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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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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 at 9:33
    
Which phenomenon in particular? I think the animation you linked was quite correct and cool. –  Envite May 8 at 9:37
    
Yeah that was. I want as if I am somewhere between sun and mercury or right behind mercury. –  Yashbhatt May 8 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 at 10:04
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@Yashbhatt Yes, It is :) –  Envite May 8 at 10:25
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