Things I Don’t Understand: Mercury’s Wandering Sun

Okay, this is a thing I’ve read about multiple times, but no matter how many times it’s been explained to me I just don’t get it.  Apparently on Mercury, the sun sometimes appears to change directions in the sky.

Let me explain what I mean.  Imagine you’re standing on the surface of Mercury (and are somehow still alive).  You see the sun rise in the east, just as it does on most planets in the Solar System.  And then over the course of a long (very, very long) Mercurian day, you watch the sun slowly (so very, very slowly) travel from east to west.

But at one point, let’s say around midday, the sun appears to stop its east-to-west motion and then, for a short while (about 4 Earth days), it wanders from west to east instead.  Then the sun stops again and continues on its original westerly path.

Why does this happen?  I know it has something to do with the length of Mercury’s solar day versus its sidereal day.  A solar day on Mercury, the time it takes for Mercury to complete a rotation relative to the Sun, is approximately 176 Earth days long. But Mercury’s sidereal day, the time it takes for Mercury to complete a rotation relative to the ecliptic, equals about 59 Earth days.  Also, Mercury’s year is 88 Earth days long, so Mercury’s solar day is roughly twice as long as its year.

Obviously this all means the sun moves very slowly through Mercury’s sky, but why should it briefly stop, turn around, and go the other way?  I just don’t get it. I guess I just can’t conceptualize why this happens.  Maybe if I were better at math, all those numbers would add up for me, and I’d understand what’s going on.

Anyway, does this make sense to anyone else, or are you just as baffled by this as I am?

Update: Looks like I have a lot of really smart readers! It’s still kind of hard for me to conceptualize why this happens, but it’s starting to make a little more sense to me. The first comment from TureNorthBricks definitely cleared up a lot for me.

16 thoughts on “Things I Don’t Understand: Mercury’s Wandering Sun

  1. The way that I have understood it (and this is probably a layman’s over simplification) is that Mercury has the most eccentric orbit of all of the planets. As it reaches the point where it is closest to the Sun, its orbital velocity actually increases significantly enough to make it seem as if the Sun moves backwards in the sky. My guess is that the apparent “stop” in motion occurs as the planet is speeding up, it reaches a moment where rotation and revolution are equal, simulating a tidal lock akin to our moon. I don’t know how accurate that is… but it makes sense to me!

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    1. Yeah… I don’t fully get it either, but the way that I’ve tried to explain it to myself is partly with what truenorthbricks says. Also, I envision it similar to the way planets double back on themselves from our perspective. Earth moves faster in its orbit than Mars, say, moves in its. So, Mars, will be trucking along just fine, and then it’ll slow and appear to move backward relative to the background stars and relative to its earlier motion as Earth speeds past it. So, on Mercury, the Sun moves one way, but the combined speed of Mercury’s orbit, which changes because of its eccentricity, and its slow rotation causes the Sun to appear to move backward as a given spot on Mercury’s surface speeds along.

      Again, I’m not sure that’s right; but that’s how I try to make sense of it.

      Liked by 3 people

      1. Yeah it reminded me of the way planets double back on themselves too, which is why I used the word “wandering” to describe what the Sun does as seen from Mercury. I just couldn’t picture why the Sun would do that. But this thing about Mercury’s eccentricity makes a whole lot of sense to me. I’ll try to read up on it more, but I feel like that’s got to be the answer.

        Liked by 2 people

    2. The explanation on the Mars Wikipedia page backs up truenorthbricks’ explanation:

      “At certain points on Mercury’s surface, an observer would be able to see the Sun peek up about halfway over the horizon, then reverse and set before rising again, all within the same Mercurian day. This is because approximately four Earth days before perihelion, Mercury’s angular orbital velocity equals its angular rotational velocity so that the Sun’s apparent motion ceases; closer to perihelion, Mercury’s angular orbital velocity then exceeds the angular rotational velocity. Thus, to a hypothetical observer on Mercury, the Sun appears to move in a retrograde direction. Four Earth days after perihelion, the Sun’s normal apparent motion resumes.”,_rotation,_and_longitude

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  2. It’s caused by the fact that Mercury doesn’t get the chance to complete its second day before it finishes a year. That causes the sun to rise twice.

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  3. I think it’s as simple as the very long day is so long it will have a cycle within it where the day has an inner cycle with it’s year. It’s day is long and year short such that within it’s year it’s day can’t keep up with the year. Not sure how to explain it as these things are my weak point but does that get you anywhere?

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    1. This is sort of a weak point for me too. I figured the thing about Mercury’s day being longer than its year would have something to do with it. A few comments also mentioned Mercury’s eccentric orbit, which causes Mercury’s orbital velocity to outpace its rotation for a short time. It’s still pretty hard to conceptualize for me, but it’s starting to make more sense.

      Liked by 1 person

  4. I’ve spent a bunch more time thinking about this since you first posted than I’d like to admit (so, thanks a heap). But… yeah, like we’ve all said, it’s the combination of Mercury’s speed when it’s around perihelion, its slow spin, and it’s short year. There’s that 3:2 spin:orbit resonance. Every Mercury year is one and a half Mercury days. and every day is two thirds of a year.

    What this means is by the time Mercury has rotated to the point that a given (and presumably very thirsty) viewer would see the Sun start to set, it’s rushed around toward the other side of the Sun. So, the Sun never really gets a chance to set, but, instead, it doubles backward.

    I kind of think I’m saying the same thing as everyone else at this point. 🙂

    Liked by 1 person

    1. I’m really glad this has provoked so much thought and conversation here in the comments. This has been bothering me for years. Or in Mercury terms, it’s been bothering me for several days. But it’s thanks to all these comments that I think I finally get why this happens! Hopefully other people learned something as well.


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