Culture

Beginners Astronomy: Atmospheric Refraction

by Duncan Lunan.

If the Earth had no atmosphere, life would be much easier for astronomers – as long as they didn’t need to breathe.  (As observatories are built at greater and greater heights on mountaintops, astronomers who suffer from altitude sickness are becoming more separated from those who don’t…)   Blue light is scattered and absorbed, making the Sun look red near the horizon; but as it gets lower it seems to slow down in its descent, its track gets nearer to horizontal, and its disc seems to become flattened at the bottom.  This is one of the major effects of refraction.

Refraction is the process which makes a stick look bent when it’s thrust into water.  The cause is that while light travels in straight lines, both in air and in water, it travels through the denser medium slightly more slowly.  (The speed of light in vacuum is the top speed limit of the Universe, but the characteristic blue glow round water-cooled nuclear reactors is caused by particles travelling faster than the speed of light in water.)   As light crosses the interface between air and water, the change of speed causes it also to change direction.  The same thing happens between air and glass: magnifying glasses, spectacles and refracting telescopes wouldn’t work otherwise.

Our atmosphere is always in motion, and it’s refraction which makes the stars seem to twinkle – in space, there’s no such effect.  When a pressure wave in the air passes between us and the star, the light path is moved around twice, according to the branch of modern mathematics called ‘catastrophe theory’.  The old verse ‘Twinkle, twinkle, little star’ is literally true.   Sirius, the brightest star in the sky, has its light split into component colours and flashes red, green and blue;  and because the planets show discs, however small, we see them as relatively steady.

With stars and planets, even if we know in theory where they should be in the sky, and the telescope can be preset with setting circles  (or these days, ‘Go To’ electronics)  to point there, we still need a ‘finder telescope’ to zero in on the target.  The higher the magnification by the telescope, the greater the need.

A finder telescope has a relatively low magnification, so that it can take in a larger area of sky around the target, making it easier to find and centre.  This can be done using setting circles, especially if the target is in an unfamiliar part of the sky or really faint, but for most purposes it will be enough to aim the telescope roughly by eye.  The finder telescope is then used to pinpoint the target – either directly, or in relation to the surrounding stars if it’s still too faint to be made out.

The finder may be fitted with cross hairs or a pointer; some models can be illuminated so that the guides can be seen against a really dark sky  (which makes all but the brightest targets hard to see.)  But what is essential is that the finder is aligned parallel to the central axis of the telescope, so that when the target is on the cross-hairs it is in the much smaller field of the main instrument.  In setting a finder, the trick is to start with some large, fixed target like a distant building, then the Moon, relatively easy to find without one.   Once that is centred in the telescope, the finder can be adjusted to match.

There are two standard mountings for telescopes.  The altazimuth mount moves straight up and down (in altitude) and right to left  (in azimuth).  It’s simple, but it isn’t the way the heavenly bodies cross the sky unless you happen to be at the north or south pole.  For the rest of us, the most effective telescope mount is the equatorial.  This is aligned parallel to the Earth’s axis at the observing site, so that the target can be followed in right ascension (east to west across the sky) while the telescope is fixed in declination, which corresponds to latitude on Earth, projected on to the sky.  With setting circles, you can point the telescope to the right ascension and declination of your target, as long as you know the precise local sidereal time – which is not the ordinary time of day, because we tell time by the Sun, not the stars – and then you’d still have to correct for refraction, which takes effect in altitude, so you still have to use the finder.  With practise, making the corrections becomes easy, even though their exact size varies with the conditions from night to night.  Some call this ‘getting the feel of the telescope’; others call it ‘learning the instrument’.   It takes a while, but it repays the effort.

But the nearer any object is to the horizon, the more refraction seems to displace it upwards – hence the flattening effect on the relatively large disc of the Sun.  Variations in atmospheric temperature, and dust and water content, can change the value of refraction dramatically near the true horizon  (zero degrees altitude).  We’re not so aware of refraction with the Moon, but it shows dramatically in a Moonrise photo taken from Earth orbit.  One consequence of it is that the Sun and the Full Moon can sometimes be seen on opposite horizons at the same time, even during lunar eclipses when they have to be diametrically opposite and the Moon shouldn’t be fully visible until the Sun has fully set. 

In designing the stone circle at Sighthill in Glasgow, the first astronomically aligned one for over 3000 years, one of the problems was there was too little time  (initially 9 months, later extended to the traditional year and a day)  in which to determine the alignments.  By the time we had found our site, gained permission to use it, located suitable stones and brought them into Glasgow, the weather had turned against us, and it was impossible to make observations at the autumn equinox or winter solstice.  Glasgow was repeatedly hit by blizzards, and December 1978 saw the worst weather in the English Channel for 25 years.  High winds on the hilltop prevented us from even making a survey until well into 1979, but at last we had a clear, still day, when the survey team was available, on February 23rd.  Once I had all the theoretical rising and setting points pinpointed, on the true horizon, I could plot their paths and determine where the Sun, Moon or star would appear on the actual horizon of the site, then factor in atmospheric refraction, using the tables in the books of Professor Alexander Thom.

Using spherical trigonometry and log tables, the standard methods of the time, at least three calculations would be required for each alignment, running to two or three pages of calculations.  Since the astronomy project had other major commitments at that time, I reckoned a month’s work at least would be needed, and I would and should have done it.  Instead, we were told that a Royal Navy helicopter would be available to fly the Sun stones, star stones and central stone into position, but it had to be done in 14 days’ time.  My boss reacted by requiring all the completed calculations to be on his desk the following morning.

With the help of megalith expert Tony Crerar from Wales, who happened to be visiting at the time, I got it done in an all-night session.  It meant that everything had to be done graphically, making assumptions about the rising and setting tracks which I knew could not be correct, but I had to hope the approximations would suffice.  Observations over the next 40 years have shown that the events all occured within the diameters of the stones  (all but one – see below), but the angles of the rising and setting tracks varied considerably around the circle, changing the horizon points significantly.  To his dying day, the late John Braithwaite  (my second-in-command)  insisted that atmsopheric refraction could account for all the differences, but that can’t be true.  Atmospheric refraction always acts upward, and no matter how cold the air is, it can’t make a heavenly body appear to be lower than its true position.

In November 2012 it was announced that the stone circle and the enture Sighthill Park would be removed to make way for new housing.  An online campaign started by Mandy Collins of Spooky Isles drew 6500 signatures, and the City Council announced that instead a new site would be found.  I was commissioned to find a new site, choosing the eastern end of the former Park, overlooking Pinkstone Road, and also had to determine the new alignments.

I didn’t have to resort to spherical trig, because I could obtain all the readings from the new ‘Interactive Sky Chart’ on the invaluable website http://www.heavens-above.com.  Initially I did it for all the alignments at the old site, reconciling them with the actual observations there, and then transferred the tracks to the ‘working photographs’ taken at the new one.  It still took several weeks, and when he looked over the fair copies of my results, Dr. Alan Cayless of the Open University said, “I can tell how much work has gone into this by the number of times that your printer has run out of ink”.  Those results have never been published till now, and they’re both interesting and instructive.

On the eastern side of the old circle, moonrise at furthest north  (every 18.61 years)  was exactly where it should be on the corrected track.  That put it behind the Sighthill high flats, which have since been demolished, so the actual moonrise wasn’t observed, but there appears to have been no refraction at all.  The same is true for midsummer sunrise, the rising of Rigel in Orion, and moonrise at furthest south.  Midwinter sunrise, photographed in 1979, was affected by refraction in the haze over steelworks in the Clyde Valley, but when it was observed on a clear horizon after the foundries closed in the 1980s, the effect had disappeared.

On the western side, the track of moonset at furthest north, photographed in late evening, was flattened by refraction, but seemingly not until it was right on the horizon.  Moonset at furthest south in early evening was flattened for the last few degrees, but was still over the marker stone.  9.3 years later in 2016, photographed through thick mist, the descending track of minor standstill southerly moonset was a degree above prediction  (half a fingertip, at arm’s length)  all the way from where the images start, enough for it to set on the right of the stone.  Either that segment of its track was all affected equally by refraction, or the stone itself was too far left – possible but unlikely, when we took so much care over the alignments, and unable to be checked because the stones were removed soon afterwards.

But midwinter sunset, equinox sunset and midsummer sunset were all well to the right of the calculated tracks, displaced upward by 1.5 to 2 degrrees, near the theoretical limit of atmospheric refraction.  The effect has since been confirmed at the new site, at midsummer sunset in 2018 and 2019, and it kicks in at 2 degrees above the true horizon.  No observations could be made in 2020 because of the lockdown, and although midsummer sunset was the most popular event for visitors to the site, it is now blocked by the new housing in the northeast and will remain so until that comes down.  When planning a stone circle one has to think in thousands of years, ignoring the temporary flickering in and out of modern buildings.

At first sight, it seems that refraction at the Sighthill circle has been and still is affected by something like invisible smog, which forms over the city on clear sunny days.  As the effect is the same in late December, late March and late December, it’s not affected by air temperature, and may be due to the action of sunlight on car exhaust fumes.  The circle was built as a tribute to the late Alexander and Archie Thom, Euan MacKie and Archie Roy;  it is a functional and educational observatory, as originally specified, and it has demonstrated that the ancient builders could have achieved the accuracies claimed by those four dedicatees.  But it now seems that it may also have an unintended rôle as a monitor of urban air quality.

1 reply »

  1. I’d like to say ‘Thank you ‘ to Duncan for his work in making sure that this circle and it’s associated memories, survives. If he hadn’t taken action, it would have been swept away and become just a memory – taking all the memories with it.
    Thank you Duncan, and all involved.

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