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

July 25, 2018

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.


My First Scientific Paper

May 17, 2017

As a follow-up to yesterday’s post on how to read a scientific paper, I wanted to share the story of my first attempt to read such a paper myself. I was doing research in preparation for the 2015 Mission to the Solar System, and I’d found a paper titled “Thermal Stability of Volatiles in the North Polar Region of Mercury.”

I was under the impression this paper was sort of a big deal as far as Mercury exploration is concerned, so I felt I ought to read it. Previously I’d only read the abstracts of papers, and occasionally the conclusions. I’d never before tried to read a scientific paper in full.

It didn’t go well. Not at first. The paper was only four pages long, but it felt like forty and may as well have been four hundred. I was particularly confused by the usage of the word volatile, as in volatile chemicals. I thought I knew what that meant. Turned out I was wrong, and it took awhile for me to figure out what volatiles really are.

I must’ve read the paper straight through three or four times before something in my brain clicked. And then…

I got it! I actually got it! NASA had found water (a volatile) on Mercury! I’d already learned about this from another source, but the fact hit me with a new weight. Suddenly I not only knew about Mercury’s water, but I also knew where the water was located (frozen inside dark polar craters), why it hadn’t melted or sublimated away (at the poles, crater rims shield it from sunlight), and how NASA had found it (by bouncing radio waves off the ice sheets).

Maybe this will sound silly, but reading that “Thermal Stability” paper was a life-altering experience for me. I’ll never forget that moment of revelation when all that sciency stuff started making sense. For the first time, Mercury felt like a real place to me. For the first time, I “got” how NASA does what it does.

And most importantly, I learned that even though I’m just a science fiction writer and don’t have any kind of scientific degrees, I can still read and comprehend scientific publications. Which means I can bypass the unreliable science reporting I saw on T.V. or the Internet and go straight to the source for my scientific knowledge.