cold moon, distant earth (tanka)

For Frank’s Haikai challenge #171 “cold moon”

Sundered by violence,
circling, locked in a dance, she
always shows one face
though her partner turns away,
as they drift slowly apart.

Okay, so a poem should be able to stand on its own. But as an educator (by profession and vocation), I can’t resist adding some explanation. (Stop here if you’re not interested in astronomy.)

The moon was formed from a collision of the Earth with a rogue planet about 4 billion years ago (probably). Bits of both Earth and the impacting body that were shattered off then accreted to become the moon. The Earth and moon are locked together by gravity, orbiting a common centre of mass (within the Earth, about ¾R(earth) of the way from the centre). The moon is tidally locked to the Earth – which is why the moon always faces the same way from our point of view – because of tides on Earth and the moon. But because of the tides, mechanical energy (potential + kinetic) is dissipated due to friction. The result is both Earth and moon are slowing down. The rotation of the Earth is slowing, so eventually (but not before the sun gives up) we would be tidally locked to the moon too. It also results in the orbit of the moon slowing. But angular momentum is a conserved quantity – and it goes like mvr where m is the mass of the moon, r is its orbital radius and v is its orbital velocity. So, if v gets smaller because of kinetic energy being lost due to the tides, then r must get bigger (mvr is constant). And that is indeed what we see – the moon is slowly fleeing its dance with the Earth. But she’s keeping an eye on us even as she slowly backs away.
And if you haven’t yet said TL:DR, you can read more on hyperphysics. (Love that site.)

Can you tell I miss teaching my science enrichment program?

17 Comments

Filed under musings, poem

17 responses to “cold moon, distant earth (tanka)

  1. Reblogged this on Frank J. Tassone and commented:
    #Haiku Happenings #3: Kate Wilson’s latest #tanka for my current #Haikai Challenge!

  2. 😀
    This post of yours deserves a standing ovation actually!

  3. I appreciate the science though I’m not really able to understand it completely. I know some folks see a face in the moon and others have stories connected to the other shadow bowls of the moon… Like the Rabbit in the Moon (an Asian folk tale).

    Hope the New Year is better for all of us! Cheers Jules.

    • To be honest, I don’t really see anything like a face or rabbits in the moon, just blotches. But I find the orbital mechanics fascinating – such a lovely example of conservation principles. 🙂
      Have a very happy 2021 Jules!

      • The blotches on the moon are like the constellations of the stars… Peoples make up their own stories to connect the dots 🙂

        We just watched an odd little show ‘https://en.wikipedia.org/wiki/Time_Trap_(film)’ (only a three star rating) But in the end there was some bit about traveling to mars and then attempting to maybe one day returning to earth.

        I also liked “Hidden Figures” that also showed how math was used to save the astronauts that were up in space. I don’t recall the name of the one principle that was used to help give the craft the push it needed to break the moons orbit and make it able for them to return home.

        Cheers!

  4. Good description in poem.

  5. I enjoyed your explanation as much as your tanka.

  6. Pingback: #Haikai Challenge #172 (1/3/21): first sun (hatsuhi)/ #173: (1/10/21) Democracy #haiku #senryu #haibun #tanka #haiga #renga – Frank J. Tassone

  7. Love this one, hi from the astronomer! 😉

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