I was reading this world building post and it got me thinking - every body eventually tidally locks with the body it rotates with.

Let's say for a moment a planet near the sun had a Uranus style axis that was in line with the solar plane. Over time the Sun would 'lock on' to this planet as it is doing with Venus, and apparently will do to us in 40 billion years (if, of course, we weren't going to get destroyed before then...) Would it simply stop spinning on that sideways axis? Or would the Sun torque it upright?


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If it was near enough, it definitely could.

Remember, that the planet's axis, beyond slow (centuries) precession, points towards the same point in space.

Let's take a planet with exactly 90 degrees axial tilt. As the planet circles the star, at one point, the north pole points towards the star, and the daily tidal forces are nearly constant, not affecting it. But 1/4 of the year later, the axis points in the same direction but the planet is 90 degrees away in the orbit - essentially, the star is right above its equator. At that time the tidal forces act at full strength arresting the spin with maximum efficiency.

Once the spin is slowed enough that the rotary momentum doesn't force the axis to remain in place, and tidal forces over the year (orbital period) begin dominating the forces over the diurnal period, the spin axis will begin shifting towards the standard 0 degrees, perpendicular to the orbital plane - until the planet spins at 1 spin per orbit - meaning, is tidally locked.

It would take longer than with planets with spin axis near-perpendicular to their orbital plane, but that's not an excessive difference - something of order of twice as long or so.

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    $\begingroup$ Does this mean that any tidally locked body has a perpendicular axis? $\endgroup$
    – corsiKa
    Aug 5, 2017 at 4:15
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    $\begingroup$ @corsiKa: Yes, if you really want to define such axis. In tidal lock, with the non-uniform body moving in orbit that's not a perfect circle, isn't ideally coplanar with sun's equator, and generally imperfect, defining such a simple abstraction as spin axis may be quite misguiding as the remaining motion ("wobble") of the satellite is quite a bit more complex than just "1 spin per 1 orbit". $\endgroup$
    – SF.
    Aug 5, 2017 at 13:27

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