# Mercury A to Z: Zero Degrees Longitude

Hello, friends!  Oh my goodness, we made it!  We made it to the end of the A to Z Challenge!  For this year’s challenge, my theme is the the planet Mercury, and in today’s post Z is for:

## ZERO DEGREES LONGITUDE

Zero degrees longitude.  The prime meridian.  It’s an imaginary line that helps define the latitude-longitude coordinate system for mapping the surface of a planet.  On Earth, the prime meridian runs through the very English Royal Observatory in Greenwich, England.  On Mars, the prime meridian runs through Airy-0, a crater named after Sir George Airy, the very English scientist who decided where Earth’s prime meridian should be.  So where is the prime meridian on Mercury?

Actually, we talked about this in a previous post.  Mercury’s 0 and 180 degree longitude lines are supposed to run through the planet’s “hot poles,” the two points along Mercury’s equator where the temperature gets highest.  But the hot poles aren’t visible surface features, like Airy-0 or the Greenwich Royal Observatory.  So in the 1970’s, when NASA’s Mariner 10 space probe arrived at Mercury, scientists were hoping they could find an obvious surface feature to serve as an official prime meridian marker.

Mariner 10 visited Mercury three times.  It flew by Mercury, looped around the Sun, then flew by Mercury again, and then again one more time, before the space probe ran out of fuel.  During each of those three visits, only half of Mercury was visible to Mariner 10’s cameras, and unfortunately it was always the same half of the planet.  As a result, Mariner 10 never saw Mercury’s prime meridian, nor could it see any surface features on or near that imaginary line.

So Mercury’s prime meridian ended up being defined in a rather awkward way.  Scientists picked a tiny crater 20 degrees west of where the prime meridian was supposed to be.  They named the crater Hun Kal, which means twenty in an ancient Mayan language (this is one of the rare craters on Mercury not named after an artist, writer, or musician).  Scientists then officially defined Mercury’s prime meridian as a line of longitude exactly 20 degrees east of the center of Hun Kal Crater.

Thanks to NASA’s MESSENGER Mission, we now have photos of the entire surface of Mercury.  Presumably this means scientists could redefine Mercury’s prime meridian, if they wanted to, but nobody seems interested in doing that.  Using Hun Kal Crater to define the prime meridian may not be ideal, but it seems to work well enough.  And if it works, why fix it?

Here’s an article from SpaceRef.com, featuring an image of Hun Kal Crater as seen by MESSENGER.

# Mercury A to Z: Year of Mercury Watching

Hello, friends!  For this year’s A to Z Challenge, I decided to talk about the planet Mercury.  I wasn’t sure at first if I’d be able to do a whole alphabet’s worth of posts about this one planet, but at this point, I think I just might pull it off!  In today’s post, Y is for:

## YEAR OF MERCURY WATCHING

My original plan for this post was to talk about Mercury’s year.  It’s 88 Earth days long, which (oddly enough) is only half the length of Mercury’s solar day.  That’s because of a spin-orbit resonance, sidereal rotation, yada yada… we’ve already talked about this stuff.  So instead, today I want to talk about the rest of this year here on Earth and tell you when the best opportunities to see Mercury will be.

The best time to see Mercury is during an “elongation,” which is when Mercury (as viewed from Earth) is as far away from the Sun as he can get.  To say that another way, if you drew an imaginary line between Mercury and the Sun, elongation is when that line would be at its longest.

Some elongations end up being a little higher (or should I say longer?) than others.  This is because of Mercury’s highly elliptical (non-circular) orbit.  The highest elongation of Mercury this year occurred on April 11.  I wish I’d known that earlier this month, because I definitely would have mentioned it.  Anyway, the next elongation will occur in the morning on May 29, 2023.  After that, there will be an elongation in the evening on August 10.  Another elongation will occur on the morning of September 22, and another will be on December 4.

Don’t worry too much about the specific dates.  You’ll still get a pretty good view of Mercury a few days before and after an elongation occurs.

I have only seen Mercury two times in my life that I know of.  I recently learned, however, that Mercury is known to twinkle like a star, so I may have seen him many times without recognizing him as a planet.  After all the Mercury research, Mercury writing, and Mercury artwork I’ve done for this year’s A to Z Challenge, I am very eager to get out there and see Mercury again (even if it means getting up before sunrise on or around May 29).

I found the dates for upcoming elongations of Mercury in this article from EarthSky.org.  The article also goes into a little more detail about how elongations work.

# Mercury A to Z: X-Class Solar Flares

Hello, friends!  For this year’s A to Z Challenge, my theme is the planet Mercury, and in today’s post, X is for:

## X-CLASS SOLAR FLARES

X was the hardest letter to find for this A to Z series.  For a while, I thought I was going to have to do something like “eXtreme temperatures on Mercury,” or maybe “eXoplanets like Mercury.”  There is a crater on Mercury named Xiao Zhao, which could have worked, but we’ve talked about so many craters already.  That seemed boring.  Then, just as I was wrapping up my research, I stumbled upon a paper titled “Modeling the Impact of a Strong X-Class Solar Flare on the Planetary Ion Composition in Mercury’s Magnetosphere.”  I have never in my life been so excited by the title of a scientific paper.

Scientists have come up with something like a Richter scale for solar flares.  The smallest, least energetic flares are called A-class solar flares.  B-class flares are ten times stronger than A-class.  C-class flares are ten times stronger than B-class.  Then, confusingly, we get M-class flares (ten times stronger than C-class) followed by X-class flares (at least ten times stronger than M-class).

Here’s how I rationalize this A-B-C-M-X system.  A, B, and C-class flares are low level flares that are too weak to affect us here on Earth.  We Earthlings can basically ignore them.  The M in M-class flare probably stands for medium. We do need to worry about these medium level flares.  If an M-class flare were aimed directly at Earth, it could damage our satellites, endanger astronauts on the International Space Station, and scramble some forms of radio communications here on Earth.  On th upside, these medium-level flares can also trigger geomagnetic storms around Earth’s poles (a.k.a. auroras).

And X-class flares are eXtreme!  If an X-class flare hit Earth, it could overwhelm our planet’s magnetic field.  Potentially, it could overload our power grids, cause worldwide communications blackouts, and basically wreck the global economy, at least for a few days.  The auroras would be truly impressive, though, possibly extending all the way to Earth’s equator.  So we’d at least be able to enjoy that while waiting for the world’s banking computers to reboot.

Scientists have a pretty good understanding of how solar flares affect Earth.  They’ve also had opportunities to see up close what powerful solar flares do to Mars, Jupiter, and Saturn.  But what about Mercury?  You’d think solar flares would be a pretty big deal on the planet closest to the Sun, but according to the paper I read, we don’t know much about how solar flares affect Mercury.  That’s surprising at first, but it makes sense when you consider how much time and energy (and money) we’ve spent exploring those other planets I mentioned compared to how little we’ve spent thus far exploring Mercury.

Solar flares don’t happen all at once; they occur in phases.  According to the paper I read, when an X-class solar flare hits Mercury, we can expect different elements of Mercury’s exosphere to ionize during different phases of the flare.  Magnesium would ionize right away, during the impulsive phase.  Other elements, like oxygen and helium, would ionize later, during what’s called the gradual phase (also known as the decay phase).  And some elements, most notably sodium, might not be affected at all.

Hopefully more research on this will come soon.  Maybe the BepiColombo Mission will be lucky enough to observe Mercury up close during an M-class or X-class solar flare (I presume BepiColombo is designed to protect itself from that kind of thing).

And here’s a brief article from one of NASA’s education/outreach websites explaining the solar flare classification system.

And here’s another article from NASA that briefly discusses the different phases of a solar flare.

# Mercury A to Z: Weird Terrain

Hello, friends!  We’re getting close to the end of this year’s A to Z Challenge, when the last few letters of the alphabet start forcing challenge participants to get weird.  My theme for this year’s challenge is the planet Mercury.  Fittingly, in today’s post, W is for:

## WEIRD TERRAIN

Mountains, canyons, plateaus, glaciers, plains, hills, deserts… we already have names for these things.  But scientists sometimes discover landscapes on other worlds that we simply don’t have here on Earth, and they have to invent new words to describe them.  There’s the spider-like araneiform terrain on Mars, or the chaos terrain on Europa, or the cantaloupe terrain on Triton, which really does make parts of Triton look like the skin of a cantaloupe.  In 1974, scientists discovered a weird, new kind of terrain on Mercury.  They decided to call it weird terrain.

If you recall my post about Caloris Basin, then you know that just shy of four billion years ago a gigantic asteroid smashed into Mercury, giving Mercury a crater larger than the state of Texas.  Mercury’s weird terrain is on the exact opposite side of the planet.  This is almost certainly not a coincidence.

Three factors probably contributed to the formation of Mercury’s weird terrain.  Some people say that Mercury’s weird terrain looks almost like something tried to punch its way up through the planet’s crust, and that may be exactly what happened.  When that giant asteroid slammed into Mercury, the force of the impact went straight through the planet and ripped up the ground on the planet’s opposite side.

Additionally, the force of the impact would have sent tremendous seismic waves rippling through the planet’s crust.  When those seismic waves converged on the exact opposite side of the planet, they further disrupted the planet’s crust in that region.

And then there’s one more thing.  The impact event that created Caloris Basin would have sent debris flying all over the planet.  Clouds of flying debris probably converged on the opposite side of the planet.  When that happened, rocky debris started to rain down on that one badly disrupted patch of land.  That one patch of land would have looked weird enough already, so the extra rubble falling from above would have made it look even weirder.

Words like “jumbled” and “haphazard” are sometimes used to describe Mercury’s weird terrain.  In some images, the landscape reminds me a little of a stucco finish.  With the ground being ripped up from below and all that debris raining down from above, it’s little wonder that weird terrain looks the way it does. As far as I know, Mercury’s weird terrain is unique in the Solar System.  I feel like I could be wrong about that, though, so if anyone knows of something similar that’s happened anywhere else, I’d love to hear about it.

This article from the Planetary Society goes into a little more detail about how Mercury’s weird terrain was discovered and how it probably formed.

# Mercury A to Z: Vulcan

Hello, friends!  Welcome to another posting of the A to Z Challenge.  For this year’s challenge, my theme is the planet Mercury, and in today’s post, logic dictates that V is for:

## VULCAN

As you know, Mercury is the planet closest to the Sun, but at one time astronomers had reason to believe that there was another planet even closer to the Sun than Mercury.  This hypothetical planet was named Vulcan, after the ancient Roman god of fire—a highly logical choice.

Our story begins with Isaac Newton and his law of universal gravitation.  Thanks to Newton, it became possible to predict the motions of the planets with extraordinary precision; however, in the centuries following Newton’s death, astronomers started having trouble using the logic of Newton’s law to predict when transits of Mercury would occur.

A transit of Mercury is when Mercury passes directly in front of the Sun, as observed from Earth.  This is one of the most exciting ways to see Mercury, provided you take the necessary precautions to protect your eyesight.  But in the 18th and 19th Centuries, Mercury started transiting the Sun at seemingly illogical times.  Mathematical predictions of Mercury transits were off by minutes, hours, or even by as much as a full day!

So French astronomer and mathematician Urbian Le Verrier hypothesized that another planet (named Vulcan) might exist, orbiting the Sun within the orbital path of Mercury.  Vulcan’s gravity might perturb the orbit of Mercury enough to explain why Mercury never seemed to transit the Sun on schedule.  Le Verrier had made a similar hypothesis, based on perturbations of the orbit of Uranus, which led to the discovery of the planet Neptune.  Thus, it seemed only logical to take Le Verrier’s Vulcan hypothesis seriously.

In the following years, a few astronomers claimed to have found Vulcan, proving Le Verrier’s hypothesis, but follow up observations could never confirm these discoveries.  Most sightings of Vulcan were probably just stars that happened to be near the Sun.  Most transits of Vulcan were probably just sunspots.  Perhaps, instead of a single planet, Vulcan might be a swarm of asteroids: the vulcanoid asteroids.  But it would require an absurd number of asteroids to account for the observed perturbations of Mercury’s orbit.  Logically speaking, an asteroid swarm that large would have already been noticed.

So Mercury kept transiting the Sun at the wrong times, according to Newton’s laws, and no one could explain why.  Not until 1915, with the publication of the theory of general relativity.  Thanks to the logic of German theoretical physicist Albert Einstein, we now know that the mass of the Sun curves the fabric of space-time.  This curvature affects the orbits of all the planets, but most especially the orbit of Mercury, because Mercury is so very close to the Sun.

Today I want to recommend this video from Astrum, one of my favorite YouTube channels.  If you love space as much as I do, it would be only logical to check out what Astrum has to offer.

# Mercury A to Z: Uplands

Hello, friends!  Welcome back to the A to Z Challenge.  For this year’s challenge, my theme is the planet Mercury, and in today’s post U is for:

## UPLANDS

Science Fiction

The year is 2059.  With the benefit of newly invented gravity manipulation technology, NASA has determined that they can safely and economically place a small rover on the surface of Mercury.  The first ever Mercury rover will land in a region just south of Mercury’s equator, part of the so-called “uplands” of Mercury.

Science Fact

There are generally two types of terrain on Mercury: the smoother, flatter volcanic plains regions, which are mostly found in the northern hemisphere, and the rougher, craggier, more heavily cratered “uplands,” which are found in Mercury’s equatorial regions and extend into the southern hemisphere.

Those smoother, flatter regions formed through a process planetary scientists call “resurfacing,” which is one of my favorite scientific euphemisms.  Resurfacing sounds like something you do to a parking lot.  What resurfacing actually means, in reference to planets, is that some sort of extreme volcanic activity covered part of a planet’s surface in lava.  The lava cooled and hardened, creating a smooth new surface and covering up whatever surface topography may have been present in the past.

Mercury is not a volcanically active world today, but it must have been at some point.  Most likely, the partial resurfacing of Mercury happened shortly after the end of the late heavy bombardment, a critical period in the history of our Solar System when the inner planets got pelted with asteroids.  Lava pooled in low elevation regions of Mercury, either filling in or totally covering up craters left by the late heavy bombardment.  But higher elevation regions—the uplands, in other words—were spared from resurfacing.

Similar upland terrain can be found on the Moon, and studying the lunar uplands has told scientists much about what the Solar System was like during the late heavy bombardment.  Comparing and contrasting the uplands of the Moon with the uplands of Mercury may give us an even clearer and more detailed picture of what that era of the Solar System’s history was like.  For this reason, a mission to explore the uplands of Mercury could be very interesting and exciting for scientists.

Science Fiction

NASA apparently failed to learn their lesson after the public naming contest for their mission to Uranus and proceeded to hold another public naming contest for the Mercury Uplands Rover.  And that is how NASA’s Up-Dog Mission officially came to be.

Here is a 2016 article from NASA announcing the first complete topographic map of the surface of Mercury.

And here is an article from Space.com about the Moon and Mercury and the things we might learn by comparing and contrasting the two.

P.S.: If you don’t understand the up-dog reference, feel free to ask me “What is up-dog?” in the comments below.

# Mercury A to Z: The Twinkling Planet

Hello, friends!  We’re in the final week of this year’s A to Z Challenge!  If you don’t know what the A to Z Challenge is, it’s a monthlong blogging event where participants write a full alphabet’s worth of blog posts about a topic of their choice.  My topic this year is the planet Mercury, and in today’s post T is for:

## THE TWINKLING PLANET

When you look up in the nighttime sky, stars twinkle, and planets don’t.  This is probably the first astronomy lesson most people learn.  It’s one of those basic facts almost everybody seems to know.  However, like most super basic facts that everybody seems to know, there are exceptions to the rule.  Mercury is a planet, and yet Mercury twinkles.

Stars twinkle because they’re very far away, and their starlight is relatively faint.  So when starlight hits Earth’s atmosphere, the atmosphere distorts the light, causing a twinkling effect.  But planets are much closer, and sunlight reflecting off a nearby planet is much brighter and more intense than the light emitted by distant stars.  Earth’s atmosphere still distorts the light reflecting off planets, but the distortion is nowhere near as noticeable.  Usually.

Two factors make Mercury different.  First, Mercury is much smaller than the other planets, so he doesn’t reflect as much sunlight our way as, say, Venus or Jupiter.  It’s probably worth mentioning that Mercury is also a darker colored planet, with much of his surface covered in graphite.  Second, because Mercury is so close to the Sun, we Earthlings usually can’t see him except just after dusk or just before dawn.  This means that whenever we see Mercury, Mercury’s light has to pass through Earth’s atmosphere at an angle.  In other words, Mercury’s light has to travel through more of Earth’s atmosphere in order to reach our eyes.

Less light plus more atmospheric distortion equals a twinkling planet.  As William Sheehan notes in his book Mercury (for the Kosmos series), “[Mercury] is more often seen, no doubt, than recognized.”

I love star gazing.  I love looking for planets in the sky, and I love the moment of realization when I recognize one of them.  I only know for certain that I’ve seen Mercury two times in my life, and I needed help from an astronomy app both times.  However, I may have seen Mercury many times than I know and just assumed he was another star, twinkling near the horizon.

The real lesson here is that there are those basic facts that everyone knows, basic facts like stars twinkle and planets don’t.  But once you learn the basic facts about something, start learning about the exceptions to the rule.  You may miss out on some really neat experiences in life if you ignore the exceptions and stick to knowing only the basic facts.

Here’s an article from EarthSky.org about why stars twinkle and planets (usually) don’t.  It’s one of the rare articles I’ve seen that notes how, under the right circumstances, planets can twinkle, too.

And also, I’m going to once again recommend Mercury by William Sheehan.  Chapter One of that book is about how Mercury twinkles, or rather how Mercury “scintillates,” to use a more scientific term.

# Mercury A to Z: Solar vs. Sidereal Days

Hello, friends!  Welcome to another posting of the A to Z Challenge.  My theme for this year’s challenge is the planet Mercury, a planet that often gets overlooked by space geeks like me.  In today’s post, S is for:

## SOLAR vs. SIDEREAL DAYS

We’ve been talking about Mercury all month long, and we’ve been talking a lot about Mercury’s rotation rate.  In some posts, I’ve told you that Mercury has a rotation rate equal to approximately 59 Earth days.  In other posts, I said a day on Mercury is about 176 Earth days long.  That seems like a contradiction, but both of those numbers are correct.  It all depends on whether we’re talking about a solar day or a sidereal day.

A solar day can be defined as the time it takes a planet to rotate once relative to the Sun, while a sidereal day is the time it takes a planet to rotate once on its own axis.  A solar day is what we Earthlings usually mean by the word “day.”  It’s the 12 a.m. on Monday to 12 a.m. on Tuesday kind of day.  A sidereal day (pronounced si-der-e-al) is more like a planet’s true rotation period, relative to the rest of the universe.  Why are these things different?  Because planets orbit the Sun.  Because as they orbit, they change position relative to the Sun.

In Earth’s case, it takes about 23 hours and 56 minutes to rotate once on its own axis, but because Earth moves through space during that time (changing positions relative to the Sun), it takes an extra 4 minutes to rotate once in reference to the Sun. And according to my math, 23 hours and 56 minutes, plus an extra 4 minutes, makes Earth’s solar day 24 hours long.

The situation is similar for Mars.  A Martian sidereal day is about 24 hours and 37 minutes, while a Martian solar day is more like 24 hours and 40 minutes.  For both Earth and Mars, the difference is small.  Most of the time, it’s not worth mentioning.  But on Mercury, a sidereal day is 59 Earth days long, while a solar day ends up being 176 Earth days in length.  You see, Mercury rotates very, very slowly.  Over the course of one sidereal day, Mercury travels two-thirds of the way around the Sun.  That puts Mercury in a very different position, relative to the Sun, at the end of a sidereal day.  As a result, Mercury’s solar day ends up being longer—a whole lot longer—than Mercury’s sidereal day.

I have to admit I had a hard time understanding the distinction between solar and sidereal days the first time I heard about it.  I hope that I have done a decent job explaining it to you.  But if anyone was wondering why I quoted different numbers in different posts for Mercury’s day/rotation period, this is the reason I did that.  For some topics, the sidereal day matters; for others, it’s the solar day that’s important.  And for the purposes of the A to Z Challenge, I wanted to save this discussion for S-day, so I just glossed over it until now.

I’m going to recommend an article from Universe Today called “How Long is a Day on Mercury?”

I also want to recommend another article from Universe Today called “How Long is a Day on Venus?” because if you think Mercury’s day is confusing, wait until you hear how messed up a day on Venus is.

# Mercury A to Z: Resonance

Hello, friends!  Welcome back to the A to Z Challenge.  My theme for this year’s challenge is the planet Mercury, and in today’s post R is for:

## RESONANCE

There are many weird coincidences in the Solar System, most of which really aren’t coincidences.  It’s usually just the force of gravity doing its thing.  Sometimes objects in space keep tugging on each other, gravitationally, pulling each other into these oddly specific mathematical relationships which astrophysicists call “resonances.”

As an example, for every three complete orbits of Neptune, Pluto completes exactly two (a 3:2 orbital resonance).  Another example: for every thirteen complete orbits of Venus, Earth completes exactly eight (a 13:8 orbital resonance).  And then there are the resonating moons of Jupiter.  For every four complete orbits of Io, Europa completes exactly two orbits, and Ganymede completes exactly one (a 4:2:1 orbital resonance).

Earth’s Moon is locked into a different kind of resonance, called a spin-orbit resonance.  For every orbit the Moon completes around the Earth, the Moon rotates (spins) exactly one time (a 1:1 spin-orbit resonance).  This happens because the Moon is not perfectly spherical.  The Moon bulges a little at the equator, kind of like a football (or a rugby ball, for readers outside the U.S.).  As the Moon circles around the Earth, Earth’s gravity keeps pulling one of the pointy ends of this football-shaped Moon towards the Earth.  This is why you always see the same side of the Moon whenever the Moon appears in the sky.

Mercury is also locked into a spin-orbit resonance, specifically a 3:2 spin-orbit resonance.  So for every two orbits around the Sun, Mercury rotates (spins) exactly three times.  This happens for two reasons:

• Like the Moon, Mercury is slightly football shaped.
• Unlike the Moon, Mercury’s orbit is highly eccentric (non-circular).

Since Mercury is slightly football-shaped, gravity tries to keep one of Mercury’s pointy ends pointed toward the Sun.  However, Mercury’s highly eccentric orbit means that Mercury’s orientation, with respect to the Sun and with respect to the Sun’s gravity, changes.  This changing orientation creates torque, which causes Mercury to spin.

So every time Mercury reaches perihelion (the point when Mercury is closest to the Sun) one end of the football-shaped planet points directly toward the Sun.  As Mercury moves away from perihelion, gravitational torque sets the planet spinning.  Mercury spins very slowly, only managing to turn 180° before reaching perihelion again, at which time gravity once again forces the football-shaped planet to point directly toward the Sun.  This has probably been happening for billions of years now, and it will probably keep happening for billions of years to come.

I want to recommend this very short video I found on YouTube.  I’ve read a lot about this topic.  Most sources I looked at get very technical very fast.  This video was, by far, the clearest and most straightforward explanation of Mercury’s spin-orbit resonance that I could find.

I also want to recommend this article from Space.com.  Some exoplanets may be locked into Mercury-like spin-orbit resonances, and for exoplanets orbiting red dwarf stars, a spin-orbit resonance has implications for potential alien life.

# Mercury A to Z: q

Hello, friends!  Welcome back to the A to Z Challenge.  My theme for this year’s challenge is the planet Mercury, and in today’s post Q is for:

## q

Today, I’m going to expand a little on something we already talked about in a previous post.  Back in the late 1800’s, Italian astronomer Giovanni Schiaparelli made a bold effort to observe and characterize the planet Mercury.  He saw several prominent surface features (or at least he thought he saw them), and he determined that Mercury has a rotation period that is approximately 88 Earth days long (we now know this is incorrect).  So what happened?  Where did Schiaparelli go wrong?

In a previous post, I told you about Schiaparelli’s five.  When he looked in his telescope, he kept seeing a surface feature on Mercury that looked like a gigantic numeral five.  Looking at photographs of Mercury today, most people can’t find Schiaparelli’s five, and it’s really unclear what the heck Schiaparelli was looking at.

Specifically, Schiaparelli saw (or thought he saw) this gigantic five whenever Mercury happened to be east of the Sun, as seen in Earth’s sky.  And whenever Mercury appeared west of the Sun, as viewed from Earth, Schiaparelli saw (or thought he saw) a different large surface feature.  On his hand drawn maps on Mercury, Schiaparelli labeled this other large surface feature “q” (always lower case).

Unlike Schiaparelli’s five, which supposedly looked like the number five, q did not look like the letter q.  In Schiaparelli’s drawings of q, it reminds me a little of that Æ symbol (the combination of A and E) that you sometimes see in fantasy novels, very old English literature, and a few modern languages like Icelandic or Norwegian.  I’m not sure why this Æ feature got named q, but Schiaparelli labeled several other surface features on Mercury with lower case letters, so there must have been some method to his madness.

The important thing is that the five and q appeared, consistently, when Mercury reached certain points in his orbital path—either east of the Sun for the five or west of the Sun for q, as viewed from Earth.  Or at least they appeared consistently whenever Schiaparelli went looking for them.

The surface of Mercury is covered in light and dark splotches, making it a bit like a Rorschach test.  You see whatever your brain wants to see in those light and dark patterns.  I have tried my best to match Schiaparelli’s hand drawn maps to actual photos of Mercury.  I can kind of see the five, some of the time, but it takes a little squinting and a lot of imagination to make things line up right. I cannot find q, no matter how hard I try.

But Schiaparelli wasn’t too far off to believe he was seeing the same surface features time and again, whenever Mercury reached specific points in his orbital path.  Schiaparelli was only half wrong about that.  Exactly half wrong, in a sense.  I will try to explain what I mean by that in tomorrow’s post.