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Astronomy Beginner’s Guide: Part 2 Compass Points in the Sky

By Duncan Lunan

When people see the maps for my astronomy column ‘The Sky Above You’, I’m often asked why east is on the left and west is on the right.   The answer is that terrestrial maps look down, sky maps look up.   The viewpoint in these maps is looking south:  then the stars and planets rising in the east will be on your left and those setting in the west on your right.  The rim of the map corresponds approximately to the horizon at 9 p.m. in the middle of the month, and a point just north of the map centre will be overhead;  this is because the maps are also intended to be usable by readers in Europe and the USA.

The monthly maps for the column were drawn for me by Jim Barker, based on similar, uncredited ones in Dr. Leon Hausman’s “Astronomy Handbook”  (Fawcett Publications, 1956).  Jim had to redraw or elongate several of them because they were drawn for mid-US latitudes, about 40o North.   The biggest change needed was in November when only Dubhe, Merak and Megrez of the Big Dipper, as the US version called it, were visible at that latitude.  In the UK, all the stars of the Plough are circumpolar, always above the horizon.  We decided to keep an insert in the January map showing the position of M42, the Great Nebula in the Sword of Orion, and for that reason, we stuck with the set time of 9 p.m., (10 p.m. BST in summer), although in central Scotland, where I live, the sky isn’t dark then during June and July, and for even longer further north.  The star patterns are the same for each month of each year, and only the positions of the planets change.  (“Astronomy Handbook” accidentally shows Saturn in Virgo during May, showing that those maps weren’t originally drawn for the Hausman book.)

To use the maps, in theory you should hold them overhead, aligning the North edge to true north, marked by Polaris and indicated by Dubhe and Merak, the Pointers.   It’s more practical to hold the map in front of you when looking south and then rotate it as you face east, south and west.    

When I was Manager of the Glasgow Parks Department Astronomy Project in 1978-79, my main brief was to build an astronomically aligned structure in one of the city’s parks, probably a copy of Stonehenge or Callanish in modern materials.   First I had to convince the Parks Dept. and the Manpower Services Commission that such a copy wouldn’t work.  The tilt of the Earth’s axis has altered by half a degree since then, precession of the Equinoxes has changed the rising and setting points of the stars, the latitude is wrong, and anyway the prehistoric sites mark where events occur on the contours of the horizon at those sites.  I would have to find a suitable site and then design a structure for it according to the ancient principles.  (My next battle would be to let me build it in stone and make it a tribute to Prof. Alexander Thom and Dr. Archie Thom of Dunlop, Dr. Euan MacKie of the Hunterian Museum, and Prof. Archie Roy of the Astronomy Department – all staff members of Glasgow University, then or previously.)   The Principal Landscape Architect, Mr. Ronnie Gray, marked 18 possible sites for an investigation.  Sighthill Stone Circle – An Update By Kenny Brophy – Urban Pre-historian.

The Miner’s Dial

To make the initial checks, I was entrusted with a Miner’s Dial – a giant compass on a heavy duty tripod, beautifully made in brass and travelling in a padded wooden box.  This device was so useful that it was to reside with the project, off and on, for more than a year.  Its one peculiarity, for which we learned to allow unthinkingly, was that it showed west to the right of the north-south line, and east to the left, with 360° bearings likewise running round the dial the “wrong” way.  The explanation I was given  (which apparently is untrue)  was that because mine galleries are often awash – especially in rescue situations where maps are most needed – mine maps are drawn to be read by the helmet lamp of a man lying on his back, holding the map up to the roof.  The Miner’s Dial was to be used above ground in conjunction with the below-ground maps.   The actual explanation is more complex and has to do with the angle of dip of the magnetic compass, which need not detain us here.

If we had used the Dial upside down, it would have matched the orientation of the sky.  In 1973 that led to considerable confusion at a lecture I gave to the British Interplanetary Society, mentioning the south-west proper motion of Arcturus.  This was challenged by Alan Bond, later the designer of the HOTOL space-plane, who said it was south-east.  My source had been Camille Flammarion’s Les Étoiles and Alan later established that the direction of proper motion was shown correctly to the right and down, as seen by a northern hemisphere observer.   We supposed that there was a printing error in Les Étoiles and a correction duly appeared in the July 1973 issue of Spaceflight.   Unfortunately Flammarion was right the first time:  the direction is south-west because it is towards the western horizon.

At the time the Miner’s Dial was made it would also have been correct for the situation on the Moon.   Historically Moon maps were drawn with south at the top, as it appears in a refracting telescope from Earth’s northern hemisphere;  but to tally with star maps, they showed west on the left and east on the right.  Thus on the Moon the Sun rose in the west, though the Moon rotates in the same direction as the Earth does.  However when the U.S. Air Force mapped the Moon by radar for the Apollo Moon landings, they didn’t want any risk of confusion which might prove fatal in an emergency, so true east and west were restored.  Because those were the best Moon maps available, their directions quickly became standard and the old convention was dropped.  

The Miner’s Dial wouldn’t work on the Moon anyway, since the Moon has no intrinsic magnetic field.  But if Moon maps, Miner’s Dials etc. survive into the future, origins forgotten, a latter-day Velikovsky or von Däniken may argue that the Earth and Moon “must” have turned the other way.   If some latter-day Thom from an engineering background tries to deduce their true function, what will archaeologist critics say about him?

Another oddity of the time was that the International Astronomical Union had never got round to ratifying the names of the lesser moons of Jupiter.  As the Pioneer 10 probe approached the planet in 1974, NASA began using the moons’ unofficial names in progress reports.  Perhaps to avoid another fait accompli, the International Astronomical Union brought out a new set of official names, whereby satellites in direct orbit were given names ending in ‘a’, and those in retrograde orbit (probably captured asteroids)  got names ending in the letter ‘e’.  What made it look like a rush job was that Jupiter’s largest satellite, named by its discoverer Galileo, has a direct orbit and its name, Ganymede, has always ended in an ‘e’.’

New York’s Grand Central Station

In a 1986 report on ‘Astronomy in New York’ I referred to the difficulty of seeing the stars – except, of course, when artificially projected on to the dome of a planetarium!   But there is one other place where one can see the stars in New York, and it’s on the roof of the main hall of Grand Central Station.  I had forgotten about that, and nobody prepared me for it, so it came as quite a shock.

credit: Arnoldius, CC BY-SA 3.0 , via Wikimedia Commons

The huge painting on the concave roof is crossed diagonally by the Ecliptic, which is the Sun’s apparent path (i.e. the plane of the Earth’s orbit).   The arc portrayed shows the Zodiac from Aquarius to Cancer, plus Pegasus and Orion.  The constellation figures are portrayed in classical style, and light bulbs represent the brighter stars.  I learned that the ceiling is false so that the bulbs can be replaced from above, and maintaining the arrangement was a precondition before Pan Am were given permission to build an office block over the station.   Nevertheless, quite a few were out when I was there.

The oldest known star maps were carved on solid spheres – after all, the sky itself was thought to be one – and as the user was then looking at the sky from outside, of course the constellations were left-to-right inverted.  Classical star maps followed the same convention, and indeed some were printed as ‘gores’ which could be cult out and pasted to a globe of your own making.  But the roof of Grand Central Station is concave, so we’re looking at the sky from inside, and the figures shouldn’t be reversed.  My informant suggested it was to avoid trouble from fundamentalists about depicting pagan figures, but the answer to that would have been to depict the sky as it really is, not artificially reversed.   I suspect that the artist simply copied an old map, without an astronomer’s guidance.

In case you missed it: Amateur Involvement in Astronomy: Part 1 of Our Beginner’s Guide

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