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.  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, featuring an image of Hun Kal Crater as seen by MESSENGER.

Mercury A to Z: Hot Poles

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


I remember a certain cartoon that I saw as a kid.  The main character wanted to go exploring the world, to discover lands that were totally new.  This character knew that somebody had already reached the North Pole and that somebody else had already been to the South Pole.  But what about the East Pole?  What about the West Pole?  Surely the East and West Poles had yet to be discovered!

Of course, Earth doesn’t have an East or West Pole.  But Mercury does… sort of.  There are two points on Mercury’s equator, on exactly opposite sides of the planet, that reach maximum temperatures higher than anywhere else on the planet.  These two points are called Mercury’s hot poles.

Now you may be wondering why would only two specific points on Mercury’s equator get extra hot?  Shouldn’t all points along Mercury’s equator get equally hot?  To answer those questions, I first need to explain two key things: Mercury’s orbit is really eccentric, and Mercury’s day is really long.

Mercury’s Eccentric Orbit

Planetary orbits are never perfectly circular.  They are always at least a little bit oval-shaped.  Eccentricity (in the context of astrophysics) is a measure of just how non-circular a planet’s orbit is, and Mercury has the most eccentric orbit of any planet in the Solar System.

As you know, Mercury is the planet closest to the Sun, but thanks to that highly eccentric orbit, sometimes Mercury gets a little extra close to the Sun.

Mercury’s closest approach to the Sun is called perihelion.  As you can see in the highly technical diagram above, whenever Mercury is at perihelion, that’s when things get extra hot.

Mercury’s Super Long Day

A year on Mercury is about 88 Earth days long.  A day on Mercury (by which I mean a solar day, not a sidereal day) is about 176 Earth days long.  That makes a day on Mercury twice as long as a Mercurian year.  In fact, a day on Mercury is exactly twice as long as a Mercurian year.

It’s really important for you to understand that, so I’m going to repeat it: a day on Mercury is exactly and precisely twice as long as a year on Mercury.  So if it’s noon (local time) on Mercury, you’ll have to wait exactly one Mercurian year (one full orbit around the Sun) before it’ll be midnight.  And once it’s midnight, you’ll have to wait another full Mercurian year (another full orbit around the Sun) before it’ll be noon again.

The Hot Poles of Mercury

Now, with those two facts about Mercury in mind, let’s imagine that Mercury is at perihelion.  Mercury is extra close to the Sun, and the dayside of Mercury is getting extra hot.  Now let’s fast forward.  Mercury has orbited all the way around the Sun and returned to perihelion.  It is one full Mercurian year later, but it has only been half of a Mercurian day.  Exactly half.  What was the daylight side of Mercury is now in darkness, and what was the nighttime side of Mercury is now in full daylight.  Where it was noon, one Mercury year ago, it is now midnight, and where it was midnight, it is now noon.

Fast forward another Mercurian year.  Mercury is at perihelion again, and the two sides of the planet have once again swapped places.  It is always like this.  Every time Mercury reaches perihelion, either one side of the planet is facing toward the Sun, or it’s the exact opposite side facing the Sun.  It’s always one way, or the other.  Never anything in between.

And so the two points along Mercury’s equator which always end up being the bullseye center of the planet (from the Sun’s point of view) during perihelion keep reaching maximum temperatures higher than anywhere else on Mercury.  Scientists call these two points the hot poles of Mercury, and they have been officially designated as zero degrees and 180-degrees longitude, for the purposes of mapping Mercury’s surface.

So in a way, these hot poles are kind of like the east and west poles of Mercury.


I’m a little disappointed that there isn’t more info on the Internet about Mercury’s hot poles.  I did find this article from, from when the MESSENGER Mission photographed one of the hot poles.

I also found this heat map of Mercury, which basically just shows how maximum temperatures are not even distributed around the planet’s equator.