MIRA 58
Autumn 2001


Sci-Fi v Reality
by Ivor Clarke, Editor 

Like most folk in our society I enjoy reading and watching Science Fiction stories in books, on the TV and in films.  And like most of you reading this, I pick out a lot of astronomical mistakes in the stories.   Most of them could be avoided if the producer had asked the average amateur astronomer with an average knowledge of the subject to check the script.  Foreinstance; how many times have you seen a žlm with instant radio and TV communication across the solar system?  Even from the Moon and back it takes 3 seconds!  From Mars can take as much as 20 minutes for a reply.  So from low Earth orbit up to our Moon are the only places in the solar system where talking one to one is possible.  That is until Sub-Space radio like in Star Trek happens.
What about faster than light travel?  Well it may be possible, but I think we‘ll be waiting a long, long time yet.  So what about very fast travel, say 99% light speed?  This doesn't brake any rules of physics, but what would it be like in a ship travelling so fast?  Well for starters you can‘t see where you‘re going, or for that matter, where you‘ve come from!  All the stars in the sky will be crammed into a starbow, a rainbow of colour all around the ship at 90° to its motion.  All the stars in front will be blue shifted and all the stars behind red shifted into a band around the ship.  As you sit there in the Captain‘s chair, light from the instruments will be bent and blue shifted also into a band around you, just as the stars outside are, so you had better know your way around that keyboard and instrument panel because you don‘t wont to press the wrong button, do you?  And if you do press a button, how long will it take for the electrons to do there work when realistic effects are taking over?  Even crawling along at only 80 or 90% light-speed things get weird.  Clocks run slow and messages from Earth or even a ship travelling along side take ages to come in and very slooowwwlllllyyyy at that.
Another gripe, why do most aliens look like us, with two eyes, a mouth, nose, ears all in the same place as ours?  Their arms and legs bend like ours in the middle and most have 4 fingers and an opposing thumb!!  And speak American!!!!!!!  The universe must be a lot smaller than I thought.  Again, most of the planets Dr. Who landed on looked very much like a quarry while most of the ones everyone else lands on have breathable air even when there‘s no sign of plant life?  What about the gravity on these planets and moons. It is always just right, not to strong or weak, mostly just like our in fact .  So why is it then in films that when astronauts are working outside on the surface of the Moon they move very slow and bounce around slowly and immediately they come inside the base they manage to move normally and everything seems Earth‘s normal pull? 





A Couple Questions & Answers to Astronomical Problems
by Steve Paine

Question   I am curious as to what causes the Earth‘s spin, and why does the Moon not spin?   Asked by: Peter Granka
Answer     Stars and planets form as a result of the gravitational collapse of accreting material. Any net translational motion of that initial material is accelerated as its radius of rotation decreases.  This is due to the same conservation of angular momentum principle that makes skaters and divers spin more rapidly when they bring their arms closer to their bodies.  In its extreme, this effect can make collapsed neutron starts spin many times each second, creating the regular beat of pulsars as observed from Earth.  Once an initial spin was established for the Earth, the same conservation principle says that its angular momentum will continue — unless it can be transferred to another object.  With no significant frictional forces to allow that to happen, the Earth can continue to spin indefinitely without any further prodding.  (More on that later.)
It is incorrect to say that the Moon does not spin. It rotates on its axis in synch with its revolution around the Earth, keeping the same face always pointed toward us.  If it did not rotate at all, we would see all of its surface over the course of one month‘s orbit.  This condition was created by tidal forces between the Earth and Moon,
gradually slowing its initial rotation until the current more stable situation evolved.  This same ”tidal lock• condition has been observed for satellite closely orbiting other planets.  Because of its elliptical orbit, a slight ”wobble• allows us to actually see slightly more than half of the Moon‘s surface over the course of a month.
Tidal forces, by the way, continue to affect the Earth/Moon system.  They exert enough frictional force on the Earth so that some of its angular momentum IS being transferred slowly to the Moon.  The net effect of that is to gradually slow down Earth‘s rotation and lengthen the day.
Where is that energy and momentum going?  The Moon is gradually receding from the Earth.  Both of these effects are small.  The Moon is receding at a rate of less than 2 inches per year and the day is getting longer by one second every 67,000 years or so.
Answered by: Paul Walorski, B.A., Part-time Physics Instructor

Question   What is meant when one says that the temperature of the cosmic background radiation is 3°K?   Asked by: Suraj
Answer     Any object with internal energy
(a temperature above absolute zero, or 0°K) radiates that energy in the form of electro-magnetic waves (light).  The theoretical distribution of wavelengths in that radiation represents ”black body" radiation, and is described mathematically by an equation called Planck‘s Law.  Plotting intensity vs. wavelength, the resulting curve peaks at a wavelength that depends on temperature ˜ the higher the temperature, the shorter its peak wavelength will be.  That consequence of Planck‘s equation is also known as Wien‘s Law.  Also, intensities increase across all wavelengths as temperature increases.
You can see this behavior as an iron rod is heated from room temperature.  At first, all of the radiation is in the infrared region, which has a wavelength too long for humans to see.  As the rod‘s temperature increases, it glows red as emitted wavelengths decrease into the visible range.  Next, you see orange, then yellow, then white as peak intensity moves to shorter and shorter wavelengths.  By analyzing the spectrum of wavelengths, temperature can be calculated.  Temperatures of astronomical objects can be determined with this technique.  Extreme  temperatures near a black hole, for example, peak in the region of very short X-Rays.
Stars like our Sun emit primarily in the visible region, and cooler objects like planets emit invisible infrared radiation.  The extremely high temperature of the Big Bang released intense, very short wavelength radiation, but the subsequent cooling of the universe has shifted those wavelengths to the microwave region.  (This can also be interpreted as the stretching of wavelengths along with the expansion of the Universe.)  Because microwaves have wavelengths longer than even invisible infrared radiation, they are observed in the radio region of the spectrum with radio telescopes.  The remnant ”background radiation• from the Big Bang is now seen coming from all directions in the Universe. If you plot this radiation‘s intensity for different  wavelengths, it matches the curve for a temperature of between 2°K and 3°K.
Answered by: Paul Walorski, B.A., Part-time Physics Instructor.






Heart of Darkness - an African Eclipse
By Mike Frost

On Thursday June 21st 2001, the first total eclipse of the new millennium swept across southern Africa, from Angola to Madagascar. After the disappointments of 1999 I was determined to see totality again, and so once again I joined an Explorers Tours expedition to view the eclipse.
The choice of viewing site was a delicate balance of judgment.  Madagascar was a non-starter because of the low altitude of the sun and the likelihood of cloud.  The climate and eclipse duration were best for Angola, but the continuing civil war there made it far too dangerous. Likewise, Mozambique was struggling to recover after years of internal strife, with the result that the infrastructure - hotels and roads - was poor to non-existent.  The southern tip of Malawi barely poked into the totality zone, so the choice came down to two countries — Zambia or Zimbabwe.
At first glance the choice seemed to be straightforward.  The eclipse track directly crossed the Zambian capital, Lusaka, and roads eastwards from the capital ran along the eclipse track.  In Zimbabwe, however, the track of totality ran across the northern part of the country, where most roads ran north to south, offering little opportunity to change location if the weather refused to co-operate.
Moreover, Zimbabwe had received unstintingly uncomplimentary press coverage throughout the previous eighteen months.  The increasing lawlessness of President Robert Mugabe‘s regime, particularly the seizure of mostly white-owned farms by so-called ”war veterans"of Zimbabwe‘s independence struggle, and the resultant collapse of the Zimbabwean economy, seemed to make Zambia the likelier choice.
But first appearances were deceptive.  Even allowing for the collapse of the Zimbabwean economy,  Zambia is a much less-developed country, with poorer and fewer facilities for visitors.  And although Zimbabwe has problems, it does not border the brutal civil war to the north in the Congo (Zimbabwe does have troops in the Congo, to protect Mugabe‘s diamond mines and keep the army from causing trouble at home). And the economic collapse meant that Zimbabwe welcomed tourists as a source of much-needed foreign currency.
In our first few days in Zimbabwe we had ample opportunity to see the promise of the country.  We flew on an Explorers charter flight from Johannesburg to Victoria Falls.  Vic Falls is a superb location, home to arguably the most impressive set of waterfalls in the world, twice as wide and twice as high as Niagara.  I enjoyed the helicopter flight and the Zambezi cruise but didn‘t have time for the white water rafting or the bungee jump.  I was particularly impressed by the wonderful rainbows to be seen in the spray from the falls.  Apparently Victoria Falls is a great place to see Moonbows: but not when it‘s so close to a New Moon!
From Victoria Falls we spent two days traveling cross-country, first to Bulawayo and then to the capital Harare.  And finally on the 21st, eleven coach loads of eclipse chasers headed north out of town to intercept the totality track, which we did just south of Mount Darwin.  The northern part of Zimbabwe is hilly and scrubby, with frequent smallholdings, tiny mud-hut villages and wooden stockades for cattle.  Shortly after the village of Rushinga, the coaches turned off the metalled road and bumped down a dusty dirt track.  After ten uncomfortable miles, we stopped just before the bridge over the Ruya River.
Our observing location was in the grounds of Maname school, a collection of buildings on the slopes heading down to the river.  We were welcomed by the head teacher, Patrick Matiza, carrying his young son; and Patrick‘s brother, who was sporting a Manchester United away shirt. Explorers Tours had made a donation to the school in return for access to the school grounds, and had encouraged us to bring spare biros and books for donation to the school; a huge pile of gifts rapidly piled up.
The viewing location was very scenic.  The Ruya River had a wide flood plain, but in mid-winter the stream was little more than a brook. The terrain was sandy and rocky, covered with scrubby brushes and few trees.  The sun was low to the north, above the hills (up to 2500m in height) that marked the border with Mozambique, 10km to the north.  The 470 eclipse chasers spread themselves along the riverbank.  I set up shop immediately next to the stream.  The sky was almost cloudless, with just a few wisps threatening to spoil the party.  Although the Sun was by no means overhead, it still felt very hot, and the sandy riverbed made for a suntrap. As we set up our equipment, the children from the school eyed us with curiosity.  Some of them were keen to observe the sun through filtered telescopes but most seemed to be more interested in the observers than the eclipse itself.  The observers tucked into a buffet lunch and waited for first contact.
At ten to two the eclipse began.  The temperature began to drop surprisingly quickly, which was a relief given the strength of the sun.  As the edge of the moon crept across the sun the breeze began to rise.  I had used my dinner plate to construct a reflection experiment, filling it with water in which I hoped to observe the sun, but the breeze rippled the surface too much for a steady image.  My pinhole array (spelling out HELLO MUM in pinhole suns) was more successful, and a few natural crescent suns could be seen in the gaps between leaves on the bushes.  There was a large flat bed of rock on my right hand side that I resolved to check for shadow bands at the end of totality, compos mentis permitting.
By ten to three, the wind had dropped, and I could clearly see the eclipsed sun reflecting dazzlingly in the dinner plate — the reflected image was too bright to look at with any magnification.  The remaining clouds had dispersed and conditions were well nigh perfect, with only a little haze.  As the light began to thin we could hear the sound of crickets, and by the river we noticed sand flies swarming in anticipation of evening.
Three o'clock — fifteen minutes to go — and I ran through my checklists, swapping around lenses (telephoto to main camera, wide-angle to my secondary, with which I hoped to shoot a panorama during totality).  Taping focuses at infinity, switching to manual exposures on main camera, making sure my binoculars and second camera were to hand.  The accelerating pace of the darkness nearly caught me by surprise but I was ready by ten past three, positioning my camera ready for the onset of totality.  And for the next five minutes I watched the sky darken and the Sun‘s crescent narrow . . .
Thirty seconds to go . . .  OK to watch the Sun directly now . . .  the crescent‘s breaking up . . .  Bailey‘s Beads —  Here comes the corona!! . . .  Yeah!!! . . .   Expletives of delight from my left hand side . . .   Off with the lens cap . . .   Check through my viewfinder . . .   Sun dead center and looking gorgeous . . .   Run through the exposures one by one . . .   pause between shots to let the camera steady . . .   check for fiery red prominences on the disk . . .   lovely one at two o'clock . . .   Quick glance at my watch . . .   One minute down!! . . .   Pick up second camera on my left hand side, stand up . . .   begin a series of wide angle panoramic shots from left to right, sun in the center shot . . .   Jupiter outstanding below and to the left of the Sun . . .   One shot looking behind me . . .   life‘s too short to look away from the Sun for long . . .   Gordon on my left has put down my binoculars . . .   Pick them up and study the disk again . . .   Fantastic prominence at three o'clock! . . .   uh-oh, two minutes down! . . .   Turn my binoculars away to the left, no sign of Mercury or Saturn, so back to the Sun . . .   group of prominences towards the trailing edge — can‘t be long to go now . . .   Time for another run through the exposures, flicking the camera back in to auto ready for the diamond ring . . .   Here comes the chromosphere!!  Pink, pinker, red . . .   And . . .   DIAMOND RING!!  One shot, two shots, pause to take it in . . .   Here comes the Sun!!  WHOOO!! . . .  Cheers and applause . . .   Look down to my right . . .    bands of dark and light running away from the Sun across the flat rock . . .  ”SHADOW BANDS ON THE ROCKS!!!" I bellow . . .   people look round . . .  YEAHHHHHHHHHHHH!!!!! . . .
I put down the binoculars and paused for breath. ”Well, only eighteen months to the next eclipse. . ."
The corona was symmetric around the Sun but there was more spikiness than I had anticipated — my impression was of a corona shaped rather like a pastry cutter.  There was plenty of structure to the corona, streamers and polar brushes, but as I write I find that I don‘t have a clear memory of the exact details — perhaps my pictures will jog the memory [they didn‘t], but definitely something to concentrate on more next time.  Likewise, I didn‘t find too much to remember about my view around the horizon, I was expecting more variety in the light looking in different directions.  I remember the prominences far better than in my previous (successful) eclipse, and the long duration of the diamond ring — five seconds plus, because of a judiciously placed lunar valley — meant that I had time to enjoy the spectacle AND run off a couple of shots.  The shadow bands were an unexpected bonus — it‘s difficult to contrast them with those I saw in Curacao, as the surface (rock instead of sand) was different.
All the fascinating phenomena which had so enthralled us up to totality — sharp shadows, crescent pinholes, crickets chirping — were still going on, of course, but the sheer overwhelming delight of totality made them seem much less important than just ten minutes before.  Nevertheless, I ran off another couple of exposures of my Hello Mum pinholes, and noticed to my surprise more bands of light flitting across the cardboard — shadow bands again?   No, sunlight reflecting off the bubbling stream — I took a minute to convince myself this wasn‘t what I had seen immediately after totality.  Then it was time for a beer!  Some hardy souls stayed to the bitter end, to complete their shots of the partial phases as the Moon gradually withdrew from the scene.  I did the rounds of my fellow observers, checking to see what they had seen.  From over by the school came the sound of singing.  The villagers were celebrating the end of the eclipse with traditional African dancing and chanting.  The children I spoke to seemed curiously unaffected by the eclipse itself — many people had walked away from the viewing site as the partial phase began, but I‘m not sure if this was to go indoors or because they wanted to leave us alone.  As I left, I noticed an agitated crowd in one corner of the eclipse site.  I strolled across to find out what was going on and discovered that the remains of the lunch buffet were being divvied out.
The coaches taking us back to Harare filled up and left one by one.  As we passed each village, the children waved goodbye with frantic excitement.  What did they make of us?  Coach after coach of rich westerners driving past — hundreds of tourists who piled out of their coaches, watched the skies darken and clear, and then headed off just as quickly as they came.  The next total solar eclipse, on December 4th 2002, starts in Africa, crossing the southern part of Zimbabwe.  But December is the rainy season for southern Africa, so eclipse chasers will go elsewhere.






Sailing to Philadelphia
by Mike Frost


One of my favourite musical artistes is Mark Knopfler.  You will probably know him as the writer and lead guitarist for Dire Straits, who had a string of hits (Sultans of Swing, Romeo and Juliet, Private Investigations, Walk of Life, and others) in the late seventies and early eighties.  In recent years he has recorded a string of idiosyncratic solo projects and written the scores for a number of movies, among them The Princess Bride, Cal and, most memorably, Local Hero (from which you‘ll have heard the main theme ”Coming Home", even if you don‘t know you have!)
I was a great fan of Dire Straits, but some critics made the claim that, as the band achieved international success, they became transatlantic, moving away from their roots and writing for an international (but mostly American) audience.
Certainly early songs such as Tunnel of Love, with its evocative description of Northumbrian seaside resorts ”from Cullercoats to Whitley Bay and Rockaway, Rockaway", made way for songs like Money for Nothing, with its plaintive chant of ”I want my MTV".
But how do you write songs that appeal to fans on both sides of the Atlantic, without losing sight of your musical roots?
On his latest album ”Sailing to Philadelphia", Mark Knopfler provides one answer.  The title track is a duet with James Taylor, another veteran of the seventies.  The song tells the story of two men, one a Geordie, one from the West Country, who gave their names to one of the historic borderlines of the United States.
The men‘s names were Charles Mason and Jeremiah Dixon.  And they were astronomers.

* * * * *

On June 8th 2004 (make a note in your diary!) you stand a great chance of viewing an event that nobody alive has ever seen — namely a transit of Venus across the face of the Sun.  Observers in England are well-placed to view this event; so, if the weather is good between about 5:20am and 11:15am, you should be able to see Venus as it  crosses between the Earth and the Sun.  You‘ll have to project the image, of course, or use a  safe solar filter, but otherwise this rare event should be straightforward to observe.
Why are transits of Venus so infrequent?  Because the orbit of the Earth around the Sun is slightly inclined to that of Venus; so a straight alignment is unlikely.  Mercury, the only other planet that can pass between Earth and the Sun, is better aligned, and so transits are rather more frequent (the last one was in December 1999). Transits of Venus occur in pairs, 8 years apart, separated by an interval of 122 years, which we just now coming to the end of.  There will be transits in 2004 and 2012, but the last ones were in 1874 and 1882.
The first person who understood planetary motion well enough to be able to forecast the occurrence of a transit was the great astronomer and mathematician Johannes Kepler.  He correctly predicted the transit of Venus in 1631 but was unable to have his prediction confirmed as Europe was on the wrong side of the globe during the transit.  Kepler failed to predict that there would be a second transit 8 years later.  The only person who realised that a transit would take  place in 1639 was an Englishman, Jeremiah Horrocks, and he and one his friends were the only people to view the momentous event.
You‘ll be hearing more about Horrocks (from me, if no-one else) in the run-up to the 2004 transit.
By the time the next pair of transits came along, in 1761 and 1769, their observation had become of huge interest to the scientific community.  Observers who view a transit of Venus from two different (widely separated) locations will see that the planet appears to cross the Sun along two different paths.  This is simply a parallax effect caused by Venus being at only one third of the distance of the Sun, but it enables astronomers to calculate a very accurate distance to the Sun, because this can then be related directly to a measurable distance on Earth.  For the first time, astronomers would be able to estimate an accurate distance to the Sun, and from that, distance scales for the whole solar system.  European astronomers, principally in England and France, prepared expeditions to the other side of the world to observe the 1761 transit.
The future Astronomer Royal, Nevil Maskelyne (now of some unjustified notoriety as the baddie of the Dava Sobel‘s bestseller Longitude) led the primary expedition to St Helena in the Southern Atlantic Ocean.  French expeditions were planned to Rodrigues in the Indian Ocean, India and Siberia (we were at war with the French at the time, but our astronomers were civilized enough to maintain cordial links with their colleagues across the Channel).  As his assistant, Maskelyne chose the assistant to James Bradley, the then Astronomer Royal, namely Charles Mason.

”He calls me Charlie Mason, a stargazer am I,
It seems that I was born, to chart the evening sky,
They‘d cut me out for baking bread, but I had other dreams instead,
This baker‘s boy from the West Country, would join the Royal Society"

Shortly before the expedition was due to set out, there was a change of plan.  Because of the risk of bad weather in St Helena, it was decided to launch a second expedition.  Maskelyne went to the South Atlantic, but Mason was dispatched instead to Sumatra in the Indonesian archipelago.  For his assistant he took Jeremiah Dixon, by profession a surveyor but an amateur astronomer as well.

I am Jeremiah Dixon, I am a Geordie Boy,
A glass of wine with you sir, and the ladies I‘ll enjoy,
All Durham and Northumberland is measured up by my own hand
It was my fate from birth, to make my mark upon the Earth

Mason and Dixon‘s first expedition did not go smoothly.  Hours after their ship, H.M.S. Seahorse, left Portsmouth, it was attacked by a French warship, ”Le Grand".  Eleven men died, thirty-seven were injured, and the Seahorse was forced back to port.  Mason was all for giving up the expedition there and then, and wrote a letter to the Royal Society - ”We will not proceed thither, let the Consequence be what it will".  The Royal Society replied that if Mason and Dixon continued to renege on their agreed expedition, they would be taken to court ”with the most inflexible resentment" and prosecuted ”with the utmost Severity of the Law".  On February 3rd 1761 H.M.S. Seahorse set sail again.
By early May they had reached Cape Town, where they received news that their intended observing site in Sumatra had been captured by the French.  This may have been fortuitous for Mason and Dixon, as they were running out of time to cross the Indian Ocean.  They decided to stay at the Cape, and on June 5th Charles Mason and Jeremiah Dixon were rewarded with a clear view of the transit.  Their observations were priceless, as the other expeditions to the Southern Hemisphere had much less fortune.  Maskelyne was clouded out in St Helena and the French expeditions suffered various setbacks, meteorological and political.  The authorities took good note of Mason and Dixon‘s ability to make excellent observations in challenging circumstances.

* * * * *

A trying piece of litigation had been occupying the British courts for decades — a dispute between two of the American colonies, Pennsylvania and Maryland, over their mutual border.  In 1632, Maryland had been granted  ”That part of the bay of Delaware which lieth beneath the fourtieth degree of north latitude".  Unfortunately, this description included the Pennsylvanian city of Philadelphia, which the Pennsylvanian colonists were not going to give up.  The resolution of the dispute was to shift the borderline to fifteen miles south of Philadelphia, and measure due west from there.
This was easier in theory than in practice.  The Pennsylvania-Maryland border rapidly ran into mountainous forest, uncultivated by the colonists and home to the hostile Iroquois Indians.  The first attempts to survey the boundary were made by Thomas Penn, in 1761, but he admitted defeat after one of the surveying telescopes warped in the rain.
The two colonies asked the government back in England to provide them with some competent surveyors, and agreed to abide by the results of the survey.  Charles Mason and Jeremiah Dixon were dispatched to do the job.
In 1763 they began work at Mr Alexander Bryan‘s plantation house, 15 miles due south of Philadelphia. Mason and Dixon worked their way steadily westwards, clearing the line of the boundary with axes, checking their latitude by the altitude of stars above the horizon, and placing limestone border posts (imported from England) every eleven and a half miles.  It was an arduous and dangerous task.  After three years, surveying was halted during an Iroquois uprising, and restarted six months later after negotiations.
Surveying finally ceased in 1767, two hundred and thirty three miles east of Mr Bryan‘s house,and thirty miles east of the present south-west corner of Pennsylvania.  Many of Mason and Dixon‘s original boundary posts survive to this day, although some have been removed and incorporated into buildings along the route.  The Mason-Dixon line has been re-surveyed twice subsequently, in 1849 and 1902, and found to be remarkably accurate.
Mason and Dixon stayed in the American colonies for a further year, making observations of a lunar eclipse and occultations of the moons of Jupiter, as well as a precise measurement of a meridian in Maryland.  They arrived back in England in November 1768, just in time to be dispatched on expeditions to observe the next transit of Venus, in June 1769.  Jeremiah Dixon went to Hammerfest Island, north of Norway, and Charles Mason to Cavan in central Ireland (this transit of Venus was also observed by Captain Cook from Tahiti in the Pacific Ocean).
On his return from Norway, Dixon reverted to his profession of surveyor, working in the north-east of England until his death in 1779.  Charles Mason remained a surveyor of the skies, charting the positions of stars and providing the timings of their occultations by the Moon.  This provided the astronomical solution to the ”Longitude problem"  that so occupied the maritime nations.  His work was highly regarded but Mason felt that the Board of Longitude had not paid him enough for it. Consequently he sailed again for Philadelphia in 1787, and spent the remaining ten years of his life there.

* * * * *

Mason and Dixon‘s surveying might have been just a curiosity of local history, but events  were to give it a much greater significance.  Mr Bryan‘s plantation doubtless was worked by slaves, as Maryland was the northernmost of the agriculturally based states in which slave ownership was permitted.  Pennsylvania, however, was  becoming an industrially based commonwealth, and through the first half of the nineteenth century, slavery was gradually abolished.
Slaves began to escape northwards to the abolitionist states.  To the west of Pennsylvania, the borderline was formed by the Ohio River, but increasingly the Mason-Dixon line became a  synonym for the boundary between freedom and slavery.  When civil war broke out in 1861, the 233 miles surveyed by Mason and Dixon became the front line.  It‘s no co-incidence that the turning point of that war, the battle of Gettysburg, took place only ten miles north of the Mason-Dixon line.
Mason and Dixon could not have known how significant their boundary would become.  To them it was simply a line to survey, but to history it would mark the boundary between the great fault-lines of nineteenth-century America  — north and south, industrial and agricultural, freedom and slavery.  Mark Knopfler‘s song conveys the  sense of adventure as two astronomers from the old world prepare for their momentous adventure in the new.

Now hold your head up Mason, see America lies there,
The morning tide has raised the capes of Delaware,
Come up and feel the Sun, a new morning has begun,
Another day will make it clear, why your stars should guide us here

It‘s great song and a good album.
 

Sources: -

”Sailing to Philadelphia" by Mark Knopfler (Mercury, 542 981-2, 2000)

”Transits, Travels and Tribulations" by J.D.Fernie. American Scientist (Sep-Oct 1997)

The Dictionary of National Biography, entries for Charles Mason and Jeremiah Dixon (OUP, 1993)

The cartoon, by Harry Dierken, features in an article on the Mason-Dixon line at
http://www.geocities.com/SoHo/1422/mas_dix.html

P.S.  Yes, I have crossed the Mason-Dixon line during my travels around Pennsylvania, although I haven‘t seen any of the boundary markers.  Just south of the Mason-Dixon line lies a small town where I had my photo taken by the town boundary.  The sign says ”Welcome to Friendly Frostburg".