<p>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.</p>

<p>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.</p>

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

CC BY-SA 3.0
2015-08-15T03:12:02.573Z
2015-08-19T20:43:20.200Z
2015-08-19T20:43:20.200Z
Anton Sherwood
<p>Both effects combined.</p>

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

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

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

CC BY-SA 3.0
2014-05-08T09:33:12.720Z
Thanks for the answer. Can you please post something which can help me visualize the phenomenon?
Yashbhatt
CC BY-SA 3.0
2014-05-08T09:37:29.777Z
Which phenomenon in particular? I think the animation you linked was quite correct and cool.
Envite
CC BY-SA 3.0
2014-05-08T09:41:34.620Z
Yeah that was. I want as if I am somewhere between sun and mercury or right behind mercury.
Yashbhatt
CC BY-SA 3.0
2014-05-08T10:04:17.070Z
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
CC BY-SA 3.0
2014-05-08T10:25:26.057Z
@Yashbhatt Yes, It is :)
Envite
CC BY-SA 3.0
2014-05-13T05:15:47.397Z
I get why the sun appears to retrace a bit. But from what I read about it in a magazine, it was not exactly like the sun reversing its pat but more like the sun making a loop as it goes. I don't know what causes that. Do you?
Yashbhatt
CC BY-SA 3.0
2014-05-13T13:57:13.037Z
The combination of two movements, one "horizontal" with forward and backward and one "vertical" caused by the inclination of the Axis, the same one we experience on Earth where Sun is higher in sky in summer and lower in winter.
Envite
CC BY-SA 3.0
2014-05-13T14:05:45.623Z
I too thought that it should be due to the axis. Well, thanks.
Yashbhatt
CC BY-SA 3.0
2014-05-08T09:05:41.453Z
2014-05-08T09:05:41.453Z
Envite
<p>I recently found <a href="http://messenger-education.org/Interactives/ANIMATIONS/Day_On_Mercury/day_on_mercury.php">this</a> 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?</p>

CC BY-SA 4.0
2023-01-23T16:56:24.343Z
@userLTK  that animation is flash-based and not functional. If you know of a more recent animation please post.
Maesumi
CC BY-SA 3.0
2015-08-15T06:27:55.150Z
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
CC BY-SA 3.0
2015-08-15T17:13:06.207Z
@LocalFluff Maybe when the simulation ends, the side facing the sun is not facing the sun at the end of it.
Yashbhatt
CC BY-SA 3.0
2015-08-15T18:58:28.940Z
Indeed, yes, Mercury behaves like that. Thank you for reminding me.
LocalFluff
CC BY-SA 3.0
2015-08-15T19:43:23.863Z
It's explained in some detail here (same animation).   http://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
<the-sun><mercury>
Motion of the sun as observed from mercury
Yashbhatt
# Motion of the sun as observed from mercury

(Asked by: username_0 on 2014-05-08)

I recently found [this](http://messenger-education.org/Interactives/ANIMATIONS/Day_On_Mercury/day_on_mercury.php) 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?

**Tags:** the-sun, mercury

> **Comments:**
> **username_2**: 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.  
> **username_0**: @username_2 Maybe when the simulation ends, the side facing the sun is not facing the sun at the end of it.  
> **username_2**: Indeed, yes, Mercury behaves like that. Thank you for reminding me.  
> **username_3**: It's explained in some detail here (same animation).   http://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.  
> **username_1**: @username_3  that animation is flash-based and not functional. If you know of a more recent animation please post.  

---

## 2 answers

### Answer by username_5 on 2014-05-08. Score: 7 | Accepted answer

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.

> **Comments:**
> **username_0**: Thanks for the answer. Can you please post something which can help me visualize the phenomenon?  
> **username_5**: Which phenomenon in particular? I think the animation you linked was quite correct and cool.  
> **username_0**: Yeah that was. I want as if I am somewhere between sun and mercury or right behind mercury.  
> **username_0**: 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?  
> **username_5**: @username_0 Yes, It is :)  
> **username_0**: I get why the sun appears to retrace a bit. But from what I read about it in a magazine, it was not exactly like the sun reversing its pat but more like the sun making a loop as it goes. I don't know what causes that. Do you?  
> **username_5**: The combination of two movements, one "horizontal" with forward and backward and one "vertical" caused by the inclination of the Axis, the same one we experience on Earth where Sun is higher in sky in summer and lower in winter.  
> **username_0**: I too thought that it should be due to the axis. Well, thanks.  

---

### Answer by username_4 on 2015-08-15. Score: 2

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; username_5 was probably thinking of the analemma.


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