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.

10 thoughts on “Mercury A to Z: Zero Degrees Longitude”

1. Congrats on making it to the letter Z. As I said at the start, I loved your original cartoons and I also learned a few things along the way. Thank you!

My A to Z Blogs
DB McNicol – Small Delights, Simple Pleasures, and Significant Memories
My Snap Memories – My Life in Black & White

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I ship Mariner 10 and Mercury. Marcury! Meriner 10?

Congratulations on getting to Z! I will definitely never do anything like that, that seems like a lot of work.

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1. It was a lot. I’ve done this a few times now, and it always leaves me burned out at the end. But it’s still a lot of fun. It’s something I look really forward to doing it every year.

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3. My Hubble Abode says:

Whoop whoop you made it to Z! I know I haven’t been interacting much, but honestly it was really nice to see your A to Z pop up in my inbox! There’s so much to talk about one little planet!

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1. Yeah, I have a new found respect for Mercury. I mean, I respected Mercury before, but I respect him even more now. There’s a lot more going on there than I realized before this challenge.

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