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I recently found this animation which shows the motion of sun as observed from mercury. It seems as if the sun stops in between, retraces a bit and then continues towards the west. What is the reason for this strange motion? Is it because of variation in the distance between mercury and the sun? Or is it because a day on Mercury is longer than Mercury's year?

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That animation starts at aphelion and a surface temperature of -200 C. It ends at aphelion too, but then the temperature is over +100 C. –  LocalFluff Aug 15 at 6:27
@LocalFluff Maybe when the simulation ends, the side facing the sun is not facing the sun at the end of it. –  Yashbhatt Aug 15 at 17:13
Indeed, yes, Mercury behaves like that. Thank you for reminding me. –  LocalFluff Aug 15 at 18:58
It's explained in some detail here (same animation). cseligman.com/text/planets/mercuryrot.htm The sun doesn't stop, or more accurately, doesn't itself move very much. It's the planet motion that gives the sun it's apparent movement anyway. What happens is Mercury's orbit when it gets close to the sun, temporarily overtakes it's rotation, which is quite slow to begin with, then as it moves further away from the sun, it's orbital speed slows down and it's rotation overtakes it's orbital speed. –  userLTK Aug 15 at 19:43

2 Answers 2

up vote 6 down vote accepted

Both effects combined.

Day being longer than year would just have retrograde motion of the Sun on sky, but no direction change.

Variation of the distance alone happens on Earth, and we have no such effect.

But the combination of both factors, in the precise amount they have on Mercury, makes this effect happen.

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Thanks for the answer. Can you please post something which can help me visualize the phenomenon? –  Yashbhatt May 8 '14 at 9:33
Which phenomenon in particular? I think the animation you linked was quite correct and cool. –  Envite May 8 '14 at 9:37
Yeah that was. I want as if I am somewhere between sun and mercury or right behind mercury. –  Yashbhatt May 8 '14 at 9:41
Okay, first when mercury is at the farthest point, there is an imbalance between the rotational speed and speed of revolution. The sun moves normally across the sky. Now, when mercury is nearest to the sun, its speed of revolution is the greatest. At that point, a balance is achieved between both these speeds such that the sun appears stationary and seems to retrace a bit but then again, as time passes, mercury slows down and so the sun moves ahead in its path. Is that right? –  Yashbhatt May 8 '14 at 10:04
@Yashbhatt Yes, It is :) –  Envite May 8 '14 at 10:25

Before about 1966, Mercury was thought to be tide-locked, almost half always sunlit and another near-half always dark – as most moons, including ours, are tide-locked to their primaries, and for the same reason. The difference in the strength of the primary's gravity between the inner and outer ‘poles’ creates a force tending to pull those points away from the center of the satellite, along the line joining it to the primary. If the satellite is ellipsoidal rather than spherical, the tide will tend to align the ellipsoid's long axis to the primary.

But Mercury's orbit is so eccentric that the strength of the solar tide varies by a ratio of 4:7 (if I've computed correctly). The rotation rate nearly matches the revolution rate at perihelion, when tide is strongest and Mercury is moving fastest; if the match were perfect (if the orbital eccentricity were a bit less), the sun's apparent path would have cusps rather than little loops. Presumably the imperfection is because the tidal effect does not vanish away from perihelion.

The loops have nothing to do with axial tilt; Envite was probably thinking of the analemma.

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