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Why is our moon exactly half when half?

  1. Aug 14, 2013 #1

    I looked up on the sky today while arriving home from work.

    There was the moon but only exactly half of it was being visible.

    With exactly half I mean, with regard to my poor english, that it was like a folded circular disc.

    Seeing a "new-moon" like we call it here in Sweden, the moon look more like the moon on the turkish flag meaning that it is curved like the curvature of earth actually is blocking the sun's rays.

    But how can it be exactly half?

    Shouldn't the curvature of earth always make the moon curved, so to speak?

    Best regards, Roger
  2. jcsd
  3. Aug 14, 2013 #2


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    The moon doesn't appear half full because of the earth's shadow (and this also applies when it is a crescent), the moon appears half full because it is lit from the side by the sun. This page has a fairly good diagram: http://www.moonconnection.com/moon_phases.phtml

    There are times when the earth blocks the sun's rays, causing the moon to go dark, but this is a separate phenomenon known as a lunar eclipse (and it can only happen when the moon would normally be full).
  4. Aug 14, 2013 #3
    Thanks cjl!

    Very nice and explanatory link!

    I do however now feel stupid regarding my actual question.

    But now I do not fully understand neither Gibbous nor Crescent :)

    Do however not feel obligated to answer this.

    Best regards, Roger
    Why isn't it always a lunar eclipse at full moon as shown in the diagram?

    Is the lunar orbit not in the same plane as the earth orbit around the sun?

    And is the lunar plane changing with time?
  5. Aug 14, 2013 #4


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    Close, but not exactly the same.... so usually the moon is a bit above or a bit below the earth/sun plane.
  6. Aug 14, 2013 #5


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    Take a ball (model Moon) and a torch (Model Sun) and turn off the lights. Shine the torch on the ball from a range of different angles and you will see that you can get all the shapes (including crescent and gibbous) that you see when you look over the Moon during its 28 day cycle.

    The Sun, Moon and Earth are not in exactly the same plane all the time. The axes of orbits are in different planes. The only time you get a Lunar or Solar Eclipse is when they happen to be in the same plane and in line. To see the Eclipses, you also need to be on the appropriate side of the Earth during the Eclipse period. The Lunar Eclipse has to be during a full Moon because the Earth must be on a line between Sun and Moon but this only happens for some of the full moon. These things are easiest to appreciate using actual models in your hand or, almost as good, looking at animations. (There are dozens of them to find if you google).
  7. Aug 14, 2013 #6
    Let's begin from scratch.

    1) I have always thought of the whole planetary system as being in one plane
    2) I have always thought of the planets moving in almost perfect circular orbits

    I know our earth's axis is off by some 16(?) degrees.

    That our moon isn't moving in an orbit in the same plane as the earth around the sun is new to me.

    But now I have learned that that is the reason a lunar eclipse happens only sometimes and not always (at full moon).

    Is there some kind of drift here regarding lunar plane vs earth/sun plane or is the "offset" constant?

    The "shape" of the moon at Crescent (C) and Gibbous (G) however still makes me kind of confused.

    I could for sure simulate it like you suggest, but I prefere being able to calculate it and thereby understanding it.

    Best regards, Roger
  8. Aug 14, 2013 #7


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    Your 1. and 2. placed you somewhere like five centuries out of date before you joined this thread - you have achieved some impressive time travel, subsequently. :wink:
    You won't "calculate" it without a lot of information and number crunching.
    Everything tends to affect everything else and the inclinations are shifting a bit, all the time afaik. There are no 'integer relationships' out there; hence all the problems with leap years and leap seconds - and that's only the tip of the iceberg.
    The so-called plane of the Ecliptic is not really a plane but it does contain the orbits of the planets, within a few degrees, at least. Orbits are Ellipses with pretty low eccentricity (i.e. almost circles) but Keppler observed that they are elliptical, five hundred years ago and worked out his law which is still used, without knowing anything about gravity, too.
    Something that really impresses me is that the 'ancients' and certainly the 'mediaevals' had this stuff tabulated and could predict Ephemeris and Tides and observe the positions of planets (in 3D) with a high degree of accuracy.
    As for the "shapes" - do it with a model. The alternative is to get really good at spherical geometry and do the sums. (Good luck with that. haha)
  9. Aug 14, 2013 #8
    The earth's axis is inclined at an angle of about 23 1/2 degrees to the axis of its orbit around the sun. The axis of the moon's orbit around the earth is inclined at an angle of about 5 degrees to the axis of the earth's orbit around the sun.
  10. Aug 14, 2013 #9

    I obviously know nothing :D

    But that is a good thing, isn't it?

    Because then I can view things in a way scientists simply aren't capable of because they know "too much".

    And if I can't "view it" in a different way, I can always come up with more stupid questions :D

    It's like growing old.

    You learn so much along the road.

    But when you have reached a certain age (like I have) you simply know too much about the consequences for doing this and that that you get repressed and kind of forget how to live.

    Best regards, Roger
  11. Aug 14, 2013 #10
    Thank you for that exact data.

    I thought it was less, obviously.

    But I am surprised about the 5 degrees.

    Does not sound so much.

    How can there be a lunar eclipse out of that?

    Best regards, Roger
  12. Aug 14, 2013 #11
    Nonscientists say this a lot. But it never had its bluff called. In the last 100 years, there was never a scientific discovery by nonscientists.
  13. Aug 14, 2013 #12
    Hi micromass!

    Don't take it so seriously.

    I am just trying to boost my self confidence regarding me knowing so very little.

    What is wrong with that?

    Best regards, Roger
  14. Aug 14, 2013 #13


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    as nugatory say ... no

    if it was, we would get a total eclipse of the Sun and the Moon every month of the year

    we can get a lunar or solar eclipse because at periodic intervals during the ascending or descending nodes of the Moon's orbit they happen to be in line between the Earth and the Sun.

    Last edited: Aug 14, 2013
  15. Aug 14, 2013 #14
    Jesus Dave, is this really you?

    Dave from EP?

    So you are interested in physics you too?

    Ain't the world small :)

    Take care and thank you for your input!

    Best regards, Roger
  16. Aug 17, 2013 #15
    Hi Chestermiller!

    I made some calculations regarding if 5 degrees was enough to not making the earth cover the moon at full moon.

    Here are my results:

    [tex]r=LU=384 000 km[/tex]

    [tex]R=6370 km[/tex]


    [tex]x=r*sin(5)=33 500 km >>R[/tex]

    Hense, a lunar eclipse has to be rear.

    Best regards, Roger
  17. Aug 17, 2013 #16


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    hahah yup its me :)
    Yes, i have a love of astronomy, geology, electronics ....

  18. Aug 17, 2013 #17


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    At least, when it does occur, more people get a chance to see it than the few who, for a brief while, can glimpse the Moon's small shadow as it races over the Earth in a Solar Eclipse.
  19. Aug 19, 2013 #18
    Yes, it is a spectacular event!

    Is the Solar Eclipse more rare(!) than the Lunar Eclipse?

    If so, why?

    With regard to the link above and what we have discussed, it feels like it shouldn't be.

    Best regards, Roger
  20. Aug 19, 2013 #19


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    They occur with approximately equal frequency, but a lunar eclipse is visible from a bit more than half the earth's surface, while a solar eclipse is only visible from a narrow track (where the moon's shadow falls on the earth), so your chances of seeing a solar eclipse from any given location are much lower than your chances of seeing a lunar eclipse.
  21. Aug 19, 2013 #20
    I would suggest you look at DISCOVER....science for the curious magazine for a whole range of (usually older) enthusiasts who would not describe themselves as scientists.
    But now I suppose they are!
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