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Tidally locked planet's sun still moves?

  1. Jul 13, 2013 #1
    Hi PF!

    So I was reading about the possibility of life on planets orbiting red dwarf stars, and it was said that the major problem was that the planet would become tidally locked. With half the planet in eternal darkness, the whole atmosphere would freeze on the dark side, leaving the planet barren.

    But it seems to me that on a tidally locked planet, the sun would still move dramatically if the planet's orbit was highly elliptical. In this drawing, I have a figure marking one point on the planet, with the planet's year marked at four equal times. From the figure's POV, the sun is directly overhead only on the first days of summer and winter. For spring and fall, the sun is almost at the horizon!

    An area around the figure would have constant daylight, and the opposing area would have constant darkness, but much of the planet would have sunrises and sunsets, with the sun at it's largest when it was low in the sky. (Thus filtering out more radiation.) Given that the planets "year/day" could be only a few days or even one day long, doesn't this look quite habitable?

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  3. Jul 13, 2013 #2


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    Tidal locking also circularises the orbit, so you can't really have one without the other.

    But the claim that the atmosphere would freeze on the dark side of the planet seems odd to me. Wouldn't air circulation between the hemispheres keep the temperature above the boiling point of most of the gasses?
  4. Jul 13, 2013 #3
    Rats! You messed up my planet.

    According to the program I saw, not unless the air pressure was higher than earth's.
  5. Jul 13, 2013 #4
    Mercury is tidally locked, and on a quite eccentric orbit.
  6. Jul 13, 2013 #5


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  7. Jul 13, 2013 #6
    As the above reference states, Mercury's orbital period is 87.98 earth days while its axial rotation period is 58.65 earth days. As I understand the term, "tidally locked" means the rotation period is equal to the orbital period.

    EDIT: I don't think a tidally locked planet or moon can have an orbit that is too eccentric because tidal locking implies a strong gravitational link with the star or planet. The tidally locked moons of Jupiter are all within 2 million km of the planet and have near circular orbits. Callisto at almost 1.9 million km is tidally locked.
    Last edited: Jul 13, 2013
  8. Jul 13, 2013 #7


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    Heck, our own moon is tidally locked to the Earth and is at 360,000 - 400,000 km away.
  9. Jul 14, 2013 #8
    Yes, and the terminator is virtually fixed indicating the earth is always visible in the same place
    from a point on the earthward side of the moon. This goes against the OP's idea that the star of a tidally locked planet would change position in the sky of the planet in any significant way. The eccentricity of moon's orbit is 0.0549 which supports the observation that tidally locked planets/moons have low orbital eccentricity although the moon's eccentricity is significantly larger than the Earth's 0.0167.
    Last edited: Jul 14, 2013
  10. Jul 14, 2013 #9


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    Of course! I wasn't implying otherwise.
  11. Jul 14, 2013 #10
    So I'll do it instead! :)


    It looks to me like there are certain points on the moon where the Earth would rise and set.
  12. Jul 14, 2013 #11
    Yes, near the terminator.

    "An earthrise that might be witnessed from the surface of the Moon would be quite unlike sunrises on Earth. Because the Moon is tidally locked with the Earth, one side of the Moon always faces toward Earth. Interpretation of this fact would lead one to believe that the Earth's position is fixed on the lunar sky and no earthrises can occur, however, the Moon librates slightly, which causes the Earth to draw a Lissajous figure on the sky. This figure fits inside a rectangle 15°48' wide and 13°20' high (in angular dimensions), while the angular diameter of the Earth as seen from Moon is only about 2°. This means that earthrises are visible near the edge of the Earth-observable surface of the Moon (about 20% of the surface). Since a full libration cycle takes about 27 days, earthrises are very slow, and it takes about 48 hours for Earth to clear its diameter.[11] During the course of the month-long lunar orbit, an observer would additionally witness a succession of "Earth phases", much like the lunar phases seen from Earth. That is what accounts for the half-illuminated globe seen in the photograph."

    Earthrise, wikipedia

    Libration is caused by the eccentricity of the moon's orbit which I noted at 0.0549 is not insignificant. However the tidally locked moons of Jupiter have much smaller eccentricities and I would expect a tidally locked planet near a red dwarf star to have low eccentricity, but I could be wrong, in say multiple star system.
    Last edited by a moderator: Apr 20, 2017
  13. Jul 14, 2013 #12
    Thanks VandeCarr!

    So how much libration is plausible on a planet orbiting a red dwarf? Also, I understand tidal locking generally, but I'm not clear on how this would circularize an orbit.
  14. Jul 14, 2013 #13
    There are several kinds of libration. With eccentricity, orbital speed changes with the variations in the distance of the orbiting body from its planet or star. Another kind is due to axial wobble. There are some other less well understood kinds. Any of these could possibly affect a tidally locked planet.

    True tidal locking (Mercury is not tidally locked) implies stronger gravitational fields and higher and possibly more constant orbital speeds. I think the real question is why orbits would not be near circular. It could be how a satellite was acquired by the central body or by the influence of nearby planets or stars ( in multiple star systems). I noted that all the tidally locked moons of Jupiter have very low eccentricities, but I can't give you a definitive answer.
    Last edited: Jul 14, 2013
  15. Jul 15, 2013 #14
    I understood that circular orbits were due to lots of collisions. The eccentricities of various bodies would tend to cancel each other out when they combine into one.

    When a planet is in the process of becoming tidally locked, the rotational energy goes into heat from tidal deformations of the planet. Where would the energy come from (or go to) from reducing eccentricity? Can an orbit be changed without a third body involved?
  16. Jul 15, 2013 #15
    All I can tell you is that the tidally locked moons of the Jupiter and Saturn systems have low eccentricities (generally less than 0.0100). Tidal locking apparently places some restrictions on the moons' librations. The Earth's moon is somewhat of an exception with an eccentricity of 0.0549. There are several possible explanations including the possibility that it's a captured satellite. The Moon is also much larger relative to Earth compared to those of Jupiter and Saturn and therefore its librations might be less restrained. As for more details as to why tidally locked satellites generally have small eccentricities, perhaps someone else will respond.
    Last edited: Jul 15, 2013
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