By Duncan Steel 10/10/2019 2


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New Zealand has prided itself for decades with regard to its lack of pollution, and all will be aware that the ‘100% Pure New Zealand‘ meme is under threat through land, water and air pollution of various causes. There is another type of contamination that the country also faces: light pollution. Astronomers are concerned that the dark skies of NZ, speckled with myriad stars and a fine view of the Milky Way, might be a thing that future generations will no longer be able to experience and enjoy. 

Let me begin by admitting that the image in the header above is not of New Zealand; it did, however, prompt this blog post. The photograph is of the Iberian Peninsula (Spain and Portugal) at night, taken by an astronaut on board the International Space Station. The European Space Agency made it available just yesterday, and spotting it prompted me to prepare this missive.

It is not only Spain and Portugal that can be seen there. Morocco and other parts of North Africa are evidenced at lower right; in the Mediterranean at right are the Balearic Islands; towards the top of the frame, southwest France glows around the Bay of Biscay and along the coast to the Riviera; and at the very top, above the limb of the Earth, the upper atmosphere shines in what is known as the airglow or nightglow: atoms and molecules de-exciting and emitting light after solar ultraviolet had energised and split them during the daytime.

On the high seas in this photograph, a few ships can be detected. Many of those are to the northwest of the Pillars of Hercules (the Strait of Gibraltar), in fact where the Battle of Trafalgar was fought in 1805. Sitting as I am in Nelson, and regularly walking along Trafalgar Street, I thought I’d point that out. Since arriving in this city I have often found it strange that there is a Trafalgar Street, and a Nile Street, but apparently no Copenhagen Street. What’s the link? Those are the three great battles fought by Admiral Lord Nelson; when I was working at the University of Lund (in southern Sweden) I would often take the hydrofoil from Malmö to Copenhagen, and as we slowed to enter the Danish capital’s harbour one could see cavities in the brick fortifications of offshore islands, caused by cannonballs from Nelson’s artillery in 1801. My puzzlement over Nelson (the city here in New Zealand) street names was remedied just last week, when I discovered that the hillock right at the very southern end of Trafalgar Street, now occupied by Fairfield Park, was once called Copenhagen Hill. Problem solved.

By the way, whilst I am throwing out useless facts, the Duke of Wellington’s horse at the Battle of Waterloo was called Copenhagen.


The nightglow (introduced above before I veered off-course) is a natural limitation for optical astronomy, effectively a noise background that means that fainter stars may escape detection. The Moon also has a profound effect on astronomical observations, being especially a problem around full moon. At one time I was running two distinct research programmes, one at Siding Spring Observatory in New South Wales (searching for near-Earth asteroids), and another at the University of Adelaide in South Australia (radar observations of meteors). Each month I would spend three weeks at Siding Spring, and then drive the 1,350 km to Adelaide to work on the radar project for eight days over bright-of-moon, when the sky was so bright that we could not do much in the way of asteroid tracking. The cycles of the heavens can affect scientific research in all sorts of ways.


Here in New Zealand the skies are relatively dark compared to North America, Europe, eastern Asia or south-eastern Australia. Just consider the following composite image of ‘the world at night’.

As per usual, NZ has almost fallen off the map. Here is an alternative version showing Australia and New Zealand a little more centrally:

From the perspective of astronomy, then, NZ does not have the problems of London, New York, Tokyo or Singapore. Nevertheless, there is a surprising amount of artificial light leaking up into the heavens from our cities and towns. The trouble is that some of those photons get scattered back downwards by the atmosphere, hindering the quests of those who would study the planets, stars and galaxies which can be seen far clearer in a dark sky.


One of the easiest ways to examine how much light is leaking upwards from NZ is to look at the photographs that astronauts take from the Space Station, as in the header to this blog post. So here we go. (Before starting, though, I might also point out that NZ has been particularly well-photographed from orbit by the German astronaut Alexander Gerst – who was Space Station Commander for some months last year – due to his affinity with this country: he took a master’s degree in earth sciences at the Victoria University of Wellington 2003.)

The South Island of NZ plus the lower North Island as seen on a dark, moonless night from the International Space Station. The structure at upper right is a robotic limb known as the Canadarm.
NZ’s central regions, with various towns and cities labelled. Note the gas burn-off from the Maui oilfield. Unlike the preceding image, this one was taken while the Moon was high in the sky, and so moonlight (simply scattered sunlight) illuminates the clouds, and also causes the brighter patch of light evident as a reflection from the Pacific to the northeast of NZ.
Looking almost straight down from the Space Station, the street lights (and other illuminations) of Christchurch form a distinctive pattern; but the lights also mean that one needs to head for the hills (or at least the Canterbury Plains) to do any serious astronomy.

In fairness I have to say that the following photograph is not really about light pollution – it was taken during daytime (so the Sun is that bright thing near the top of the image) – but I do think that it is a most wonderful framing of a picture of southernmost New Zealand from far above.

Southern NZ seen through a window on the Space Station.

Finally, I should point out that there are other forms of natural light pollution (as such) than just the airglow of the atmosphere at night and possible moonlight, depending on the phase of the synodic month. Here I am referring to something that is itself of considerable scientific interest: the aurorae. The aurora australis (‘Southern Lights’) are certainly a wonderful thing to witness, and most easily seen the further south you are in New Zealand, and so closer to the southern magnetic pole. From the Space Station, though, astronauts get a magnificent view of the aurorae from above, as in the next image.

Auroral arcs and spikes seen from the Space Station; note the dark external equipment on the right (and top) of the photograph. The higher-altitude red glow is due to atomic oxygen, at two wavelengths near 630 nm; the lower-altitude greenish emission is also due to atomic oxygen, occurring at a wavelength of 557.7 nm.

I wrote finally above, meaning that I was introducing the last of the photographs. I do have another thing to say, however.

Although we cannot do much about the natural glows in the sky that can limit optical astronomy, we can perhaps reduce or contain the amount of artificial light leaking upwards. Globally, this has become a major quest for astronomers: to alert the public that their skies are ‘being lost’, and that there are simple (often energy-saving) ways that steps can be taken to reduce this deleterious effect of modern technological life. There is an International Dark-Sky Association (IDA):

Shortly, on October 20-23 (Sunday-Wednesday), there will be held in Tekapo the New Zealand Starlight Conference (see also here), under the auspices of the IDA. Speakers from around the astronomical world will be coming to tell of how they are organising dark-sky reserves in their own countries and principalities. The Aoraki Mackenzie International Dark Sky Reserve (see also here and here) is an example of such a development in NZ, and there are several others. More power to their arms.

 


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