Science

Time by the Stars

By Duncan Lunan

We know by the Moon that we are not too soon,
We know by the sky that we are not too nigh,
 We know by the stars that we are not too far,
 And we know by the ground, that we are within sound…

Gower Wassail  (traditional).*

I quoted those lines in my book Children from the Sky, which is an investigation into a mediaeval mystery which I’ll discuss later.  When I visited East Anglia for research in 1993, I was staying in a guest house at Thurston, and went to the village of Woolpit to visit the Bygones Museum.  I then walked up to the A45  (now the A14)  to locate various landmarks, and back to eat at the Bull Inn.  There was a Guinness promotion that evening, and I was invited to a game of darts, so the time passed very pleasantly.  Woolpit reminded me of Sanquhar, the Dumfriesshire village where my parents lived, so I thought I knew the ropes.

In Sanquhar, however, local taxis drove a brisk trade when the pubs closed.  My first surprise in Woolpit came when I went to call one, and learned that there was no local taxi service, at night at least.  There wasn’t one nearer than Bury St. Edmunds, and the cost of a 7-mile call-out would be astronomical.  However, that suggested the answer:  being an amateur astronomer has let me find my way out of trouble before, including one memorable occasion in downtown Los Angeles.  The sky was hazy, but a few brighter stars were visible, enough to give me my bearings;  and I had an Ordnance Survey map, though I’d have to memorise the route because it was unlit almost the whole way.

It took two hours, twenty minutes.  The northern sky was obscured but I could see the Summer Triangle of Vega, Deneb and Altair in the west, and the traffic noise from the A45 gave me a bearing when they were clouded over.  The sound of a freight train in the distance ahead was reassuring, because the line goes through Thurston.  A briefly clearing sky in the east showed me the rising Pleiades and Aldebaran, and at the only junction where I was uncertain, a phone box told me where I was and let me re-check the map.  Coming into Thurston, I found the street which led north through it to the guest house – and at that point, theatrically, the northern sky cleared to reveal the Plough and the Pole Star, straight ahead.  The only possible comment on that was, ‘A fat lot of good you’ve been tonight!’

Navigating from Woolpit to Thurston by the stars was the kind of thing any 12th century countryman or woman might have had to do, in the period I was researching.  With no paved roads, it would be a long hike even in daylight, with good weather.  There would be no O.S. map – maps showing roads were only just coming in, and the magnetic compass wasn’t used in the UK till the 15th century.  And at night, there would be no handy illuminated phone-box, half-way, in which to check the route;  no noise from the A45 or the railway for additional referents – and much more awareness of the wind and the animals in the fields, where there might still be wolves.  And apart from that, only the stars.  If they were faithful friends to me in my minor inconvenience, they would mean a great deal more to anyone who had to do it eight centuries ago.  People did travel by night, especially when the sky was clear:  in 1179 Henry II and the king of France travelled to Canterbury during a lunar eclipse, and in 1366 some monks on the road turned in a dramatic account of the Leonid meteors.  Even in the monasteries, they often had to be up for prayer and vigil during the hours of darkness.  At St. Albans in 1254, a remarkable display of aurora or parhelia was seen by monks who were “inspecting the stars to see if it was the hour to sing matins”:  this was more than eighty years before Abbot Richard of Wallingford built his great clock at St. Albans to tell the hours of prayer, as well as showing the movements of the Sun and Moon.  

Our divorce from the night sky is extremely recent, even in the cities:  I had an 1869 star atlas in my collection which showed the full panoply of stars, month by month, from due south of St. Paul’s in London, and a 1930 book which showed the brighter stars from Westminster Bridge, month by month, above the gas lamps.  In an old encyclopaedia I’ve seen a plate of the stars from Blackpool beach, without the Illuminations, with the Tower silhouetted against the natural background glow of the sky.  Old astronomy books and Scout handbooks used to have diagrams showing how to tell the time by the position of the Plough, even in a city;  in 1926 H.V. Morton wrote of  “the Plough flinging its clear symbol over a powdered sky” above the Embankment.  I used to have a guide to the constellations south of the equator, which similarly showed how to find the time by the Southern Cross.  In Polynesia, where the navigators between the islands used ‘star-ropes’ overhead, instead of constellations, they would have known exactly what stars rose in what order and how far along the night was.  In the corresponding season for caravan travel in the Middle East, Suhail  (Sirius)  rose before dawn, and the experienced caravan-masters would make sure to take the first watch, then telling the younger men to wake them ‘at the rising of Suhail’.  Rigel, at the foot of Orion, rose earlier, and evidently the novices had trouble telling one blue-white star from another, because one of the alternative names for Rigel was ‘Suhail of the Oaths’.

As the clocks go forward and back, nowadays, the sky performs the trick we impose on it every spring and autumn – seeming to jump more than a fortnight, so that in winter, stars which rose at ‘10 p.m.’ in summer now rise at their true time of nine.  Well – 8.56, if you want to be really precise, because the stars rise four minutes earlier every night.  But then again, as astronaut Michael Collins points out in his autobiography Carrying the Fire, it really means that you should be standing further east…or does it?  As the first man to orbit the Moon alone, his scientific workload was comparatively light, and he had time to reflect on what he was seeing.  The Sun doesn’t rise or set, it shines all the time while the Earth turns under it;  and your watch isn’t fast or slow, it’s just that you’re standing too far east or west.

Apollo 11 Ascent Stage return by Michael Collins

This is one to try on your friends.  The Earth turns through roughly one degree every four minutes.  If Saiph in Orion, say, right on the eastern edge of our April map, is due to rise at 8.56 p.m. and when you see it, your watch says 9.00, where should you be standing – one degree east, or one degree west?

You can’t do it:  the correct answer is ‘yesterday’.  Saiph rose at 9 p.m. last night, and tomorrow it will rise at 8.52.  Tonight, everywhere, the star rises at 8.56.  The moral of the story is that in the modern world, we don’t usually tell time by the stars.  There’s one major exception in the late Archie E. Roy’s novel The Curtained Sleep, where the central character has been kept under drugs for months, and brainwashed into taking his own life.  What saves him is recognising the Summer Triangle of Vega, Deneb and Altair overhead at dusk, so it must be Autumn, though the last thing he remembers is in springtime.  Once he realises that the stars are ‘wrong’, he starts to question other things.

Collins was talking about how we normally tell time, by the movement of the Sun.  If your watch says 12 noon and the Sun isn’t yet on the meridian, you are indeed standing too far west  (but see below).  But if your watch is slow by Saiph, or any other star, you just have to adjust the watch – as astronomers did with the clocks, in every observatory on Earth, until clocks became more precise than the rotation of the Earth itself.  That four minutes  (actually a little less, on average) corresponds to a movement of just under a degree by the Earth in its orbit.  365¼ of them bring us back to where we were.

Airy Transit Circle, Greenwich, 1851

In older observatories there are still ‘transit instruments’, beautiful telescopes made only for timekeeping, moving only in a very accurate north-south axis, up and down the local meridian  (see ‘Compass Points in the Sky’ early in this series).  You also find two clocks, one showing Universal Time, the other Sidereal Time – time by the stars, measured from the Vernal Equinox, aka ‘the first point of Aries’, although for the whole of the Christian era it has been in Pisces.  If you know your Local Sidereal Time, and the Right Ascension of your target, and your telescope mount is equipped with accurate ‘setting circles’, you can point your telescope with great precision to any star in the catalogue before you open the dome.  If you don’t have a sidereal clock, you can still do it by the local solar time.  When the ‘new’ Airdrie Observatory was commissioned by Baillie James Lewis in 1925, it was provided with an entire book of stellar transit times through the year, calculated for the longitude of Airdrie, from which to work that out.  The longitude of Airdrie is 3.9626 degrees west, which means that the Sun and stars all transit the meridian nearly 16 minutes later than they do at Greenwich.  Events like lunar eclipses, which are visible all over the side of the Earth facing them, are not affected;  but in solar eclipses, planetary occultations and other lunar-generated events, the Moon is close enough to us for the beginning and end times of those events to be significantly altered.

Great Western Railway clocks.

Before the coming of the railways, local solar time was good enough for most practical purposes, and it could be checked against any proper sundial, set to the latitude of its site.  The only complication was that the Earth’s orbit around the Sun is not a perfect circle, so it moves fastest when nearest the Sun  (perihelion)  and more slowly at its furthest  (aphelion).  At perihelion the Sun transits early, and at perihelion it transits late.  Over the year, the midday Sun traces a figure-of-eight in the sky around the meridian which is called the Analemma, and truly accurate sundials and globes have it printed or carved on them so that the daily difference  (called the Equation of Time)  can be allowed for.  At a site with good enough year-round weather, if you point your camera due south on a tripod and mask off the foreground, taking a photograph every day at noon, and removing the mask for the last one, that will give you a photograph of multiple suns marking out the Analemma.  It’s been done for Mars as well, but that takes 687 days and makes a less interesting D-shape.

The Equation of Time has a maximum value of 16 minutes 33 seconds, equivalent to 4.125 degrees in longitude.  That makes a big difference in navigation, if you’re comparing the local transit time with Greenwich time to calculate your longitude – one degree out makes a difference of around 60 kilometres.  Dava Sobell’s best-selling book Longitude explains just what a struggle it was to invent chronometers which told Greenwich time with the necessary accuracy.  Once they were available, there was a big international fuss over just where the Prime Meridian should be.  France made a strong claim for the Paris meridian, which I mentioned last week.  But in the end, Britain had the biggest navy, and there was the added convenience that making Greenwich’s the Prime Meridian meant that the International Date Line was almost entirely over water.  Marking off from the Meridian, the Earth’s surface was divided into time zones, each covering 15 degrees of longitude, one hour apart by Greenwich Mean Time, which is the average motion of the ‘Mean Sun’, i.e. what the real Sun would do if Earth’s orbit was a perfect circle.  Nowadays, to shed the British imperial connotation, Greenwich Mean Time is often called ‘Universal Time’, which is a bit grandiose when it applies to just one 15-degree segment of the Earth. 

As mentioned above, Summer Time complicates the issue still more, especially because not all nations adopt it, and those that do, do so on different dates.  In 1968-71 BST became British Standard Time, lasting all year, to make our clocks compatible with most of Europe’s.  I didn’t like it because for me, the stars were in the wrong place all winter.  When British Summer Time was adopted in 1916, and doubled during World War 2, it must have been equally disconcerting for the agricultural workers it was supposed to benefit.  There are still calls from the south of England to reintroduce ‘Standard Time’, regardless of the inconvenience for Scotland – and in the USA there are calls to abolish ‘Daylight Saving’ altogether, on the grounds that nowadays nobody needs it.  That wouldn’t suit me either, since I’ve been used to British Summer Time all my life.

Victory Cigar Congress Passes DST, 1918

Important as Universal Time is for running railways and navigating ships, its varying difference from local solar time aren’t important for another staple of Boy Scout handbooks – how to find south with a wrist or pocket watch.  With a 12-hour dial, the hour hand is moving twice as fast as the Sun, which is approximately due south at midday.  So if you take off the watch and hold it flat with the hour hand pointing at the Sun, south will be midway between that and 12 o’clock on the dial – or one o’clock if you’re on Summer Time!  When digital watches first came in, I got bored with people showing me the functions like stopwatch and alarm which were exactly the same on each, so I took to asking, ‘How would a Boy Scout find south with it?’  I was enjoying the blank looks until a chap called Graeme Duncan, who was a keen yachtsman, replied, “Listen, sunshine, if I know what time it is, I know where the bloody Sun is!”   Which maybe served me right, but it illustrates the basic point that we mostly tell the time by the Sun.

For hunter-gatherer societies, before the advent of agriculture, it can be argued that the Moon would have been still more important for timekeeping.  One of the oldest known astronomical depictions is thought to be a carving of the Moon’s phases, on an engraved bone plaque from the Dordogne, c.27,000 BC;  and the next oldest a similar tally carved on a bone tusk at Gontzi, in the Ukraine, 15-10,000 BC.  But the most famous cave paintings of all, at Lascaux in France c.17,000 BC, are in a cave which is dome-shaped and illuminated by the Sun only at summer solstice;  of 130 known caves of the period decorated with paintings or carvings, only four are not aligned with the solstices or equinoxes, while the orientation of undecorated caves appears random.  The Lascaux sequence matches the Zodiacal constellations with the Bull, a unicorn representing Capricornus and horses fitting Sagittarius. The figure representing Taurus appears to show the position of Aldebaran, between us and the Hyades cluster, as it was in 15,300 BC, while rows of dots below the horse and stag figures may represent the lunar cycle.

From his detailed survey in Megalithic Sites in Britain  (Oxford University Press, 1967) the late Prof. Alexander Thom produced histograms of their astronomical alignments, indicating that the ancient builders had a 16-part solar calendar marking the solstices, equinoxes and cross-quarter days in between them  (pages 71 and 113 of the late Euan W. MacKie’s book, Professor Challenger and His Lost Neolithic Worldwhich Bernie Bell has been so ably promoting in recent Orkney News issues.)   A detailed carving of it has since been found on stone K15 of the passage grave of Knowth, on the Bend of the Boyne  (pp. 110-111), and there’s growing evidence  (including testimony of Julius Caesar’s, p. 63),  that the 16-part calendar was inherited by the Druids and survived into relatively modern times.

Caesar reported that the Druids held “long discussions about the heavenly bodies and their movements”, and we can be sure that those discussions included attempts to reconcile the solar and lunar calendars, as they must have done in the Neolithic.  That exercise has been a preoccupation for much of the cultural history of mankind, with no satisfactory answer because the two are not commensurate – and just as well for us.  If such a resonance did arise, the motion of the Moon could become chaotic, even expelling it from orbit or causing a collision with the Earth.  In a 1990 lecture, ‘How Safe Is the Moon?’, the late Prof. Archie Roy said that the real mystery is why the Moon is still here, or why we are still here, given that the Moon’s motion should have become chaotic at least twice, as it receded from the Earth during the history of the Solar System to date.

Gower Wassail words are in A.L. Lloyd, “Folk Song in England”, Panther Arts, 1969;  sung by Steeleye Span on “Ten Man Mop, or Mr. Reservoir Butler Rides Again”, Pegasus Records PEG 9,1971;  quoted in Duncan Lunan, Children from the Sky, a speculative investigation of a mediaeval mystery, Mutus Liber, Edinburgh, 2012, and in The Stones and the Stars, building Scotland’s newest megalith, Springer, New York, 2012.

April blank map, after a drawing by Jim Barker.