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?