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Question regarding sun/moon angle

  1. Jul 9, 2007 #1
    I just want to start off by saying that my knowledge of Astronomy (and physics, for that matter) is pretty limited, so please don't laugh too hard if I make an ass of myself.

    I'm taking the first Astronomy course at my University, and today my professor briefly mentioned that the arc angle when looking at the sun is .5 inches and it is the exact same for the moon (I may be off on the measurement, but that's not the point I'm driving at). Because this is so, a lunar (and/or?) solar eclipse is possible.

    As someone who probably thinks too often in Darwinian terms, I asked my professor what the natural advantage of this is. He said it was a good question, but he did not know the answer.

    Does this help anything (sun, moon, the 'orbit' itself, earth, etc.) in any way? Long ago, was there perhaps some sieving process which left us with a moon that could block out the sun so as to help itself and/or the moon (or something else)?

    So basically, why is it that the moon and sun have the same arc angles? Is it just random chance? Is it because it helped it to survive over billions of years? I know that I might be applying this thinking to too broad of a question, but I just can't convince myself it happened by accident. Or maybe it did...

    If there is no answer for this question I would appreciate an educated assumption. Yours would certainly be better than mine.

    Thanks for your time.
  2. jcsd
  3. Jul 9, 2007 #2
    As far as I know it's just a matter of dumb luck: the orbit of the moon is getting wider over time - very very gradually, due to the transfer of angular momentum from the rotation of the earth th the orbital velocity of the moon. It leads to the average earth-moon distance (measured over the course of one moon orbit) increasing by roughly 3 centimetres per year. Hence, in a couple of tens of millions of years all solar eclipses will either be partial eclipses or annular eclipses. :)

    To give a more specific answer: I don't think there is any 'advantage' (to anyone or anything) to the moon having approximately the same angular size as the sun, seen from Earth - at least, I can't think of one at all. It hasn't always been the case (the moon used to be more close by) and it won't remain the case either. We're lucky to be living in this period so we can enjoy the best total solar eclipses!
    Last edited: Jul 9, 2007
  4. Jul 9, 2007 #3


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    Just FYI: The " symbol does not mean inches in this context. 0.5" would mean 0.5 arcseconds or seconds of arc. As you have correctly stated, arcseconds and arcminutes are units of angular measure. There are 60 arcminutes (symbol ') in one degree, and 60 arcseconds in one arcminute.

    Furthermore, the angular diameter of the sun is more like 30' (thirty arcminutes), so I'm not sure where you got 0.5" from.

    EDIT: Oh sorry. You said in your original post that the number may have been off but that wasn't the point you were driving at. Sorry I missed that.

    Well, lunar and solar eclipses would still occur. The sun just wouldn't be perfectly obscured (if the lunar disc were smaller). In this case, we might not call it an eclipse, but a transit (e.g. when Venus passes in front of the sun, we call it a transit). Furthermore, if the lunar disc were *larger*, then clearly solar eclipses would still occur (perhaps total eclipses would be even more frequent).

    Ummmm...if you are thinking in *Darwinian* terms, then you should be thinking about how human characteristics have evolved to take advantage of our natural surroundings, NOT how our natural surroundings may have been prearranged to be advantageous to us.

    That would imply that the arrangement of bodies in the solar system did not occur as a result of the physical laws governing the system (gravity) but was preordained by some intelligence. This is not scientific.

    In a manner of speaking, yes. The reason why the Sun, Earth and Moon are in the relative positions that they occupy today is because of physics. The reason why the sun and moon appear the same size from Earth is because they are occupying those positions and happen to have the sizes that they do. The fact that the relative distances of the sun and moon from the Earth result in them appearing the same size is therefore indeed coincidence.

    Helped WHAT to survive?
    Last edited: Jul 9, 2007
  5. Jul 10, 2007 #4


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    This is the wrong way round! There are theories that our moon is necessary for long term life on Earth by stabilising the tilt, producing tides and protecting Earth from impacts.
    That theory has taken a bit of a dive since the discovery of extremo-philes which suggest that life will pretty much always find a way!
  6. Jul 10, 2007 #5


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    but the higher life-forms, mgb, the higher

    we aren't extremo-philes, we're pretty soft actually
    we like our stabilized tilt, steady temperature regimes etc.
  7. Jul 11, 2007 #6


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    We manage to live in L.A. and Toronto (well perhaps not higher life forms!) so we are prretty adaptable.
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