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The terminator od the Moon

  1. Oct 12, 2013 #1
    Hello everyone,
    I have one simple question. How quickly moves the terminator on the Moon? It is the same speed as the Moon rotates? I can't imagine it a lot.
    Thank you very much.
  2. jcsd
  3. Oct 12, 2013 #2
    Ideally for a tidally locked satellite, the terminator doesn't move at all relative to the satellite surface. However because of its orbital eccentricity, a slight axial tilt and the earth's rotation, the terminator of the moon does move over the course of its orbital period exposing about 59% of its surface to view from the earth. The rate of change is generally constant over that 29 earth day period, but the very small amount due to the earth's rotation is diurnal. These movements of the terminator are called librations.

    Note: This thread probably belongs in the General Astronomy forum.
    Last edited: Oct 12, 2013
  4. Oct 12, 2013 #3


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    Moved from GP.

  5. Oct 13, 2013 #4
    So to answer the original question, yes the speed of the terminator is approximately as fast as the Moon rotates, although we'd have to stipulate that we are referring to rotation measured vs. the sun, not the background stars (synodic vs. sidereal). Per wikipedia, the equatorial rcircumference is 10,921 km and the synodic period is 29.5 days, which works out to 4.28 m/s at the equator.
    Last edited by a moderator: Oct 13, 2013
  6. Oct 13, 2013 #5


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    thanks Barakn

    that's one of those random and interesting bits of info that I hadnt considered in all my years of lunar observing

  7. Oct 13, 2013 #6
    The terminator is the boundary between the illuminated and dark portion of a celestial body. You're describing the movement of an arbitrary point on the moon's surface as a function of the moon's rotation. As a tidally locked body, the "movement" of the terminator is best described by the moon's librations. Even the apparent movement of the terminator from a vantage point on the earth is dependent on lunar latitude and is affected by the librations.

    Last edited: Oct 13, 2013
  8. Oct 13, 2013 #7


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    Sorry, SW VandeCarr, but you seem to be confused here.
    You describe the situation as if the Earth was the source of Lunar illumination. It's obviously the Sun, and as such the terminator position is a function of Moon's rotation with respect to the Sun and not the Earth.

    Libration has no bearing on the movement of the terminator across the apparent(non-physical) disc of the Moon, and is only a minor contribution to the movement of the terminator with respect to the surface of the Moon. It would be the dominant, and only contribution, if the Moon were tidally locked with the source of illumination.

    The wikipedia article on libration has got a nice composite animation showing the monthly movement of the terminator as well as libration. This should help visualise the contributions of each of the two effects.

    The bottom line is, you appear to have fallen victim to the age-old dark side of the Moon/far side of the Moon confusion.
  9. Oct 13, 2013 #8
    Sorry. Yes. You're obviously correct. My bad. Barakn is also right. I don't know what a terminator is unless it's the terminator for "earthlight". :(
    Last edited: Oct 13, 2013
  10. Oct 14, 2013 #9


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    from one of my many Lunar photos
    Lunar Terminator
    the line separating the day and night areas on the lunar surface as seen from earth


    another way to think of this, is its the sunrise line (Sunset line - see below) if you were on the lunar surface

    Viewed from earth, you dont see a terminator line at New Moon, as the face of the Moon that we would normally see is in darkness. As the Moon goes through its orbit around the earth, we see the Moon becoming more and more illuminated by the Sun as sunrise advances across the Moons surface. Then when Full Moon is reached, we again no longer see the terminator line.
    From Full Moon back to New Moon, we again observe the terminator line moving across the face of the Moon. This time tho, its the advancing sunset.


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    Last edited: Oct 14, 2013
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