge" of the moon at the start of the eclipse for them will see the two cirlces just touching. Later, that person starts to see a total solar eclipse and at the same time a person on the other "edge" of the moon will see the eclipse just starting. Everyone else on the moon at that time will see a partially eclipsed (darkened) sun. And everyone on earth at that time will see the moon nearly exactly in the penumbra and therefore partially, but unevenly darkened.

    But how much? Well, because these are circles overlapping, they start slow and end quickly. In addition, our eyesight is not linear in terms of our ability to detect differences in brightness:

    So when you see the first sliver of umbra getting cut out, the area around it is significantly darkened, but the area on the opposite side of the moon is still near full brightness. It isn't darkened enough for you to notice. Specifically, from the Hyperphysics link: a spot in the center of the moon at that time can still see well over half the sun, which makes it appear to Earth eyes somewhere around 80% of full brightness.
  17. Dec 6, 2016 #16


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    The sharpness of the pictures has to do with the exposure. If you want to see some details in the umbra, you need to overexpose the penumbra, which gives its edge a sharper look.

    Go outside on a sunny day and cast shadows with your hand. Notice that if your hand is close to the ground, you get sharp shadows. If you hold your hand above your head, it casts a fuzzy shadow on the ground.
  18. Dec 6, 2016 #17


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    Here's a little more accurate representation of the Umbra and Penumbra shadows
  19. Dec 7, 2016 #18
    anyway the biggest concern about moon eclipses that we discussed in class was why nobody made an eclipse diagram with a degradation i the penumbra

    the one kindly provided by janus is the first i see

    is like for every 10000 diagrams just one makes a gradient

    ill use this to back up the teacher point that so far people is been just memorizing stuff without thinking by themselves and as the first person who did the eclipse diagram made it without gradient all the rest copy him wrongly

    also do you think the earth atmosphere plays any role in sharpening the edge of the shadow or the photographs sharp edge have all overexposure?
  20. Dec 7, 2016 #19


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    That's cool, but isn't that a solar eclipse, not a lunar eclipse?
  21. Dec 7, 2016 #20


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    If anything I think the atmosphere does the opposite and blurs the edges more than they otherwise would be since it scatters lots of light.
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