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← Why is The Moon Moving Away From Us? - Ask The Experts #20

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Showing Revision 1 created 04/09/2014 by Natalie Crino.

  1. Hello, Spacelab! I'm Lucianne Walkowicz.
  2. I'm an astrophysicist at Princeton University, where I work on studying stars
  3. and planets with NASA's Kepler Mission and I'm here today to answer your questions.
  4. The first question is "If we were to add a lot of mass to the moon for its gravity to get stronger,
  5. what would that mean to the rotation of the earth?"
  6. So, this is a great question and in order to understand how the rotation of the earth would change
  7. if the moon was much more massive, it helps to know how the moon actually affects Earth's rotation currently.
  8. So, the moon and the earth interact through gravity and the way that the moon affects the Earth's rotation
  9. is that it actually causes the earth to be ever so slightly egg-shaped
  10. because it exerts a pull on the earth from one side to the other, what we call a tidal force.
  11. Now, this affects the actual ground of the earth a very tiny bit,
  12. but it affects our oceans a lot more.
  13. And it causes the oceans to pile up in one sense on one side of the planet.
  14. So, we actually get what we call a tidal bulge.
  15. Now this bulge, as the earth rotates, gets dragged just a little bit forward of where
  16. the moon is in the sky, but the moon still pulls on it, so through gravity it actually
  17. creates a force that makes the earth slow down over time.
  18. Now the slow-down is really, really small.
  19. It's really only about 1 500th of a second every century, but over time it adds up.
  20. If we were to wait a long enough time, the earth and the moon would actually
  21. eventually become completely locked so that one side of the earth was always facing the same side of the moon
  22. and therefore only one side of the earth would ever be able to see the moon.
  23. So, if you were to make the moon much more massive than it is now,
  24. you would actually exert a greater force on the earth and you would cause that slow-down
  25. to the rotation of the earth to happen even faster.
  26. The second question is, "If smaller objects are being pulled by larger objects,
  27. then why does the moon get further and further away from Earth?"
  28. So the answer to this question is actually related to the answer to the first question.
  29. As the moon and the earth interact through gravity and the moon slows down
  30. the earth's rotation over time, that means that it's causing Earth to lose energy
  31. as the moon itself gains that energy.
  32. That gain in energy means that the moon starts to orbit further and further away from the earth.
  33. So it actually moves further away, even through the interaction of gravity.
  34. Now over time, that means that the moon is spiraling outwards
  35. and we can measure that happening because we do experiments where we bounce
  36. laser beams actually off of the corner reflector mirrors that the Apollo astronauts left behind.
  37. So we can measure that the moon is moving further away and this also means that
  38. we live in a sort of special time, where the moon is actually the same apparent size
  39. as the sun on the sky and that means that we live in a time where we can have total solar eclipses.
  40. So, years and years and years from now, the apparent size of the moon will actually
  41. be smaller than that of the sun and so total solar eclipses will cease to happen.
  42. The third question is, "Why are celestial bodies like starts, planets and moons, always spherical in shape?"
  43. So the answer is, they're actually NOT all spherical in shape.
  44. Even though you're probably familiar with some of the bigger bodies in our solar system,
  45. like the sun and the earth or even other planets like Jupiter that are spherical,
  46. many of the bodies in our solar system are actually sort of oblong or pock-marked
  47. and look a little bit more like potatoes.
  48. A good example is the asteroids.
  49. These bodies are small enough that most of them didn't have enough mass
  50. for gravity to mold them into a spherical shape when they were forming.
  51. So even though gravity brought the material that formed them together and holds them together today,
  52. it's in fact impossible for things that are very small to become spherical in shape.
  53. Thanks for the great questions.
  54. If you have more questions for the next expert, please leave them in the comments below.