Tuesday, May 31, 2011

Sunrise on Mercury

The Strange and Surprising World(s) Around Us

We live in a strange and beautiful world, a world hard to explain at times. (Just look at Joshua trees . . . and then think about the now-extinct ground sloth that used to browse their tops.)

While reviewing a new grammar book, I came across this astronomy-based writing sample, below. It’s on pages 224 and 225 of the Little Seagull Handbook from WW Norton. (That company uses a seagull at a corporate logo, so their comp books are branded with gull themes. Houghton-Mifflin used to have a “Dolphin Reader,” after their own logo, even though there were not any cetacean-based stories inside the collection).

Here’s the amazing scene, from a sample student paper quoting from something by Edward Bell:

“Sunrise and sunset on Mercury are spectacles to behold. Two and one half times larger in the sky than seen on Earth, the sun appears to rise and set twice during a Mercurian day. It rises, then arcs across the sky, stops, moves back toward the rising horizon, stops again, and finally restarts its journey toward the setting horizon. These aerial maneuvers occur because Mercury rotates three times for every two orbits around the sun and because Mercury’s orbit is very elliptical.”

This seemed almost too good to be true, and sent me to the websites for JPL and Wikipedia, among other sources.

For a more technical explanation, I combined several sources to get this, just for my own private notes. (I keep a journal that’s mostly a collection of amazing runs of language. The text below qualified for star status.) Here’s the science, more exactly:

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

The image below is by William Hartmann, from The Grand Tour: A Traveler’s Guide to the Solar System.



It turns out that the grammar book’s bouncing sunrise is not the half of it. The passage below is mostly adapted from Wikipedia (edited a bit, for clarity). “Messenger” refers to a space mission that is above Mercury right now. Check this out:

“During its second flyby of the planet on October 6, 2008, MESSENGER discovered that Mercury’s magnetic field can be extremely ‘leaky.’ The spacecraft encountered magnetic tornadoes—twisted bundles of magnetic fields connecting the planetary magnetic field to interplanetary space—that were up to 500 miles wide.

These twisted magnetic flux tubes, technically known as ‘flux transfer events,’ form open windows in the planet's magnetic shield through which the solar wind may enter and directly impact Mercury's surface.

The process of linking interplanetary and planetary magnetic fields, called magnetic reconnection, is common throughout the cosmos. It occurs in Earth's magnetic field, where it generates magnetic tornadoes as well. However, the MESSENGER observations show the reconnection rate is ten times higher at Mercury.”

Well, hot dog. Since Mercury is about the size of our Moon but denser, I was wondering what I would weigh. (Don’t we all secretly like those planets on which we not only weigh less but could thus bound around like a kangaroo on too much Red Bull?) The Griffith Observatory and other sources imply that on Mercury we would all be about a third our present weights, plus of course the added bulk of the space suit. (Excluding bone loss from the long journey, you would weight the same on Mars as on Mercury. Go ahead, have an extra slice of cheesecake.)




In Roman mythology, Mercury was a god of travel and thievery. Our planet Mercury has no moons but could (despite being 800 degree Fahrenheit in parts) have a hidden ice cap at its North Pole. A strange and wonderful sky, up there, isn’t it? The books I grew up were almost too timid in their suppositions and imaginary societies. Still, they started something that NASA and JPL are only now starting to develop.



Of course in my lifetime we have lost one planet (Pluto) but gained hundreds more (the so-called exo-planets, orbiting stars other than ours; there are over 500 candidates now). We also have gained an elephant, if you’re among those progressive taxonomists who split the African elephant into two species, the savannah form and a rare, shy, dwarf West African form, the Forest Elephant. I have looked for the forest species but so far all I have soon is footprints and hefty mounds of green poop. Story of my life. (All parties agree that the African species, whether one or two, differs from the Indian species, and that in turn these are different again from the mammoths and mastodons.) How many rhinos are there, really? Who knows. Right now there’s just one species of giraffe but DNA evidence and minor morphological differences tell us there maybe should be a revision, and that really there are ten species of giraffes. (See my journal note on this, below, which came originally from National Geographic.) If that giraffe change ever goes through, I would expect a few years later another revision, and have the number collapse back down again by half.





The more things change, the more they stay the same, and some of the exo-planets will turn out to be errant moons or clusters of long-lost car keys, but many may be legitimate. For now, though, we have to make do with the eight local planets we have left, including Mercury, being explored now by the Messenger probe, and Mars, the planet the color of the sky in Mojave on a windy day. Mars has two moons to our one, and Mercury, as mentioned above, has multiple sunrises. Poor old Earth, just stuck with one of everything—though as sunrises go, ours ain’t too shabby. Just before and after the sun’s up or down is a good time to look for Mercury (you can even see it with binoculars). It’s also a good time to take pictures of giraffes, to write in your journal, or to go to sunrise service at church.

Strange and beautiful planet, in an ever-more-surprising Solar System. It’s kind of cool that a Somebody up there (and/or random evolution, take your pick) was kind enough to let us hang out and enjoy it.


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