Results 1 to 9 of 9
  1. #1
    Platinum Lounger
    Join Date
    Nov 2001
    Location
    Melbourne, Victoria, Australia
    Posts
    5,016
    Thanks
    0
    Thanked 0 Times in 0 Posts

    The monthly moon.

    Here's a poser I posted in another thread on time zones. It probably belongs here:

    Another of those oft' asked questions is "Why is there a dark side of the moon"? And the answer is not because that's what Pink Floyd decided to call it when they released it! It's because the moon rotates on its own axis at the same rate as it revolves around the Earth - about once a month. But this begs the deeper question: is this some extraordinary coincidence, or is there something behind it? The Earth, for instance, spins once a day, but revolves around the sun once a year.

    No prizes for Googling either. <img src=/S/grin.gif border=0 alt=grin width=15 height=15>

    Alan

  2. #2
    Silver Lounger
    Join Date
    Mar 2001
    Location
    Dallas, Texas, USA
    Posts
    1,680
    Thanks
    0
    Thanked 1 Time in 1 Post

    Re: The monthly moon.

    I couldn't help myself...I googled! And still, All I got was the Pink Floyd solution. <img src=/S/igiveup.gif border=0 alt=igiveup width=31 height=23>
    - Ricky

  3. #3
    4 Star Lounger
    Join Date
    Sep 2002
    Location
    Stafford, Staffordshire, England
    Posts
    585
    Thanks
    0
    Thanked 0 Times in 0 Posts

    Re: The monthly moon.

    It's not a coincidence since most of the moons of the Solar System keep one face pointing towards their parents. The parent's gravity affects its moon causing tidal bulges. The planet pulls on the closest bulge trying to keep it aligned with the planet. The overall effect is to slow down the rotation of the moon until it matches exactly the orbital period.
    I should know. I've been there! <img src=/S/grin.gif border=0 alt=grin width=15 height=15>
    <font color=blue><font face="Script MT Bold"><big>Rob</big></font face=script></font color=blue>

  4. #4
    WS Lounge VIP sdckapr's Avatar
    Join Date
    Jul 2002
    Location
    Pittsburgh, Pennsylvania, USA
    Posts
    11,225
    Thanks
    14
    Thanked 342 Times in 335 Posts

    Re: The monthly moon.

    Mercury also sees this phenomenon since it is close to the sun.

    Steve

  5. #5
    4 Star Lounger
    Join Date
    Sep 2002
    Location
    Stafford, Staffordshire, England
    Posts
    585
    Thanks
    0
    Thanked 0 Times in 0 Posts

    Re: The monthly moon.

    I wasn't aware of that, Steve. Thanks.
    <font color=blue><font face="Script MT Bold"><big>Rob</big></font face=script></font color=blue>

  6. #6
    Platinum Lounger
    Join Date
    Nov 2001
    Location
    Melbourne, Victoria, Australia
    Posts
    5,016
    Thanks
    0
    Thanked 0 Times in 0 Posts

    Re: The monthly moon.

    <hr>I should know. I've been there! <hr>Well, there's no arguing with that then! Both good answers from you and Steve. Basically it all boils down to imperfect sphericity of all natural "spheres", so that the centre of mass is not really at the centre of a sphere. At equilibrium, the minimum potential energy configuration of such a (2 body) system is for the centres of mass separations to be at a minimum. This is a stable situation, since gravity always tends to restore it if there is a fluctuation. Mercury, under intense gravitation, has attained this equilibrium status. So has our moon, with its mass very much less than mother Earth. Many other planetary moons have done the same. Maybe you could check a few out if you chance to drop in Rob.

    Alan

  7. #7
    Plutonium Lounger
    Join Date
    Mar 2002
    Posts
    84,353
    Thanks
    0
    Thanked 29 Times in 29 Posts

    Re: The monthly moon.

    I'm afraid this isn't entirely true. In the mid 60's of the 20th centrury, it was discovered that Mercury rotates around its axis in 2/3 of its "year", or conversely, it rotates about its axis one-and-a-half times while it makes one turn around the sun. So there is a resonance effect due to the proximity of Mercury to the sun, but it doesn't always turn the same face to the sun.

    Added: A Google search for mercury rotation will yield more info.

  8. #8
    WS Lounge VIP sdckapr's Avatar
    Join Date
    Jul 2002
    Location
    Pittsburgh, Pennsylvania, USA
    Posts
    11,225
    Thanks
    14
    Thanked 342 Times in 335 Posts

    Re: The monthly moon.

    I guess that what I get for learning science from old textbooks! <img src=/S/grin.gif border=0 alt=grin width=15 height=15>

    Steve

  9. #9
    Platinum Lounger
    Join Date
    Nov 2001
    Location
    Melbourne, Victoria, Australia
    Posts
    5,016
    Thanks
    0
    Thanked 0 Times in 0 Posts

    Re: The monthly moon.

    I guess it's lucky then that Pink Floyd didn't release "Dark Side of Mercury". <img src=/S/grin.gif border=0 alt=grin width=15 height=15> That might have spelled class action lawsuits for misrepresentation.

    Alan

    Like Steve, I was under the impression that Mercury had a syncronous rotation/ revolution... hence the two temperature extremes on the planet. But I'm sure the information came from post-1960s sources ???

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •